No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Formation of HO2 radicals from the photodissociation of H2O2 at 248 nm
The Journal of Chemical Physics
Abstract
New experiments have been performed in order to evaluate the primary quantum yield of pathway by direct observation of HO2 radical formation after photolysis of H2O2 at 248 nm. The quantum yield for OH production at 248 nm has been determined by two different groups including Vaghijani and Ravishankara who have measured the quantum yields of OH, 0, and H relative to the known quantum yields of 03 and CH 3SH photolysis and have obtained a quantum yield. The IUPAC subcommittee has also recommended a quantum yield of two OH radicals at photolysis wavelengths of λ>230 nm. Part of the He flow in some experiments has been replaced by O2.
... The OH concentration is twice the depletion of the H 2 O 2 concentration because each H 2 O 2 molecule yields 2 OH radicals on photodissociation. 33,34 The O 2 concentration is calculated directly from the used flow. For the initial HCl concentration we assume 100% conversion of 2CE to VA/AC + HCl and set it equal to the 2CE concentration calculated from the used flows. ...
... ,34 We used a flow of30 sccm H 2 O 2 in He, delivered from a bubbler of Urea ·H 2 O 2 held at 40°C and 30 Torr. The H 2 O 2 was depleted by 0.5% yielding OH radicals at 1% of the H 2 O 2 concentration. ...
The concentration of formic acid in Earth's troposphere is underestimated by detailed chemical models compared to field observations. Phototautomerization of acetaldehyde to its less stable tautomer vinyl alcohol, followed by the OH-initiated oxidation of vinyl alcohol, has been proposed as a missing source of formic acid that improves the agreement between models and field measurements. Theoretical investigations of the OH + vinyl alcohol reaction in excess O2 conclude that OH addition to the α carbon of vinyl alcohol produces formaldehyde + formic acid + OH, whereas OH addition to the β site leads to glycoaldehyde + HO2. Furthermore, these studies predict that the conformeric structure of vinyl alcohol controls the reaction pathway, with the anti-conformer of vinyl alcohol promoting α OH addition, whereas the syn-conformer promotes β addition. However, the two theoretical studies reach different conclusions regarding which set of products dominate. We studied this reaction using time-resolved multiplexed photoionization mass spectrometry to quantify the product branching fractions. Our results, supported by a detailed kinetic model, conclude that the glycoaldehyde product channel (arising mostly from syn-vinyl alcohol) dominates over formic acid production with a 3.6:1.0 branching ratio. This result supports the conclusion of Lei et al. that conformer-dependent hydrogen bonding at the transition state for OH-addition controls the reaction outcome. As a result, tropospheric oxidation of vinyl alcohol creates less formic acid than recently thought, increasing again the discrepancy between models and field observations of Earth's formic acid budget.
... The setup has been described in detail before [19][20][21][22][23] and is only briefly discussed here ( Figure 1). can be quantified reliably) or to C2H5O2, the absorption cross-section of C2H5O2 is determined relative to the one of HO2. ...
... The setup has been described in detail before [19][20][21][22][23] and is only briefly discussed here ( Figure 1). The setup consisted of a 0.79 m long flow reactor made of stainless steel. ...
The absolute absorption cross-section of the ethyl peroxy radical C2H5O2 in the Ã←X˜ electronic transition with the peak wavelength at 7596 cm−1 has been determined by the method of dual wavelengths time resolved continuous wave cavity ring down spectroscopy. C2H5O2 radicals were generated from pulsed 351 nm photolysis of C2H6/Cl2 mixture in presence of 100 Torr O2 at T = 295 K. C2H5O2 radicals were detected on one of the CRDS paths. Two methods have been applied for the determination of the C2H5O2 absorption cross-section: (i) based on Cl-atoms being converted alternatively to either C2H5O2 by adding C2H6 or to hydro peroxy radicals, HO2, by adding CH3OH to the mixture, whereby HO2 was reliably quantified on the second CRDS path in the 2ν1 vibrational overtone at 6638.2 cm−1 (ii) based on the reaction of C2H5O2 with HO2, measured under either excess HO2 or under excess C2H5O2 concentration. Both methods lead to the same peak absorption cross-section for C2H5O2 at 7596 cm−1 of σ = (1.0 ± 0.2) × 10−20 cm2. The rate constant for the cross reaction between of C2H5O2 and HO2 has been measured to be (6.2 ± 1.5) × 10−12 cm3 molecule−1 s−1.
... At higher photon energies, the reaction H 2 O 2 + hn yields H(2S) + HO 2 (1 2 A 00 ) thus producing an excited hydroperoxy species for further studies (Ref. [3] and references therein). In this work, however, we focus on the dissociation of H 2 O 2 into two OH radicals upon its excitation at 266 nm since this reaction is characterized in numerous experiments by applying linear spectroscopic techniques and is well suited for an analysis of the complementary vectorial features offered by the four-wave mixing method. ...
... As seen from the plots in Fig. 4, the relaxation time scale at which these effects evolve depends on pressure since the latter linearly enhances the collision rate. The XXXX or XYYX DFWM components' intensity decrease with the delay can have two reasons, the first one being that the spectra may be contributed to by the anisotropic orientation of the angular momentum J and by its alignment to v, [3] which are eliminated by collisions. Second, collisions thermalize the rotational energy distribution and suppress the lines that correspond to rapidly rotating molecules. ...
The potential of a non-linear spectroscopic technique – degenerate four-wave mixing (DFWM) spectroscopy – for photo-dissociation studies is investigated. By applying a carefully considered control of the input beam polarizations, the technique is used to measure the line shapes of specific rovibrational transitions in the A 2Σ+ − X 2Π (0,0) band of nascent OH radicals upon pulsed photo-dissociation of H2O2. The photofragmentation is performed by linearly polarized radiation at 266 nm, and DFWM spectra are observed at various time delays. In contrast to linear spectroscopic methods, isotropic and anisotropic components of the non-linear susceptibility tensor are separately accessible by using appropriate polarization geometries. In particular, four-wave mixing signals can be generated that are exclusively due to the occurrence of the transient anisotropy of the recoil velocity and angular momentum vector distributions of the fragments emerging from the dissociation reaction. The observed line shapes in isotropic and anisotropic DFWM spectra are governed by the Doppler effect and vectorial correlations between the parent H2O2 molecule's transition dipole moment, the OH fragments’ recoil velocity and angular momentum, as well as by the processes of collision-induced rotational and translational relaxation. Copyright © 2013 John Wiley & Sons, Ltd.
... En raison de son rôle majeur dans l'atmosphère, plusieurs travaux précédents ont contribué à la détection du radical HO2 dans la gamme spectrale du proche infrarouge. En ce qui concerne les transitions vibrationnelles, notons, en particulier, que la bande de la première harmonique 2•1 pour le radical HO2, se trouve à environ 6600cm -1 et a été précédemment étudiée [47, 56, 57] et que sa bande d'absorption à 6638,205 cm -1 a déjà été bien décrite et utilisée dans le cadre d' études en laboratoire[58,59,60]. ...
La dégradation des polluants organiques volatils, tels que les Composés Organiques Volatils (VOCs) dans les conditions troposphériques est généralement initiée par le principal oxydant qui est le radical OH, suivie par la formation des radicaux hydroproxyles HO2 et alkylperoxyles RO2 par réaction des produits de réaction avec l’oxygène. Le devenir de ces radicaux joue un rôle important dans la chimie de la troposphère. Ils sont étroitement liés au cycle qui contrôle la capacité oxydante de l’atmosphère et la formation d’ozone troposphérique. Dans un environnement pollué, la chimie des radicaux peroxyles est bien connue et de nombreux résultats expérimentaux sont disponibles dans la littérature. Dans un environnement propre (où la concentration en oxyde d’azote NOx (x=1,2) est faible) la réactivité entre HOx (x=1,2) et RO2 contrôle la chimie de la troposphère. Cependant, cette chimie n’est pas encore bien connue. Dans le cadre de cette thèse, des études cinétiques expérimentales ont été effectué afin de mieux comprendre les mécanismes d’oxydation de ces espèces. Un dispositif expérimental de photolyse laser couplée à des techniques spectroscopiques de détection résolues dans le temps : Spectroscopie à temps de déclin d’une cavité optique (cw-CRDS : continuous wave Cavity Ring-Down Spectroscopy) permettant la détection des radicaux HO2 et RO2 et Fluorescence induite par Laser (FIL) pour la détection des radicaux OH a été utilisé.Différents systèmes de réaction ont été étudié en utilisant la technique expérimentale mentionnée ci-dessus :1) la réaction de CH3C(O)O2 + CH3C(O)O2, et CH3C(O)O2 + CH3O2, 2) CH3C(O)CH2O2 + CH3C(O)CH2O2 et pour la première fois la réaction Cl + CH3C(O)CH2O2, 3) DO2 + DO2 et pour la première fois la réaction HO2 + DO2. Les constantes de vitesse ont été déterminé pour ces six réactions à température ambiante. Pour les quatre premières réactions différentes voies réactionnelles sont possible, et nous avons également déterminé le rapport de branchement de la voie menant à la formation des radicaux pour ces réactions.
... An excimer laser (Lambda Physik LPX 201) at 248 nm was used to photolyze H 2 O 2 generating OH radicals exclusively. 26 The photolysis laser was operated at a repetition rate of 0.3 Hz and a laser fluence of B15 mJ cm À2 . The low pulsing frequency of the excimer laser ensured a complete replenishment of the gaseous mixtures inside the flow reactor before the next photolysis event. ...
This work presents the OH-initiated oxidation kinetics of 1,4-cyclochexadiene (1,4-CHD). The temperature dependence of the reaction was investigated by utilizing a laser flash photolysis flow reactor and laser-induced fluorescence (LPFR/LIF) technique over the temperature range of 295-438 K and a pressure of ∼50 torr. The kinetics of the reaction was followed by measuring the LIF signal of OH radicals near 308 nm. The reaction of OH radicals with 1,4-CHD exhibited a clear negative temperature dependence. To discern the role of various channels, ab initio and RRKM-based ME calculations (RRKM-ME) were performed over temperatures of 200-2000 K and pressures of 0.76-7600 torr. The computed energy profile revealed that the reaction proceeds via the formation of a pre-reaction van der Waals complex at the entrance channel. The complex was found to be more stable than that usually seen in other alkenes + OH reactions. Both the addition channel and the abstraction reaction of allylic hydrogen were found to have negative energy barriers. Interestingly, the abstraction reaction exhibited a negative temperature dependence at low temperatures and contributed significantly (∼37%) to the total rate coefficients even under atmospheric conditions. At T ≥ 900 K, the reaction was found to proceed exclusively (>95%) via the abstraction channel. Due to the competing channels, the reaction of OH radicals with 1,4-CHD displays complicated kinetic behaviours, reflecting the salient features of the energy profile. The role of competing channels was fully characterized by our kinetic model. The calculated rate coefficients showed excellent agreement with the available experimental data.
... The setup has been described in detail before [32][33][34][35][36][37] and is only briefly discussed here. The setup consisted of a 0.79 m long flow reactor made of stainless steel. ...
The self-reaction of acetylperoxy radicals (CH3C(O)O2•) (R1) as well as their reaction with methyl peroxy radicals (CH3O2•) (R2) have been studied using laser photolysis coupled to a selective time resolved detection of three different radicals by cw-CRDS in the near-infrared range: CH3C(O)O2• was detected in the Ã-X˜ electronic transition at 6497.94 cm−1, HO2• was detected in the 2ν1 vibrational overtone at 6638.2 cm−1, and CH3O2• radicals were detected in the Ã-X˜ electronic transition at 7489.16 cm−1. Pulsed photolysis of different precursors at different wavelengths, always in the presence of O2, was used to generate CH3C(O)O2• and CH3O2• radicals: acetaldehyde (CH3CHO/Cl2 mixture or biacetyle (CH3C(O)C(O)CH3) at 351 nm, and acetone (CH3C(O)CH3) or CH3C(O)C(O)CH3 at 248 nm. From photolysis experiments using CH3C(O)C(O)CH3 or CH3C(O)CH3 as precursor, the rate constant for the self-reaction was found with k1 = (1.3 ± 0.3) × 10−11 cm3s−1, in good agreement with current recommendations, while the rate constant for the cross reaction with CH3O2• was found to be k2 = (2.0 ± 0.4) × 10−11 cm3s−1, which is nearly two times faster than current recommendations. The branching ratio of (R2) towards the radical products was found at 0.67, compared with 0.9 for the currently recommended value. Using the reaction of Cl•-atoms with CH3CHO as precursor resulted in radical profiles that were not reproducible by the model: secondary chemistry possibly involving Cl• or Cl2 might occur, but could not be identified.
... It is relatively straightforward to produce many paramagnetic species of interest in the gas phase, with well-established methods including the photodissociation of closed-shell molecular precursors, electron beam irradiation, and electrical discharges (12)(13)(14)(15). The primary challenge lies in isolating the state-selected paramagnetic species of interest from all other contaminant species present in the sample (for example, from precursor molecules, seed gases, and other fragments). ...
Since external magnetic fields were first employed to deflect paramagnetic atoms in 1921, a range of magnetic field–based methods have been introduced to state-selectively manipulate paramagnetic species. These methods include magnetic guides, which selectively filter paramagnetic species from all other components of a beam, and magnetic traps, where paramagnetic species can be spatially confined for extended periods of time. However, many of these techniques were developed for atomic—rather than molecular—paramagnetic species. It has proven challenging to apply some of these experimental methods developed for atoms to paramagnetic molecules. Thanks to the emergence of new experimental approaches and new combinations of existing techniques, the past decade has seen significant progress toward the manipulation and control of paramagnetic molecules. This review identifies the key methods that have been implemented for the state-selective manipulation of paramagnetic molecules—discussing the motivation, state of the art, and future prospects of the field. Key applications include the ability to control chemical interactions, undertake precise spectroscopic measurements, and challenge our understanding of chemical reactivity at a fundamental level.
Expected final online publication date for the Annual Review of Physical Chemistry, Volume 72 is April 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
... Gas-phase radicals are readily formed by photolysis, electron beam irradiation or electric discharge methods, [1][2][3] and as such a number of other (unwanted) species are typically present in the beam. It is the filtering of these radicals-the removal of all unwanted species from the beam-that is experimentally challenging. ...
Radicals are prevalent in gas-phase environments such as the atmosphere, combustion systems, and the interstellar medium. To understand the properties of the processes occurring in these environments, it is helpful to study radical reaction systems in isolation-thereby avoiding competing reactions from impurities. There are very few methods for generating a pure beam of gas-phase radicals, and those that do exist involve complex setups. Here, we provide a straightforward and versatile solution. A magnetic radical filter (MRF), composed of four Halbach arrays and two skimming blades, can generate a beam of velocity-selected low-field-seeking hydrogen atoms. As there is no line-of-sight through the device, all species that are unaffected by the magnetic fields are physically blocked; only the target radicals are successfully guided around the skimming blades. The positions of the arrays and blades can be adjusted, enabling the velocity distribution of the beam (and even the target radical species) to be modified. The MRF is employed as a stand-alone device-filtering radicals directly from the source. Our findings open up the prospect of studying a range of radical reaction systems with a high degree of control over the properties of the radical reactants.
... It is notoriously difficult to prepare a pure, state-selected ensemble of radical reactants. Typical methods of forming radicals, such as through photodissociation [1], electron beam irradiation [2] or discharge methods [3,4], can also produce unwanted fragments and thus yield mixtures of precursor molecules, the radicals of interest, other dissociation products and seed gases. Mass and velocity selection of negative ions followed by electron photodetachment is a viable method for generating a pure beam of radicals [5,6]. ...
A Halbach array composed of 12 permanent magnets in a hexapole configuration is employed to deflect hydrogen atoms as they exit a Zeeman decelerator. The ability to preferentially manipulate H atoms is very useful, as there are currently very few techniques that are appropriate for purifying a beam of H atoms from precursor molecules (such as molecular hydrogen or ammonia), seed gases, and other contaminant species. The extent to which hydrogen atoms are deflected by a single Halbach array when it is tilted or shifted off the main beam axis is characterised experimentally and interpreted with the aid of a simple mathematical model. A radical beam filter is subsequently introduced, where four Halbach arrays arranged in series serve to deflect H atoms away from the main beam axis and around skimming blades; all other components of the incoming beam are blocked by the blades and are thus not transmitted through the magnetic guide. The properties of the guide, as established by experimental measurements and complemented by detailed simulations, confirm that it is a highly effective beam filter—successfully generating a pure and velocity-selected beam of H atoms.
... Millimeter wave spectroscopy of rotational transitions [5][6][7] has been used to quantify HO 2 in interstellar space 8 The absorption spectra of HO 2 and DO 2 in the near infrared region have first been observed by Hunziker and Wendt 10 and consist of vibrational overtone transitions as well as of a low-lying electronic transition, typical for peroxides. The overtone transition of HO 2 near 6638 cm -1 has been recorded [11][12][13][14] and already used for various laboratory studies [15][16][17] . The low-lying rovibronic à → ̃ 000-000 band has been recorded by Fink and Ramsay for both, HO 2 18 and DO 2 19 using FTIR emission spectroscopy. ...
The absorption spectrum of the à ²A’ ← ²A” 000-000 band of HO2 and DO2 radicals has been measured in the range 6941 – 7077 cm⁻¹ with a resolution of around 0.005 cm⁻¹. HO2 and DO2 radicals were generated from the reaction of Cl-atoms with CH3OH and CD3OD, respectively, whereby Cl-atoms were generated by pulsed photolysis of (COCl)2 at 248nm. One time-resolved absorption curve was measured by cw-Cavity Ring Down Spectroscopy (cw-CRDS) for each wavelength. The relative absorption coefficient for each wavelength was then obtained from the ring down time τ, extrapolated to t = 0 s with respect to the photolysis pulse, and the ring down time τ0 before the photolysis pulse. Absolute absorption coefficients were quantified for several selected lines relative to a well characterized absorption line of HO2 at 6638.21 cm⁻¹ by a second cw-CRDS absorption path. The maximum absorption cross section at 50 Torr Helium was for HO2 the transition N Ka Kc J = 11 0 11 11.5 ← 11 1 11 11.5 with σ7000.28 cm-1 = 2.12 × 10⁻¹⁹ cm² and for DO2 the doublet transition N Ka Kc J = 18 1 18 18.5 ← 18 0 18 18.5 and 18 1 18 17.5 ← 18 0 18 17.5 with σ7019.83cm-1 = 2.97 × 10⁻¹⁹ cm².. The full spectrum has been very well reproduced by employing spectroscopic data from earlier works.
... Another photolytic HO 2 source can be the above mentioned photolysis of H 2 O 2 , leading finally to HO 2 radicals by the subsequent reaction (Thiebaud et al. 2007): ...
Elementary chemical reaction steps, relevant to combustion and environmental chemistry, involve in many cases the reaction between a radical species and a closed-shell molecule. Rate constants and product branching ratios of such elementary reactions are important input parameter in complex models, describing combustion processes or the chemistry of the atmosphere. Different experimental setups, widely used to study such reactions, are described in this chapter. The main difference consists in the way radicals are generated (either pulsed or continuous) and the way the time resolution is obtained. The coupling of these different experimental setups with commonly used detection methods is also described.
... Finally, UV plays an important role in the production of OH and other reactive species via photodissociation of hydrogen peroxide (see (2.46) through (2.48) [298]). ...
The use of atmospheric pressure plasmas in gases and liquids for purification of liquids has been investigated by numerous researchers, and is highly attractive due to their strong potential as a disinfectant and sterilizer. However, the fundamental understanding of plasma production in liquid water is still limited. Advancements in the field will rely heavily on the development of innovative diagnostics.
This dissertation investigates several aspects of electrical discharges in gas bubbles in water. Two primary experimental configurations are investigated: the first allows for single bubble breakdown analysis through the use of an acoustic trap. The second experiment investigates the resulting liquid phase chemistry that is driven by a dielectric barrier discharge in the bulk liquid.
Breakdown mechanisms of attached and unattached gas bubbles in liquid water were investigated using the first device. The breakdown scaling relation between breakdown voltage, pressure and dimensions of the discharge was studied and a Paschen-like voltage dependence was discovered. High-speed photography suggests the phenomenon of electrical charging of a bubble due to a high voltage pulse, which can be significant enough to prevent breakdown from occurring.
The resulting liquid-phase chemistry of the plasma-bubble system was also examined. Plasma parameters such as electron density, gas temperature, and molecular species production are found to have both a time-dependence and gas dependence. These dependencies afford effective control over plasma-driven decomposition. The effect of plasma-produced radicals on various wastewater simulants is studied. Various organic dyes, halogenated compounds, and algae water are decomposed and assessed. Toxicology studies with melanoma cells exposed to plasma-treated dye solutions are completed; treated dye solution were found to be non-toxic.
Thirdly, the steam plasma system was developed to circumvent the acidification associated with gas-feed discharges. This steam plasma creates its own gas pocket via field emission. This steam plasma has strong decontamination properties, with continued decomposition of contaminants lasting beyond two weeks.
Finally, a ???two-dimensional bubble??? was developed and demonstrated as a novel diagnostic device to study the gas-water interface, the reaction zone. This device is shown to provide convenient access to the reaction zone and decomposition of various wastewater simulants is investigated.
... The quantum yield of OH is known to be 2 at this wavelength, with negligible generation of H and HO 2 . 10,11 The photolysis of DHFs and dark reaction of DHF with H 2 O 2 were found to be absent in our experimental conditions. The products of the reaction of OH with the DHFs were identified using GC−MS (Shimadzu-GCMS-QP2010) by comparing the fragmentation pattern of the product with standard mass spectral library (NIST-05 and Wiley) data. ...
Experimental characterisation of products during OH-initiated oxidation of dihydrofurans (DHF) confirms the formation of furan accompanied by the formation of HO2 to be a significant channel in 2,5-DHF (21 ± 3 %), whereas it is absent in 2,3-DHF. Theoretical investigations on the reaction of OH with these molecules are carried out to understand this difference. All possible channels of reaction are studied at M06-2X level with 6-311G* basis set, and the stationary points on the potential energy surface are optimised. The overall rate coefficients calculated using conventional TST with Wigner tunneling correction for 2,5-DHF and 2,3-DHF are 2.25 x 10-11 and 4.13 x 10-10 cm3 molecule-1 s-1, respectively, in the same range as the previously determined experimental values. The branching ratios of different channels were estimated using the computed rate coefficients. The abstraction of H atom, leading to dihydrofuranyl radical, is found to be a significant probability, equally important as the addition of OH to the double bond in the case of 2,5-DHF. However, this probability is very small in the case of 2,3-DHF, because the rate coefficient of the addition reaction is more than 10 times that of the abstraction reaction. This explains the conspicuous absence of furan among the products of the reaction of OH with 2,3-DHF. The calculations also indicate that the abstraction reaction, and hence furan formation, may become significant for OH-initiated oxidation of 2,3-DHF at temperatures relevant to combustion.
... In line with this, gas phase H 2 O 2 has been found to dissociate at wavelengths shorter than ca. 320 nm [22] with two ground-state hydroxyl radicals being the dominating photolysis products [23]. Experimental studies on aqueous hydrogen peroxide using 'far UV' (253 nm or shorter wavelengths) have revealed that OH and HO 2 radicals are the major products [ In this paper we report on the possible roles of H 2 O 2 in plants in response to UV-B irradiation. ...
Solar UV-B (280-315 nm) radiation is a developmental signal in plants but may also cause oxidative stress when combined with other environmental factors. Using computer modelling and in solution experiments we show that UV-B is capable of photosensitizing hydroxyl radical production from hydrogen peroxide. We present evidence that the oxidative effect of UV-B in leaves is at least two-fold: (i) it increases cellular hydrogen peroxide concentrations, to a larger extent in pyridoxine antioxidant mutant pdx1.3-1 Arabidopsis and (ii) is capable of a partial photo-conversion of both ‘natural’ and ‘extra’ hydrogen peroxide to hydroxyl radicals. As stress conditions other than UV can increase cellular hydrogen peroxide levels, synergistic deleterious effects of various stresses may be expected already under ambient solar UV-B.
... OH radicals were co-generated by the simultaneous photolysis of an appropriate precursor: in most experiments, O 3 has been photolysed in the presence of H 2 O: The possible influence of these O( 3 P) on the OH-decays will be discussed further down. In a few complementary experiments, H 2 O 2 photolysis at 248 nm, known as a clean OH-source [16,17], has been used as a precursor, thus excluding possible complications due to O( 3 P) chemistry. Relative time-resolved OH radical concentration profiles were detected by high repetition rate LIF (10 kHz) [14]. ...
... Also some application of CRDS for time-resolved plasma research have been reported [64][65][66][67]. Whereas most experiments were performed using pulsed laser sources, which are ideally suited for pump-probe experiments, also some cw-CRDS applications have been reported using NIR diode lasers [64,65,[68][69][70]. As has been described in detail by Thiébaud and Fittschen [69], the cw variant requires a more demanding time-control of photolysis laser and ringdown event. ...
Cavity ringdown spectroscopy (CRDS) and frequency modulation spectroscopy (FMS) are sensitive absorption based detection methods that have found widespread applications in gas phase reaction kinetics. In part 2 of this review, the theoretical foundations of CRDS are addressed with a special emphasis on quantitative time-resolved measurements of concentration profiles. A complementary review of FMS can be found in part 1 (Z. Phys. Chem. 222 (2008) 1-30). Practical aspects, possible pitfalls, attainable sensitivities, and modern trends are discussed. Recent kinetic studies based on CRDS measurements as a time-resolved detection tool are briefly reviewed and a bibliography with 136 entries is included to facilitate the access to the large body of original literature.
The radical–radical reaction between OH and HO2 has been considered for a long time as an important reaction in tropospheric photochemistry and combustion chemistry. However, a significant discrepancy of an order of magnitude for rate coefficients of this reaction is found between two recent experiments. Herein, we investigate the reaction OH + HO2 via direct spectral quantification of both the precursor (H2O2) and free radicals (OH and HO2) upon the 248 nm photolysis of H2O2 using infrared two-color time-resolved dual-comb spectroscopy. With quantitative and kinetic analysis of concentration profiles of both OH and HO2 at varied conditions, the rate coefficient kOH+HO2 is determined to be (1.10 ± 0.12) × 10–10 cm³ molecule–1 s–1 at 296 K. Moreover, we explore the kinetics of this reaction under conditions in the presence of water, but no enhancement in the kOH+HO2 can be observed. This work as an independent experiment plays a crucial role in revisiting this prototypical radical–radical reaction.
Absolute line strengths of several transitions in the ν1 fundamental band of the hydroxyl radical (OH) have been measured by simultaneous determination of hydrogen peroxide (H2O2) and OH upon laser photolysis of H2O2. Based on the well-known quantum yield for the generation of OH radicals in the 248-nm photolysis of H2O2, the line strength of the OH radicals can be accurately derived by adopting the line strength of the well-characterized transitions of H2O2 and analyzing the difference absorbance time traces of H2O2 and OH obtained upon laser photolysis. Employing a synchronized two-color dual-comb spectrometer, we measured high-resolution time-resolved absorption spectra of H2O2 near 7.9 µm and the OH radical near 2.9 µm, simultaneously, under varied conditions. In addition to the studies of the line strengths of the selected H2O2 and OH transitions, the kinetics of the reaction between OH and H2O2 were investigated. A pressure-independent rate coefficient kOH+H2O2 was determined to be [1.97 (+0.10/−0.15)] × 10−12 cm3 molecule−1 s−1 at 296 K and compared with other experimental results. By carefully analyzing both high-resolution spectra and temporal absorbance profiles of H2O2 and OH, the uncertainty of the obtained OH line strengths can be achieved down to <10% in this work. Moreover, the proposed two-color time-resolved dual-comb spectroscopy provides a new approach for directly determining the line strengths of transient free radicals and holds promise for investigations on their self-reaction kinetics as well as radical–radical reactions.
During the oxidation of hydrocarbons using hydrogen peroxide solutions, the evolution of gaseous oxygen is a side and undesirable process, in which the consumption of the oxidizer is not associated with the formation of target products. Therefore, no attention is paid to the systematic study of the chemical composition of the gas and the mechanisms of its formation. Filling this gap, the authors discovered a number of new, previously unidentified, interesting facts concerning both gas evolution and the oxidation of hydrocarbons. In a 33% H2O2/Cu2Cl4·2DMG/CH3CN system, where DMG is dimethylglyoxime (Butane-2,3-dione dioxime), and is at 50 °C, evidence of significant evolution of gaseous hydrogen, along with the evolution of gaseous oxygen was found. In the authors’ opinion, which requires additional verification, the ratio of gaseous hydrogen and oxygen in the discussed catalytic system can reach up to 1:1. The conditions in which only gaseous oxygen is formed are selected. Using a number of oxidizable hydrocarbons with the first adiabatic ionization potentials (AIPs) of a wide range of values, it was found that the first stage of such a process of evolving only gaseous oxygen was the single electron transfer from hydrogen peroxide molecules to trinuclear copper clusters with the formation, respectively, of hydrogen peroxide radical cations H2O2•+ and radical anions Cu3Cl5•− (AIP = 5 eV). When the conditions for the implementation of such a single electron transfer mechanism are exhausted, the channel of decomposition of hydrogen peroxide molecules into gaseous hydrogen and oxygen is switched on, which is accompanied by the transition of the system to an oscillatory mode of gas evolution. In some cases, the formation of additional amounts of gaseous products is provided by the catalytically activated decomposition of water molecules into hydrogen and oxygen after the complete consumption of hydrogen peroxide molecules in the reaction of gaseous oxygen evolution. The adiabatic electron affinity of various forms of copper molecules involved in chemical processes is calculated by the density functional theory method.
Hydrogen peroxide (H2O2) is a unique molecule that is applied in various fields, including energy chemistry, astrophysics, and medicine. H2O2 readily forms clusters with water molecules. In the present study, the reactions of ionized H2O2-water clusters, H2O2 +(H2O) n , after vertical ionization of the parent neutral cluster were investigated using the direct ab initio molecular dynamics (AIMD) method to elucidate the reaction mechanism. Clusters with one to five water molecules, H2O2-(H2O) n (n = 1-5), were examined, and the reaction of [H2O2 +(H2O) n ]ver was tracked from the vertical ionization point to the product state, where [H2O2 +(H2O) n ]ver is the vertical ionization state (hole is localized on H2O2). After ionization, fast proton transfer (PT) from H2O2 + to the water cluster (H2O) n was observed in all clusters. The HOO radical and H3O+(H2O) n-1 were formed as products. The PT reaction proceeds directly without an activation barrier. The PT times for n = 1-5 were calculated to be 36.0, 9.8, 8.3, 7.7, and 7.1 fs, respectively, at the MP2/6-311++G(d,p) level, indicating that PT in these clusters is a very fast process, and the PT time is not dependent on the cluster size (n), except in the case of n = 1, where the PT time was slightly longer because the bond distance and angle of the hydrogen bond in n = 1 were deformed from the standard structure. The reaction mechanism was discussed based on these results.
Radicals are abundant in a range of important gas-phase environments. They are prevalent in the atmosphere, in interstellar space, and in combustion processes. As such, understanding how radicals react is essential for the development of accurate models of the complex chemistry occurring in these gas-phase environments. By controlling the properties of the colliding reactants, we can also gain insights into how radical reactions occur on a fundamental level. Recent years have seen remarkable advances in the breadth of experimental methods successfully applied to the study of reaction dynamics involving paramagnetic species-from improvements to the well-known crossed molecular beams approach to newer techniques involving magnetically guided and decelerated beams. Coupled with ever-improving theoretical methods, quantum features are being observed and interesting insights into reaction dynamics are being uncovered in an increasingly diverse range of systems. In this highlight article, we explore some of the exciting recent developments in the study of chemical dynamics involving paramagnetic species. We focus on low-energy reactive collisions involving neutral radical species, where the reaction parameters are controlled. We conclude by identifying some of the limitations of current methods and exploring possible new directions for the field.
The rate constants of the reactions of DO2 + HO2 (R1) and DO2 + DO2 (R2) have been determined by the simultaneous, selective, and quantitative measurement of HO2 and DO2 by continuous wave cavity ring‐down spectroscopy (cw‐CRDS) in the near infrared, coupled to a radical generation by laser photolysis. HO2 was generated by photolyzing Cl2 in the presence of CH3OH and O2. Low concentrations of DO2 were generated simultaneously by adding low concentrations of D2O to the reaction mixture, leading through isotopic exchange on tubing and reactor walls to formation of low concentrations of CH3OD and thus formation of DO2. Excess DO2 was generated by photolyzing Cl2 in the presence of CD3OD and O2, small concentrations of HO2 were always generated simultaneously by isotopic exchange between CD3OD and residual H2O. The rate constant k1 at 295 K was found to be pressure independent in the range 25–200 Torr helium, but increased with increasing D2O concentration k1 = (1.67 ± 0.03) × 10−12 × (1 + (8.2 ± 1.6) × 10−18 cm3 × [D2O] cm−3) cm3 s−1. The rate constant for the DO2 self‐reaction k2 has been measured under excess DO2 concentration, and the DO2 concentration has been determined by fitting the HO2 decays, now governed by their reaction with DO2, to the rate constant k1. A rate constant with insignificant pressure dependence was found: k2 = (4.1 ± 0.6) × 10−13 (1 + (2 ± 2) × 10−20 cm3 × [He] cm−3) cm3 s−1 as well as an increase of k2 with increasing D2O concentration was observed: k2 = (4.14 ± 0.02) × 10−13 × (1 + (6.5 ± 1.3) × 10−18 cm3 × [D2O] cm−3) cm3 s−1. The result for k2 is in excellent agreement with literature values, whereas this is the first determination of k1.
Sulfuric acid (H2SO4) is the seed molecule for formation of stratospheric sulfate aerosol layer that assists ozone depletion by activation of halogen species. The impact of increased stratospheric sulfate aerosols due to large volcanic eruptions and possible side effect claimed in the geoengineering scheme of global climate using man-made injected stratospheric sulfate aerosols is ozone depletion. Given that both volcanic eruptions and geoengineering scheme are ultimately connected with increased upper stratospheric concentrations of H2SO4, here we show by theoretical approach that the pressure-independent H2SO4 + O(¹D) insertion/addition reactions via barrierless formation of peroxysulfuric acid (H2SO5) or HSO4 + OH radicals or sulfur trioxide (SO3) + hydrogen peroxide (H2O2) molecules are the potential routes towards H2SO4 loss above the stratospheric sulfate aerosol layer, and for the regeneration or transportation of consumed lower-middle stratospheric OH radical in the upper stratosphere at the cost of O(¹D)/ozone.
The reaction of peroxy radicals, RO 2 , with OH radicals has long been ignored to play any role in atmospheric chemistry. Recent experimental and modeling studies show however that these reactions are fast and can play a role in remote atmospheres with low NO concentrations when the major fate of peroxy radicals, its reaction with NO, slows down. The present article summarizes recent work on this class of reactions and its implications in different environments.
The reaction between CH3O2 and OH radicals has been shown to be fast and to play an appreciable role for the removal of CH3O2 radials in remote environments such as the marine boundary layer. Two different experimental techniques have been used here to determine the products of this reaction. The HO2 yield has been obtained from simultaneous time-resolved measurements of the absolute concentration of CH3O2, OH and HO2 radicals by cw-CRDS. The possible formation of a Criegee intermediate has been measured by broadband cavity enhanced UV absorption. A yield of ϕHO2= (0.8±0.2) and an upper limit for ϕCriegee= 0.05 has been determined for this reaction, suggesting a minor yield of methanol or stabilized trioxide as a product. The impact of this reaction on the composition of the remote marine boundary layer has been determined by implementing these findings into a box model utilizing the Master Chemical Mechanism v3.2, and constraining the model for conditions found at the Cape Verde Atmospheric Observatory in the remote tropical Atlantic Ocean. Inclusion of the CH3O2+OH reaction into the model results in up to 30% decrease in the CH3O2 radical concentration while the HO2 concentration increased by up to 15%. Production and destruction of O3 are also influenced by these changes, and the model indicates that taking into account the reaction between CH3O2 and OH leads to a 6% decrease of O3.
Reaction rate coefficients for the reaction of hydroxyl (OH) radicals with nine large branched alkanes (i.e., 2-methyl-3-ethyl-pentane, 2,3-dimethyl-pentane, 2,2,3-trimethyl-butane, 2,2,3-trimethyl-pentane, 2,3,4-trimethyl-pentane, 3-ethyl-pentane, 2,2,3,4-tetramethyl-pentane, 2,2-dimethyl-3-ethyl-pentane and 2,4-dimethyl-3-ethyl-penatane) are measured at high temperatures (900 – 1300 K) using a shock tube and narrow line-width OH absorption diagnostic in the UV region. In addition, room-temperature measurements of six out of these nine rate coefficients are performed in a photolysis cell using high repetition laser-induced fluorescence (LIF) of OH radicals. Our experimental results are combined with previous literature measurements to obtain three-parameter Arrhenius expressions valid over a wide temperature range (300 – 1300 K). The rate coefficients are analyzed using the next-nearest-neighbor (N-N-N) methodology to derive nine tertiary (T003, T012, T013, T022, T023, T111, T112, T113, and T122) site-specific rate coefficients for the abstraction of H atoms by OH radicals from branched alkanes. Derived Arrhenius expressions, valid over 950 – 1300 K, are given as (the subscripts denote the number of carbon atoms connected to the next-nearest-neighbor carbon): T_003=2.47×〖10〗^(-10) exp(-3311 K/T)〖 cm〗^3 〖molecule〗^(-1) s^(-1) T_012=1.12×〖10〗^(-10) exp(-3313 K/T)〖 cm〗^3 〖molecule〗^(-1) s^(-1) T_013=5.78×〖10〗^(-10) exp(-4561 K/T)〖 cm〗^3 〖molecule〗^(-1) s^(-1) T_022=2.59×〖10〗^(-10) exp(-4132 K/T)〖 cm〗^3 〖molecule〗^(-1) s^(-1) T_023=5.59×〖10〗^(-11) exp(-2707 K/T)〖 cm〗^3 〖molecule〗^(-1) s^(-1) T_111=5.70×〖10〗^(-11) exp(-1538 K/T)〖 cm〗^3 〖molecule〗^(-1) s^(-1) T_112=1.58×〖10〗^(-11) exp(-1551 K/T)〖 cm〗^3 〖molecule〗^(-1) s^(-1) T_113=1.08×〖10〗^(-9) exp(-4979 K/T)〖 cm〗^3 〖molecule〗^(-1) s^(-1) T_122=3.64×〖10〗^(-11) exp(-1838 K/T)〖 cm〗^3 〖molecule〗^(-1) s^(-1)
The absorption cross section of an overtone transition of OH radicals at 7028.831 cm-1 has been measured using an improved experimental set-up coupling laser photolysis to three individual time-resolved detection techniques. Time resolved relative OH radical profiles were measured by laser induced fluorescence (LIF) while their absolute profiles have been obtained by cw-cavity ring down spectroscopy (cw-CRDS). HO2 radicals were quantified simultaneously at the well characterized absorption line at 6638.21 cm-1 by a second cw-CRDS absorption path. Initial OH concentrations and thus their absorption cross sections have been deduced from experiments of 248nm photolysis of H2O2: OH and HO2 profiles have been fitted to a simple kinetic model using well-known rate constants. The rate constant of the reaction between OH and HO2 radicals turned out to be sensitive to the deduction of the initial OH concentration and has been revisited in this work: OH decays have been observed in the presence of varying excess HO2 concentrations. A rate constant of (1.02±0.06)×10-10 cm3s-1 has been obtained, in good agreement with previous measurements and recent recommendations. An absorption cross section of σOH = (1.54±0.1)×10-19 cm2 at a total pressure of 50 Torr helium has been obtained from consistent fitting of OH and HO2 profiles in a large range of concentrations.
A thorough investigation on the homolytic photolysis of hydrogen peroxide (H2O2) is helpful for understanding many reactions in biochemistry and environmental chemistry. There is still debate regarding this reaction's mechanism, although many types of experimental and theoretical studies have been performed. High quality potential-energy curves (PECs) are necessary to deduce the dissociation mechanism. In the current study, the PECs of H2O2 with respect to the O-O bond distance for the ground state and the low-lying excited states are calculated using multistate second order multiconfigurational perturbation theory. The vertical excitation energies and the dissociation energy of H2O2 are predicted at a high computational level. The current study is able to assign homolytic photolysis pathways of H2O2 observed experimentally at three regions of incident wavelength. The solvent effect is also considered in this study.
The HO2 radical yield in the OH initiated oxidation of SO2 has been determined by direct observation of HO2 concentration time profiles following the 248 nm photolysis of H2O2/SO2/O-2 mixtures. Initial OH radical concentrations have been deduced from a fit of the absolute HO2 concentration time profiles after 248 nm photolysis of H2O2 in the absence of SO2, an increase in the HO2 concentration upon addition of SO2 to this reaction mixture is observed and can be explained by a decrease of HOx-radical losses due to a faster decay of OH radicals in the presence of SO2. Simulations of these profiles using recommended rate constants in a simple model are in agreement with an HO2-yield of 1.0 +/- 0.1 from the OH initiated oxidation of SO2.
The rate constant of the reactionC2H5O2 + OH → Products
has been measured at T = 296 K using laser photolysis coupled to CRDS and LIF. C2H5O2 was generated from the reaction of Cl-atoms with C2H6 at 55 Torr O2, whereby Cl-atoms were generated by 248 nm photolysis of (COCl)2. OH radicals were generated simultaneously by photolysing H2O2. The initial Cl-concentration (and hence C2H5O2) was determined in separate experiments by replacing C2H6 by CH3OH and thus conversion of Cl-atoms to HO2. The time-resolved OH decay was detected in excess of C2H5O2. A fast rate constant of k = (1.2 ± 0.3) × 10⁻¹⁰ cm³ s⁻¹ was determined.
The reactivity of Cl atom with triple bonded molecules were examined by determining the rate coefficients of reactions of four triple bonded alcohols (TA), namely 2-propyn-1-ol, 3-butyn-1-ol, 3-butyn-2-ol and 2-methyl-3-butyn-2-ol, using relative rate method, at 298 K. The rate coefficients (k) of reaction of the four alcohols with Cl vary in the range of 3.5 - 4.3 × 10-10 cm3 molecule-1 s-1. These values imply significant contribution of Cl reaction in the tropospheric degradation of TAs in the conditions of marine boundary layer. A striking difference is observed in the reactivity trend of Cl from that of OH/O3. While the reactivity of OH/O3 is lower with triple - bonded molecules, as compared to the double bonded analogues, the reactivity of Cl atom is similar for both. For a deeper insight, the reactions of Cl and OH with the simplest TA, 2-propyn-1-ol, are investigated theoretically. Conventional transition state theory is applied to compute the values of k, using the calculated energies at QCISD and QCISD(T) levels of theory of the optimized geometries of the reactants, transition states (TS) and the product radicals of all the possible reaction pathways at MP2/6-311++G(d,p) level. The k values calculated at QCISD level for Cl and QCISD (T) level for OH reactions are found to be very close to the experimental values at 298 K. In the case of Cl reaction, the abstraction of alpha -H atoms as well as addition at the terminal and middle carbon atoms have submerged TS and contribution of abstraction reaction is found to be significant at room temperature, at all levels of calculations. Addition at terminal carbon atom is prominent compared to that at middle carbon. In contrast to Cl reaction, only addition at middle carbon is associated with such low lying TS in the case of OH. The individual rate coefficients of addition and abstraction of OH are lower than that of Cl. The negative temperature dependence of the computed rate coefficients in the temperature range of 200 - 400 K shows that the difference in the TS energy of Cl and OH affects the pre-exponential factor more than the activation energy.
OH and HO2 radicals play a major role in atmospheric and combustion chemistry, their time-resolved, simultaneous detection is highly desirable. We present a new experimental set-up combining cw-CRDS and high repetition-rate LIF to Laser Photolysis.
Radical quantum yields have been measured following the 248 nm photolysis of acetaldehyde, CH3CHO. HCO radical and H atom yields have been quantified by time resolved continuous wave Cavity Ring Down Spectroscopy in the near infrared following their conversion to HO2 radicals by reaction with O2. The CH3 radical yield has been determined using the same technique following their conversion into CH3O2. Absolute yields have been deduced for HCO radicals and H atoms through fitting of time resolved HO2 profiles, obtained under various O2 concentrations, to a complex model, while the CH3 yield has been determined relative to the CH3 yield from 248 nm photolysis of CH3I. Time resolved HO2 profiles under very low O2 concentrations suggest that another unknown HO2 forming reaction path exists in this reaction system besides the conversion of HCO radicals and H atoms by reaction with O2. HO2 profiles can be well reproduced under a large range of experimental conditions with the following quantum yields: CH3CHO + hν248nm → CH3CHO*, CH3CHO* → CH3 + HCO ϕ1a = 0.125 ± 0.03, CH3CHO* → CH3 + H + CO ϕ1e = 0.205 ± 0.04, CH3CHO* →o2 CH3CO + HO2 ϕ1f = 0.07 ± 0.01. The CH3O2 quantum yield has been determined in separate experiments as ϕCH3 = 0.33 ± 0.03 and is in excellent agreement with the CH3 yields derived from the HO2 measurements considering that the triple fragmentation (R1e) is an important reaction path in the 248 nm photolysis of CH3CHO. From arithmetic considerations taking into account the HO2 and CH3 measurements we deduce a remaining quantum yield for the molecular pathway: CH3CHO* → CH4 + CO ϕ1b = 0.6. All experiments can be consistently explained with absence of the formerly considered pathway: CH3CHO* → CH3CO + H ϕ1c = 0
ABSTRACT: Rate and equilibrium constants for the reaction of HO2 with formaldehyde
CH2O+HO2 ↔ HOCH2O2 (R1)
and acetaldehyde
CH3CHO+HO2 ↔ CH3CH(OH)O2 (R2)
havebeendirectlymeasured.TheconcentrationofHO2 radicalswasfollowedinatime-resolved method by coupling cw-CRDS (cavity ringdown spectroscopy) to laser photolysis. The reaction of HO2 with CH2O (R1) was measured at 50 Torr helium over the temperature range 292–306 K, whereas the reaction of CH3CHO with HO2 (R2) was measured in 50 Torr He but at only 294 K. The observed HO2 decay profiles were modeled to take into account secondary chemistry, especially that of the reaction products, hydroxyl-peroxy radical adducts. The rate constants for forward and back reactions of (R1) at 297 K were found to be k1 =(3.3±0.6)×10−14 cm3 molecule−1 s−1 and k−1 =(55±5) s−1, respectively, both roughly a factor of two slower than earliermeasurements(possiblyduetofalloffeffects),whiletheequilibriumconstantwasfound to be K1 =(6.0±1.8)×10−16 molecule−1 cm3 at 297 K, in good agreement with earlier, more indirect determinations. The equilibrium constant of the reaction with CH3CHO was found to be K2 =(1.7±0.5)×10−17 molecule−1 cm3 at 294 K, with the forward rate constant k2 =(1.5 ±0.75)×10−14 cm3 molecule−1 s−1 and the rate constant for the back reaction k−2 =(900± 450) s−1. C
Two-dimensional measurements of primarily hydroperoxyl radicals (HO2) are, for the first time, demonstrated in flames. The measurements are performed in different Bunsen-type premixed flames (H2/O2, CH4/O2, and CH4/air) using photofragmentation laser-induced fluorescence (PF-LIF). Photofragmentation is done by laser radiation at 266 nm, and the generated OH photofragments are probed through fluorescence induced by a laser tuned to the Q1(5) transition at 282.75 nm. The signal due to naturally occurring OH radicals, recorded by having the photolysis laser blocked, is subtracted, providing an image that reflects the concentration of OH fragments generated by photolysis, and hence the presence of primarily HO2, but also smaller contributions from H2O2 and, for the methane flames, CH3O2. For the methane flames the measured radial profiles of OH photofragments and natural OH agree well with corresponding profiles calculated for laminar, one-dimensional, premixed flames using CHEMKIN-II with the Konnov detailed C/H/N/O reaction mechanism. An interfering signal contribution is observed in the product zone of the methane flames. It is concluded that the major source for the interference is most likely hot CO2, from which O atoms are produced by photolysis, and OH is rapidly formed as the O atoms react with H2O and H2. This conclusion is supported by the fact that the interference is absent for the hydrogen flame, but appears when CO2 is seeded into the flame. Another strong indication is that the Konnov mechanism predicts a similar buildup of OH after photolysis.
A new experimental set-up for the simultaneous, time-resolved measurement of OH and HO2 radicals by coupling high repetition rate Laser Induced Fluorescence (LIF) and cw-Cavity Ring-Down Spectroscopy (cw-CRDS)
detection techniques to a laser photolysis reactor has been installed. The first experiments concern the well-known photolysis
of H2O2: the absorption cross sections of H2O2 lines in the near-infrared at 6636.75 and 6639.90 cm−1 have been determined by combined results of spectroscopic and kinetic measurements of OH and HO2 radicals produced by the photolysis of H2O2 at 248 nm. These experiments will enable the measurement of absolute H2O2 concentrations in future experiments with the same set-up.
Absolute absorption cross sections of formaldehyde, CH2O, have been measured at total pressures of 14.5 and 66 mbar Helium by cw-CRDS for two selected lines at around 6625 cm−1. Absolute CH2O concentrations have been determined in situ by measuring the pseudo-first order decays of OH radicals using high repetition rate laser induced fluorescence (LIF). OH radicals have been generated by laser photolysis of H2O2 in the presence of CH2O and from the well-known rate constant of the reaction of CH2O with OH radicals, the absolute CH2O concentration has been determined. Concentrations between 1.5 and 4 × 1014 cm−3 have been used. The line strengths for the lines at 6624.779 cm−1 and 6625.248 cm−1 have been found to be (9.1 ± 1.8) × 10−24 cm and (5.3 ± 1.0) × 10−24 cm, respectively. The broadening coefficients in Helium have also been determined for both lines. Converting the line strengths to absorption cross sections it is found that these values are a factor of 2 smaller than the only known determination of these absorption cross sections in the literature (Staak et al., J. Molec. Spectroscopy 229 (2005) 115–121).
The formation of OH radicals and their diffusion into the gas phase after UV-excitation of TiO2 in the presence of H2O has been studied using the very sensitive and selective detection method of laser-induced fluorescence (LIF). The time-resolved evolution of the OH radical concentration has been observed at different pressures and at varying distances between the photocatalytic surface and the detection volume. H2O2 has been indirectly detected by LIF. The influence of O2, hydrocarbons, and excitation laser wavelength on the evolution of both species profiles has been studied in this work. The quantum yield for the formation of OH and H2O2 has been estimated by comparison with signals obtained after photolysis of H2O2 in the gas phase.
This work describes the first ever direct detection of HO2 radicals in the gas phase above photocatalytic surfaces. A glass plate covered with TiO2 has been illuminated in the presence of H2O2 by a 20 W fluorescence lamp centred at 365 nm. The activity of the photocatalytic material has been proven through direct, time resolved observation of the degradation of H2O2 by following its concentration by the very sensitive and selective technique of cw-Cavity Ring Down Spectroscopy (cw-CRDS). An absorption line of H2O2 at 6639.89 cm−1 has been used, permitting a detection limit of [H2O2]min = 1.3 and 3.6 × 1013 cm−3 for 50 and 200 Torr of synthetic air, respectively. A lower limit of the quantum yield for H2O2 degradation has been estimated to ϕmin = 0.0024. Under the same conditions, the formation of HO2 radicals has been detected directly and selectively in the gas phase, using the same technique. HO2 radicals have been observed at up to 4 cm above the surface and at total pressures of up to 230 Torr.Graphical abstractResearch highlights▶ Direct detection of HO2 radicals in the gas phase above TiO2 surface. ▶ In situ sensing of the gas phase above surfaces by cw-CRDS. ▶ Quantitative detection of H2O2 by cw-CRDS.
We present in this work the direct observation of HO2 radicals after irradiation of benzene C6H6 at 248nm in the presence of O2. HO2 radicals have been unambiguously identified using the very selective and sensitive detection of continuous wave cavity ring-down
spectroscopy (cw-CRDS) coupled to a laser photolysis reactor. HO2 radicals were detected in the first vibrational overtone of the OH stretch at 6638.20cm-1, using a DFB diode laser. This reaction might be important because 248nm photolysis of H2O2 has often been used in the past for studying the OH•-initiated degradation of C6H6, often using a large excess of C6H6 over H2O2. The possible importance of the title reaction with respect to these former laboratory studies has been quantified through
comparison with HO2
• signals obtained from 248nm photolysis of H2O2: one obtains under our conditions (excess O2 and total pressure of 6.6kPa helium) from the 248nm irradiation of identical initial concentrations [C6H6]=[H2O2] the following relative initial radical concentrations: [HO2
•]=(0.28±0.05)×[OH•]. Experiments with various O2 concentrations have revealed that the origin of the HO2 radicals is not the reaction of H-atoms with O2, but must originate from the reaction of O2 with excited C6H6
*. The quantum yield of C6H6
* formation has been deduced to ϕ=0.2±0.1.
The decay of OH concentration following photolysis of room-temperature vapor-phase hydrogen peroxide is studied as a function
of photolysis fluence at 266nm in an open air environment. The rate of decay is found to increase with increasing photolysis
fluence, i.e., with increasing number of photodissociated H2O2(g) molecules. Single-exponential functions approximate the OH concentration decay rather well, even for higher photolysis
levels, and the decay time is shown to be inversely proportional to the H2O2(g) concentration. For fluences of about 450mJ/cm2 the difference between a single-exponential decay and measured data is becoming evident after approximately 150μs. Calculations
based on a chemical kinetics model agree well with experimental data also for times >150μs. By combining the model with measurements,
the actual photolysis levels used in experiments are estimated. The best fit between measured data and the model suggests
that about 1.1% of the H2O2(g) molecules are dissociated with a photolysis fluence of ∼450mJ/cm2, in reasonable agreement with a Beer–Lambert based estimation. Excitation scans did not unfold any differences between OH
spectra recorded at different photolysis fluences.
Absolute absorption cross sections for selected lines of the OH stretch overtone 2ν(1) of the cis-isomer of nitrous acid HONO have been measured in the range 6623.6-6645.6 cm(-1) using the continuous wave cavity ring-down spectroscopy (cw-CRDS) technique. HONO has been generated by two different, complementary methods: in the first method, HONO has been produced by pulsed photolysis of H(2)O(2)/NO mixture at 248 nm, and in the second method HONO has been produced in a continuous manner by flowing humidified N(2) over 5.2 M HCl and 0.5 M NaNO(2) solutions. Laser photolysis synchronized with the cw-CRDS technique has been used to measure the absorption spectrum of HONO produced in the first method, and a simple cw-CRDS technique has been used in the second method. The first method, very time-consuming, allows for an absolute calibration of the absorption spectrum by comparison with the well-known HO(2) absorption cross section, while the second method is much faster and leads to a better signal-to-noise ratio. The strongest line in this wavelength range has been found at 6642.51 cm(-1) with σ = (5.8 ± 2.2) × 10(-21) cm(2).
The energy dependence of HO(2) radical formation from the irradiation of benzene (C(6)H(6)) in the presence of oxygen (O(2)) at 248 nm is studied. We investigate the origin of the HO(2) radicals, that is, whether they originate from the reaction of O(2) with products obtained by one- or two-photon excitation of C(6)H(6). The concentration-time profiles of HO(2) radicals are monitored by continuous-wave cavity ring-down spectroscopy (cw-CRDS) coupled to a laser photolysis reactor. HO(2) radicals are detected in the first vibrational overtone of the OH stretch at 6638.20 cm(-1), using a distributed feedback (DFB) diode laser. Two well-distinguished HO(2) radical-formation phases are observed: a fast initial formation of HO(2) radicals followed by a slower secondary formation. While the concentration of the initially formed HO(2) species increases linearly with the excitation energy, the concentration of the secondary slow HO(2) radicals appears to vary in accordance with a two-photon process.
This article, the first in the series, presents kinetic and photochemical data evaluated by the IUPAC Subcommittee on Gas Kinetic Data Evaluation for Atmospheric Chemistry. It covers the gas phase and photochemical reactions of Ox, HOx, NOx and SOx species, which were last published in 1997, and were updated on the IUPAC website in late 2001. The article consists of a summary sheet, containing the recommended kinetic parameters for the evaluated reactions, and five appendices containing the data sheets, which provide information upon which the recommendations are made.
The quantum yields of the products, OH(X 2Π), O(3P) [plus O(1D)] and H(2S), in the photolysis of H2O2 and CH3OOH at 248 nm and 298 K have been measured. OH was directly observed by laser‐induced fluorescence while the atomic species were detected by cw‐resonance fluorescence. All quantum yield measurements were made using relative methods. The quantum yields of OH, O, and H in H2O2 photolysis were measured relative to the well known quantum yields of O(1D) and O(3P) in the photodissociation of O3, and H(2S) in CH3SH. The values we obtain are, 2.09±0.36, <0.002 and <0.0002 for OH, O, and H, respectively. For CH3OOH photolysis, the quantum yield of OH was measured relative to our value for OH quantum yield in H2O2 photolysis, and the quantum yields of O and H relative to those in O3 and CH3SH photodissociation, respectively. The values we obtain are, 1.00±0.18, <0.007 and 0.038±0.007 for OH, O, and H, respectively. In both H2O2 and CH3OOH photolysis, the observed O and H quantum yields showed an apparent dependence on the fluence of the photolysis light, the possible origin of which is discussed. The large quantum yield of OH we measure is consistent with the known continuous and unstructured absorption spectra of these molecules in this wavelength region, where the most important process is the dissociative ( 1A← 1A) transition to give OH(X 2Π, v″=0) fragment.
This article, the first in the series, presents kinetic and photochemical data evaluated by the IUPAC Subcommittee on Gas Kinetic Data Evaluation for Atmospheric Chemistry. It covers the gas phase and photochemical reactions of O x , HO x , NO x and SO x species, which were last published in 1997, and were updated on the IUPAC website in late 2001. The article consists of a summary sheet, containing the recommended kinetic parameters for the evaluated reactions, and five appendices containing the data sheets, which provide information upon which the recommendations are made.
The absolute quantum yields (Φ) for OH production from 193 and 248 nm photolysis of HNO3 and H2O2 are measured at room temperature using flash kinetic spectroscopy in a flow tube. The OH radicals are produced by excimer laser photolysis and probed via direct absorption of high resolution, tunable IR laser light. The resulting quantum yields are found to be Φ193HNO3 = 0.47 ± 0.06, Φ193H2O2 = 1.22 ± 0.13, Φ 248HNO3 = 0.75 ± 0.10, and Φ248H2O2 = 1.58 ± 0.23. These results indicate quantum yields for both precursors at both wavelengths which are less than the maximum possible values of 1 for HNO3 and 2 for H2O2. The present measurements are discussed in light of contrasting results suggested from other work.
The authors investigate the correlation between the rotational states of OH radicals coincidently prepared in the photodissociation of HâOâ at 193 nm. The calculations use classical mechanics and ab initio potential energy surfaces for the low lowest excited states. For very low HâOâ temperatures the two OH rotamers are found to be highly correlated (jâ /approximately/ jâ) and the correlation is gradually lifted as the temperature rises. This effect can be explained by the influence of initial HâOâ rotation about the O-O axis. Comparison with preliminary, only partly resolved measurements is satisfactory.
Photodissociation of H2O2 at 248 nm produces vibrationally and rotationally cold OH(X2Π) fragments (ƒv < 3%, ƒR= 11%), the bulk of the energy release going into product translational excitation. The OH fragment translational anisotropy was studied by Doppler spectroscopy, identifying the dissociative state as a 1A state. The translational anisotropy is high, β being close to its limiting value of –1, indicating a prompt dissociation. Doppler profiles were found to be sensitive to the polarisation of the probed transition and to the probe/photolysis geometry, indicating a correlation between JOH and VOH, fragment rotation being aligned along the recoil direction. Fragment rotation is predominantly generated by torsion about the O–O axis, either from breaking of the ν4 torsional mode or due to a torsional dependence to the first excited 1A potential.
Doppler profiles of H atoms from the photodissociation of H2O2 at 193 nm were measured by a laser-induced fluorescence method at 121.6 nm. On the average, 149 kJ mol-1 of energy is released as translational energy, which corresponds to about to 60% of the available energy. Doppler width anisotropy data show that the anisotropy parameter, beta, for the photofragment angular distribution is -0.26+/-0.11. This result implies that the H2O2 A1A state is responsible for H atom formation.
Flash kinetic spectroscopy in a flow tube is used to measure at room temperature the absolute yields for OH production from 193 and 248 nm photolysis of HNO3 and H2O2. The OH radicals are produced by excimer laser photolysis and probed via direct absorption of high resolution tunable IR laser light. The results indicate quantum yields for both precursors at both wavelengths which are less than the maximum possible values of 1 for H2O2. The present measurements are discussed in light of contrasting results suggested from other work.
In this paper we present measurements of the air-broadening coefficients of HO2 at room temperature in the 2ν1 band around 1.5 microns. The HO2 radicals were created by flash photolysis of SOCl2 in a flow of O2/CH3OH mixtures. To observe air-broadening, N2 (79%) and O2 (21%) were added using calibrated flow controllers and a total pressure controller. The total pressure was monitored in parallel using a capacitive pressure gauge. Air-broadening coefficients at 296K were determined for 34 absorption lines between 6631 and 6671cm−1. The air-broadening coefficients of HO2 show a rotational dependence (decreasing from about 0.14cm−1/atm for N″=3 to about 0.09cm−1/atm for N″=10). No evidence for collisional narrowing was observed.
The dynamics of the photodissociation of hydrogen peroxide has been analyzed by a complete characterization of the scalar and vectorial properties of the OH fragment using Doppler and polarization spectroscopy. When hydrogen peroxide is optically excited at 193 nm the hydroxyl radicals are formed exclusively in the X 2Π3/2,1/2 ground state with 84% of the available energy (Eav=417 kJ/mol) being released as OH recoil translation. The remaining energy is transferred in product rotation showing a strongly inverted rotational state distribution peaking at N″=12. Vector correlations between the transition dipole moment of the parent H2O2 and the OH product rotational and translational motions were observed by Doppler broadened spectral lines and evaluated in terms of four bipolar moments. The quantitative contribution of two different electronic excited states in the dissociation process could be determined by analyzing the vector properties of the fragment. 62% of the OH products evolve from the 1A electronic excited state while 38% of the fragments are formed via the 1B state when hydrogen peroxide is excited at 193 nm. The OH rotational state distributions when produced from the 1A and the 1B state show no remarkable difference. The vector correlation of the recoil velocity vOH and the rotation JOH is strongly positive and increases with increasing JOH indicating a strong preference towards vOH and JOH being parallel to one another. The major part of product rotation is caused by a strong dependence on the torsion angle of the two upper potential surfaces.
The authors investigate the correlation between the rotational states of OH radicals coincidently prepared in the photodissociation of H/sub 2/O/sub 2/ at 193 nm. The calculations use classical mechanics and ab initio potential energy surfaces for the low lowest excited states. For very low H/sub 2/O/sub 2/ temperatures the two OH rotamers are found to be highly correlated (j/sub 1/ /approximately/ j/sub 2/) and the correlation is gradually lifted as the temperature rises. This effect can be explained by the influence of initial H/sub 2/O/sub 2/ rotation about the O-O axis. Comparison with preliminary, only partly resolved measurements is satisfactory.
We present in this work a new experimental set-up for sensitive detection of reactive species: continuous wave cavity ring-down
spectroscopy (cw-CRDS) as a detection method in laser photolysis reactor. HO2 radicals were generated by using a 248nm photolysis of SOCl2/CH3OH/O2 mixtures and were detected in the first vibrational overtone of the OH stretch around 6625cm-1, using a DFB diode laser. In order to perform the spectroscopic and kinetic measurements of the HO2 radical, two different timing schemes have been used. The absorption line strength of the transition at 6625.784cm-1 has been extracted from kinetic measurement to (5.2±1.0)×10-21cm2 molecule-1cm-1. The detection limit for the actual set-up is 2×1012molecules cm-3.