[show abstract][hide abstract] ABSTRACT: The photodissociation of Cl2O has been studied at 248 and 193nm using photofragment translational spectroscopy (PTS) experiments with tunable VUV photoionization detection. The sole products observed were Cl, O and ClO fragments. Based on the derived translational energy distributions for the ClO and Cl photofragments we conclude that at 248nm 15% of Cl2O excitation results in three-body dissociation. At 193nm no Cl2 fragments are observed and we conclude that the oxygen atoms arise solely from three-body dissociation. Dissociation geometries derived from forward convolution fitting suggest two qualitatively distinct three-body dissociation pathways: asymmetric concerted dissociation and symmetric concerted dissociation in agreement with recent theoretical predictions.
Journal of Photochemistry and Photobiology A-chemistry - J PHOTOCHEM PHOTOBIOL A-CHEM. 01/2010; 209(1):56-60.
[show abstract][hide abstract] ABSTRACT: We report isomer-selective kinetics and mechanistic details for the hydroxyl radical-initiated oxidation of isoprene, in the presence of O(2) and NO, employing complementary experimental and theoretical techniques. Using a recently demonstrated photolytic route to initiate isomer-selective kinetics in OH-initiated oxidation of unsaturated hydrocarbons via the UV photolysis of iodohydrins, the photolysis of 1-iodo-2-methyl-3-buten-2-ol results in a single isomer of the possible four OH-isoprene adducts, specifically the minor channel associated with OH addition to one of the inner carbon atoms. Employing both the laser-photolysis/laser-induced fluorescence (LP/LIF) technique and time-dependent multiplexed photoionization mass spectrometry, we find clear experimental evidence supporting the prompt rearrangement of the initially formed beta-hydroxyalkyl radicals to alpha-hydroxyalkyl radicals, in agreement with Rice-Ramsperger-Kassel-Marcus (RRKM)/master equation predictions. We have determined a rate constant of (3.3 +/- 0.5) x 10(-11) cm(3) molecule(-1) s(-1) for molecular oxygen to abstract a hydrogen atom from the alpha-hydroxyalkyl radical to form 4-penten-2-one and HO(2). This reaction provides a mechanistic route to C(5) carbonyl species as first-generation end products for the addition of hydroxyl radical to isoprene in the presence of O(2) and NO.
The Journal of Physical Chemistry A 11/2009; 114(2):904-12. · 2.77 Impact Factor
[show abstract][hide abstract] ABSTRACT: We have developed a multiplexed time- and photon-energy-resolved photoionization mass spectrometer for the study of the kinetics and isomeric product branching of gas phase, neutral chemical reactions. The instrument utilizes a side-sampled flow tube reactor, continuously tunable synchrotron radiation for photoionization, a multimass double-focusing mass spectrometer with 100% duty cycle, and a time- and position-sensitive detector for single ion counting. This approach enables multiplexed, universal detection of molecules with high sensitivity and selectivity. In addition to measurement of rate coefficients as a function of temperature and pressure, different structural isomers can be distinguished based on their photoionization efficiency curves, providing a more detailed probe of reaction mechanisms. The multiplexed three-dimensional data structure (intensity as a function of molecular mass, reaction time, and photoionization energy) provides insights that might not be available in serial acquisition, as well as additional constraints on data interpretation.
The Review of scientific instruments 11/2008; 79(10):104103. · 1.52 Impact Factor
[show abstract][hide abstract] ABSTRACT: The photodissociation of vinyl iodide has been investigated at several wavelengths between 193 and 266 nm using three techniques: time-resolved Fourier transform emission spectroscopy, multiple pass laser absorption spectroscopy, and velocity-mapped ion imaging. The only dissociation channel observed is C-I bond cleavage to produce C2H3 (nu, N) + I (2P(J)) at all wavelengths investigated. Unlike photodissociation of other vinyl halides (C2H3X, X = F, Cl, Br), in which the HX product channel is significant, no HI elimination is observed. The angular and translational energy distributions of I atoms indicate that atomic products arise solely from dissociation on excited states with negligible contribution from internal conversion to the ground state. We derive an upper limit on the C-I bond strength of D0(C2H3-I) < or = 65 kcal mol(-1). The ground-state potential-energy surface of vinyl iodide is explored by ab initio calculations. We present a model in which the highest occupied molecular orbital in vinyl halides has increasing X(np) non-bonding character with increasing halogen mass. This change leads to reduced torsional force around the C-C bond in the excited state. Because the ground-state energy is highest when the CH2 plane is perpendicular to the CHX plane, a reduced torsional force in the excited state correlates with a lower rate for internal conversion compared to excited-state C-X bond fission. This model explains the gradual change in photodissociation mechanisms of vinyl halides from the dominance of internal conversion in vinyl fluoride to the dominance of excited-state dissociation in vinyl iodide.
Physical Chemistry Chemical Physics 02/2008; 10(5):713-28. · 3.83 Impact Factor
[show abstract][hide abstract] ABSTRACT: The reactions of the ethynyl radical (C(2)H) with propyne and allene are studied at room temperature using an apparatus that combines the tunability of the vacuum ultraviolet radiation of the Advanced Light Source at Lawrence Berkeley National Laboratory with time-resolved mass spectrometry. The C(2)H radical is prepared by 193-nm photolysis of CF(3)CCH and the mass spectrum of the reacting mixture is monitored in time using synchrotron-photoionization with a dual-sector mass spectrometer. Analysis using photoionization efficiency curves allows the isomer-specific detection of individual polyynes of chemical formula C(5)H(4) produced by both reactions. The product branching ratios are estimated for each isomer. The reaction of propyne with ethynyl gives 50-70% diacetylene (H-C[triple bond]C-C[triple bond]C-H) and 50-30% C(5)H(4), with a C(5)H(4)-isomer distribution of 15-20% ethynylallene (CH(2)=C=CH-C[triple bond]CH) and 85-80% methyldiacetylene (CH(3)-C[triple bond]C-C[triple bond]CH). The reaction of allene with ethynyl gives 35-45% ethynylallene, 20-25% methyldiacetylene and 45-30% 1,4-pentadiyne (HC[triple bond]C-CH(2)-C[triple bond]CH). Diacetylene is most likely not produced by this reaction; an upper limit of 30% on the branching fraction to diacetylene can be derived from the present experiment. The mechanisms of polyynes formation by these reactions as well as the implications for Titan's atmospheric chemistry are discussed.
Physical Chemistry Chemical Physics 09/2007; 9(31):4291-300. · 3.83 Impact Factor
[show abstract][hide abstract] ABSTRACT: The reactions of the ethynyl radical (C2H) with propyne and allene are studied at room temperature using an apparatus that combines the tunability of the vacuum ultraviolet radiation of the Advanced Light Source at Lawrence Berkeley National Laboratory with time-resolved mass spectrometry. The C2H radical is prepared by 193-nm photolysis of CF3CCH and the mass spectrum of the reacting mixture is monitored in time using synchrotron–photoionization with a dual-sector mass spectrometer. Analysis using photoionization efficiency curves allows the isomer-specific detection of individual polyynes of chemical formula C5H4 produced by both reactions. The product branching ratios are estimated for each isomer. The reaction of propyne with ethynyl gives 50–70% diacetylene (H–CC–CC–H) and 50–30% C5H4, with a C5H4-isomer distribution of 15–20% ethynylallene (CH2CCH–CCH) and 85–80% methyldiacetylene (CH3–CC–CCH). The reaction of allene with ethynyl gives 35–45% ethynylallene, 20–25% methyldiacetylene and 45–30% 1,4-pentadiyne (HCC–CH2–CCH). Diacetylene is most likely not produced by this reaction; an upper limit of 30% on the branching fraction to diacetylene can be derived from the present experiment. The mechanisms of polyynes formation by these reactions as well as the implications for Titan’s atmospheric chemistry are discussed.
Physical Chemistry Chemical Physics 08/2007; 9(31):4291-4300. · 3.83 Impact Factor
[show abstract][hide abstract] ABSTRACT: The photoionization of alkylperoxy radicals has been investigated using a newly developed experimental apparatus that combines the tunability of the vacuum ultraviolet radiation of the Advanced Light Source at Lawrence Berkeley National Laboratory with time-resolved mass spectrometry. Methylperoxy (CH(3)OO) and ethylperoxy (C(2)H(5)OO) radicals are produced by the reaction of pulsed, photolytically produced alkyl radicals with molecular oxygen, and the mass spectrum of the reacting mixture is monitored in time by using synchrotron-photoionization with a double-focusing mass spectrometer. The kinetics of product formation is used to confirm the origins and assignments of ionized species. The photoionization efficiency curve for CH(3)OO has been measured, and an adiabatic ionization energy of (10.33 +/- 0.05) eV was determined with the aid of Franck-Condon spectral simulations, including ionization to the lowest triplet and singlet cation states. Using the appearance energy of CH(3)(+) from CH(3)OO, an enthalpy of formation for CH(3)OO of Delta(f) (CH(3)OO) = (22.4 +/- 5) kJ mol(-1) is derived. The enthalpy of formation of CH(3)OO(+) is derived as Delta(f) = (1019 +/- 7) kJ mol(-1) and the CH(3)(+)-OO bond energy as (CH(3)(+) - O(2)) = (80 +/- 7) kJ mol(-1). The C(2)H(5)OO(+) signal is not detectable; however, the time profile of the ethyl cation signal suggests its formation from dissociative ionization of C(2)H(5)OO. Electronic structure calculations suggest that hyperconjugation reduces the stability of the ethylperoxy cation, making the C(2)H(5)OO(+) ground state only slightly bound with respect to the ground-state products, C(2)H(5)(+) and O(2). The value of the measured appearance energy of C(2)H(5)(+) is consistent with dissociative ionization of C(2)H(5)OO via the Franck-Condon favored ionization to the ã (1)A' state of C(2)H(5)OO(+).
Journal of the American Chemical Society 11/2006; 128(41):13559-67. · 10.68 Impact Factor
[show abstract][hide abstract] ABSTRACT: Nitromethane (CH(3)NO(2)) and its chlorinated analogue, chloropicrin (CCl(3)NO(2)), were photolyzed at 193, 248, and 266 nm, and the products were observed by time-dependent Fourier transform infrared emission spectroscopy. At 193 and 248 nm, the primary photodissociation pathway for nitromethane was cleavage of the C-N bond to produce CH(3) + NO(2)(A (2)B(2)). At 266 nm, weak emission was observed following photodissociation of nitromethane, but an infrared spectrum could not be obtained. The photodissociation of chloropicrin at 193 nm produced the analogous product channel CCl(3) + NO(2)(A (2)B(2)) in addition to several other product channels. At 248 and 266 nm, only CCl(3) + NO(2)(A (2)B(2)) was observed. The production of phosgene (CCl(2)O) from chloropicrin photodissociation was not observed in this study.
The Journal of Physical Chemistry A 05/2006; 110(13):4405-12. · 2.77 Impact Factor
[show abstract][hide abstract] ABSTRACT: We have studied the vinyl + NO reaction using time-resolved Fourier transform emission spectroscopy, complemented by electronic structure and microcanonical RRKM rate coefficient calculations. To unambiguously determine the reaction products, three precursors are used to produce the vinyl radical by laser photolysis: vinyl bromide, methyl vinyl ketone, and vinyl iodide. The emission spectra and theoretical calculations indicate that HCN + CH2O is the only significant product channel for the C2H3 + NO reaction near room temperature, in contradiction to several reports in the literature. Although CO emission is observed when vinyl bromide is used as the precursor, it arises from the reaction of NO with photofragments other than vinyl. This conclusion is supported by the absence of CO emission when vinyl iodide or methyl vinyl ketone is used. Prompt emission from vibrationally excited NO is evidence of the competition between back dissociation and isomerization of the initially formed nitrosoethylene adduct, consistent with previous work on the pressure dependence of this reaction. Our calculations indicate that production of products is dominated by the low energy portion of the energy distribution. The calculation also predicts an upper bound of 0.19% for the branching ratio of the H2CNH + CO channel, which is consistent with our experimental results.
The Journal of Physical Chemistry A 07/2005; 109(22):4921-9. · 2.77 Impact Factor
[show abstract][hide abstract] ABSTRACT: We have performed photodissociation experiments on CHBr3 at 248 nm using VUV ionization photofragment translational spectroscopy. Prompt C−Br bond fission is the dominant single-photon dissociation channel. In addition to primary Br and CHBr2 signals, we observe Br, CHBr, CBr, HBr, and Br2 products attributed to secondary photodissociation of CHBr2 and CHBr. There are three competing fragmentation channels from the photodissociation of CHBr2: CHBr + Br, CH + Br2, and CBr + HBr. The conclusion that Br2 fragments do not arise from a single-photon channel in appreciable yield is supported by transient FM absorption measurements of the CHBr radical. Because the molecular HBr and Br2 detachment channels are multiphoton processes, they will have very little impact on the atmospheric chemistry of CHBr3. We conclude that the most important photodissociation channel of CHBr3 in the UV region is C−Br bond breaking.
[show abstract][hide abstract] ABSTRACT: We explore the mechanism of the HCCO+O2 reaction using time-resolved Fourier transform spectroscopy. Utilizing an isotopically labeled reactant (18O2) and state-selective product detection, we determine the relative flux through different paths on the potential energy surface leading to a single asymptote: H+CO+CO2. In the labeled reaction, the dominant isotopic products are C18O and 16OC18O. Combined with data from the corresponding reaction in natural isotopic abundance, these results show that at least 85% of the reactive flux passes through a four-membered OCCO ring intermediate. An alternative reaction path through an energetically allowed three-membered COO ring intermediate represents less than 15% of the total reactive flux. The average energy deposited in vibration of CO and CO2 is in reasonable agreement with a statistical model using the separate statistical ensembles method.
Physical Chemistry Chemical Physics 01/2004; 6(8). · 3.83 Impact Factor
[show abstract][hide abstract] ABSTRACT: A theoretical study of the low-lying singlet and triplet electronic states of BrONO2 is presented. Calculations of excitation energies and oscillator strengths are reported using excited-state coupled cluster response methods, as well as the complete active space self-consistent field method with the full Breit-Pauli spin-orbit operator. The calculations predict that there is only one singlet state for BrONO2, the (A) over tilde (1)A(') state, that is accessible at wavelengths longer than 300 nm. At energies below the first singlet state, i.e., lambda>330 nm, the calculations reveal two triplet states with significant oscillator strength. Therefore, we propose that the origin of absorption in the long wavelength region from 350 to 500 nm, responsible for the majority of atmospheric photolysis, is due to transitions to triplet states and not singlet states. A comparison of the reported benchmark coupled cluster calculations (CCSD) with the results of (1) configuration interaction with all single substitutions and a perturbative correction for the double substitutions [CIS(D)] and (2) time-dependent density-functional (TDDF) calculations is provided. For the lowest energy excitations, CIS(D) calculations provide quantitative agreement with the CCSD results, while TDDF calculations yield qualitative agreement. (C) 2003 American Institute of Physics.
The Journal of Chemical Physics 10/2003; 119(15):7864-7870. · 3.16 Impact Factor
[show abstract][hide abstract] ABSTRACT: CCSD(T) basis set limit bond dissociation energies by extrapolation for ClONO2 and CCSD(T)/cc-pV5Z estimates of bond dissociation energies for BrONO2 were calculated by determining correction factors to MP2/cc-pVXZ (X = 2−5) basis set energies. To obtain the MP2 basis set limit energies, MP2/cc-pVXZ (X = 2−5) level calculations were extrapolated to the basis set limit using either polynomial or exponential functional forms. Correlation effects were taken into account by calculating the difference in energies at the MP2/cc-pVTZ and CCSD(T)/cc-pVTZ levels. Subsequent corrections for the spin−orbit energy of the atomic fragment and zero point energy were applied to yield the final bond dissociation energies. The theoretical results are in good agreement with available experimental values and theoretical values calculated using isodesmic methods.
Journal of Physical Chemistry A - J PHYS CHEM A. 01/2003; 107(6):888-896.
[show abstract][hide abstract] ABSTRACT: The photodissociation of ClONO2 at 235 nm was investigated using resonance enhanced multiphoton ionization time-of-flight mass spectrometry, which permits the state selective detection of Cl(3P3/2) (Cl) and Cl(3P1/2) (Cl*) atom products. The angular and speed distributions for the Cl, Cl*, and ClO channels were derived from forward convolution fitting of time-of-flight spectra. The anisotropy parameters for the Cl elimination channels are 0.5 ± 0.15 and 1.2 ± 0.15 for the atomic ground and excited states respectively, indicating that the states arise from different electronic excited states. The anisotropy parameter for the ClO + NO2 channel is 1.0 ± 0.15. On the basis of simulations to the TOF data, we have determined a total Cl atom quantum yield of 0.42 ± 0.1 and a ClO quantum yield of 0.58 ± 0.1. The fraction of nascent NO3 fragments that undergo secondary dissociation was calculated based on the measured internal energy distribution. The NO2 + O channel has a quantum yield of 0.20, and NO + O2 channel has a quantum yield of 0.006. The results are discussed in light of previous measurements at both longer and shorter wavelengths and suggest that at short wavelengths the spontaneous secondary dissociation of NO3 to yield NO2 + O is important and will substantially reduce the impact of the Cl + NO3 channel on stratospheric ozone depletion.
Journal of Physical Chemistry A - J PHYS CHEM A. 01/2002; 106(6).
[show abstract][hide abstract] ABSTRACT: The photodissociation of 1,2 dibromo-tetrafluoroethane (Halon-2402) has been investigated at 193 nm using photofragment translational spectroscopy with vacuum ultraviolet ionization and at 193, 233, and 266 nm using state-selected translational spectroscopy with resonance-enhanced multiphoton ionization. The product branching ratios, angular distributions, and translational energy distributions were measured at these wavelengths, providing insight into the ultraviolet photodissociation dynamics of CF2BrCF2Br. The total bromine atom quantum yields were found to be 1.9±0.1 at both 193 and 233 nm and 1.4±0.1 at 266 nm. The first C–Br bond dissociation energy was determined to be 69.3 kcal/mol from ab initio calculations. The second C–Br bond dissociation energy was determined to be 16±2 kcal/mol by modeling of the bromine quantum yield. In addition, variational Rice–Ramsperger–Kassel–Marcus theory was used to calculate the secondary dissociation rates for a range of dissociation energies above threshold. These results suggest that CF2CF2Br photofragments with sufficient internal energies will undergo secondary dissociation prior to collisional stabilization under atmospheric conditions. Based on the measured translational energy distributions and product branching ratios, a model is proposed to describe the wavelength-dependent bromine quantum yield and the implications of these results to atmospheric chemistry are discussed.
The Journal of Chemical Physics 11/2000; 113(17). · 3.16 Impact Factor
[show abstract][hide abstract] ABSTRACT: The photodissociation dynamics of chlorobromomethane (CBM) were investigated between 193 and 242 nm
by resonance-enhanced multiphoton ionization (REMPI) with time-of-flight mass spectrometry (TOFMS). Translational energy distributions, anisotropy parameters, and Br:Br* branching ratios were determined at 193
and 235 nm to explore the non-adiabatic dynamics near the avoided crossing. Additional measurements were made at intermediate wavelengths to characterize the wavelength dependence of the Br and Br* anisotropy parameters. The non-adiabatic crossing probabilities calculated by applying a one-dimensional Landau–Zener model were relatively insensitive to the excitation wavelength, indicating that the avoided crossing between 3A′ and 4A′ potentials lies in the exit channel. Additionally, we have determined the partial absorption cross sections for the excited states that contribute to the ultraviolet absorption spectrum of CBM.
Physical Chemistry Chemical Physics 01/2000; 2(17):3785-3790. · 3.83 Impact Factor
[show abstract][hide abstract] ABSTRACT: We have investigated the ultraviolet photodissociation of IBr using core-sampled state-selective ion time-of-flight mass spectrometry on the iodine and bromine atom products. The branching ratios and anisotropy parameters were determined for the I(2P3/2)+Br(2P3/2), I(2P3/2)+Br(2P1/2), and I(2P1/2)+Br(2P3/2) channels at 248, 267, and 304 nm. We find no evidence for the I(2P1/2)+Br(2P1/2) channel at any wavelength. The results provide information on the relative transition probabilities for the 3Π1(2341), 3Π0+(2341), and 1Π1(2341) excited states in the absorption band centered at 270 nm. We have further evaluated the nonadiabatic curve crossing probability for the avoided crossing between the 3Π0+(2341) and 3Σ−0+(2422) states over a range of wavelengths from 250 to 270 nm. The wavelength-dependent curve crossing probability for the avoided crossing is modeled using one-dimensional Landau–Zener theory in order to estimate the location of the avoided crossing and the splitting between adiabats. A comparison with recent work at 267 and 304 nm and analogous interhalogen molecules is also provided.
Chemical Physics - CHEM PHYS. 01/1999; 249(2):237-248.