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ABSTRACT: The dissociation of the hydroxymethyl radical, CH(2)OH, and its isotopolog, CD(2)OH, following the excitation of high OH stretch overtones is studied by quasi-classical molecular dynamics calculations using a global potential energy surface (PES) fitted to ab initio calculations. The PES includes CH(2)OH and CH(3)O minima, dissociation products, and all relevant barriers. Its analysis shows that the transition states for OH bond fission and isomerization are both very close in energy to the excited vibrational levels reached in recent experiments and involve significant geometry changes relative to the CH(2)OH equilibrium structure. The energies of key stationary points are refined using high-level electronic structure calculations. Vibrational energies and wavefunctions are computed by coupled anharmonic vibrational calculations. They show that high OH-stretch overtones are mixed with other modes. Consequently, trajectory calculations carried out at energies about ~3000 cm(-1) above the barriers reveal that despite initial excitation of the OH stretch, the direct OH bond fission is relatively slow (10 ps) and a considerable fraction of the radicals undergoes isomerization to the methoxy radical. The computed dissociation energies are: D(0)(CH(2)OH → CH(2)O + H) = 10,188 cm(-1), D(0)(CD(2)OH → CD(2)O + H) = 10,167 cm(-1), D(0)(CD(2)OH → CHDO + D) = 10,787 cm(-1). All are in excellent agreement with the experimental results. For CH(2)OH, the barriers for the direct OH bond fission and isomerization are: 14,205 and 13,839 cm(-1), respectively.
The Journal of chemical physics 02/2012; 136(8):084304. · 3.09 Impact Factor
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ABSTRACT: The dissociation of the hydroxymethyl radical, CH(2)OH, and its isotopolog, CD(2)OH, following excitation in the 4ν(1) region (OH stretch overtone, near 13,600 cm(-1)) was studied using sliced velocity map imaging. A new vibrational band near 13,660 cm(-1) arising from interaction with the antisymmetric CH stretch was discovered for CH(2)OH. In CD(2)OH dissociation, D atom products (correlated with CHDO) were detected, providing the first experimental evidence of isomerization in the CH(2)OH ↔ CH(3)O (CD(2)OH ↔ CHD(2)O) system. Analysis of the H (D) fragment kinetic energy distributions shows that the rovibrational state distributions in the formaldehyde cofragments are different for the OH bond fission and isomerization pathways. Isomerization is responsible for 10%-30% of dissociation events in all studied cases, and its contribution depends on the excited vibrational level of the radical. Accurate dissociation energies were determined: D(0)(CH(2)OH → CH(2)O + H) = 10,160 ± 70 cm(-1), D(0)(CD(2)OH → CD(2)O + H) = 10,135 ± 70 cm(-1), D(0)(CD(2)OH → CHDO + D) = 10,760 ± 60 cm(-1).
The Journal of chemical physics 02/2012; 136(8):084305. · 3.09 Impact Factor
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ABSTRACT: The 365 nm pulsed laser photolysis of nitrosyl chloride adsorbed on a rough MgO(100) surface at 90 K has been studied. Mass spectrometric detection was used to record time-of-flight (TOF) spectra by monitoring Cl+ and NO+. These ions can derive from parent ClNO, which fragments completely in the mass spectrometer, as well as from Cl and NO photofragments. The TOF distributions are considerably slower than for the corresponding gas phase photodissociation process. NO was also detected state selectively by using resonance enhanced multiphoton ionization (REMPI), and a channel corresponding to direct adsorbate photolysis was identified. The state selective detection of NO molecules that emerge from the surface following photolysis shows unambiguously that their rotational degrees of freedom reflect the surface temperature (Trot = 100−140 K), even at low coverages. At similar photolysis wavelengths, gas phase ClNO photodissociation is known to produce highly rotationally excited NO with a distinctive non-statistical distribution peaked at J″ = 46.5. Our studies suggest that, contrary to the gas-phase photolysis results, Cl and NO are not ejected rapidly following photolysis of surface-bound species on a repulsive potential energy surface. We postulate that ClNO grows in islands, with MgO defect sites serving as nucleation centers. Photofragment rotational and translational excitations are quenched efficiently due to strong attractive interactions that equilibrate NO to the surface temperature. Desorption of intact ClNO may also take place, but following (i.e., not during) the photolysis pulse. Such desorbed species can contribute to the TOF spectra, but not the REMPI spectra.
Canadian Journal of Chemistry 02/2011; 72(3):737-744. · 1.24 Impact Factor
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ABSTRACT: The unimolecular decomposition of expansion-cooled isocyanic acid (HNCO) via channels (1) 3NH + CO, (2) H + NCO, and (3) 1NH + CO [where 3NH and 1NH denote NH(X3σ−) and NH(a1 Δ), respectively] has been investigated following photoexcitation to the S1(1A ) state in two energy regimes: (i) in the region of the 1NH + CO threshold (41 700–45 500 cm−1; 240–220 nm), and (ii) ˜ 3200 cm−1 above D0(1NH + CO), at around 46000 cm−1 (217.6 nm). Several complementary experiments are presented: NCO, 3NH and 1NH photofragment yield spectra and relative 1NH/3NH branching ratios are obtained by laser induced fluorescence (LIF); photo-fragment ion imaging is used to record CO angular recoil distributions, and 1NH rotational distributions correlated with specific CO (v, J) levels, HNCO excited to S1 undergoes complex dynamics reflecting simultaneous decomposition on several potential energy surfaces, and including internal conversion (IC) and intersystem crossing (ISC). In energy region (i), a progressive loss of structure in the 3NH yield spectrum is observed above the opening of channel (3), and is interpreted as the imprint of short-time dynamics characteristic of the ISC step. State selectivity in the photodissociation is revealed by comparing the photofragment yield spectra of the three channels. In region (ii), product state distributions for channel (3) exhibit clear dynamical signatures, as expected for dissociation on S1. At low excess energies channel (2) derives from dissociation on S0, but the respective roles of S0 and S1 at higher energies are not well established yet. The results are discussed in terms of vibronic levels of mixed electronic character coupled directly or via radiationless decay to the various continua. The competition between the different processes depends sensitively on photolysis energy and excitation conditions.
Berichte der Bunsengesellschaft für physikalische Chemie. 06/2010; 101(3):469 - 477.
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ABSTRACT: Experimental observations of D fragments from the predissociation of rovibrationally excited partially deuterated 2-hydroxyethyl radicals, CD(2)CD(2)OH, are reported, and possible dissociation channels are analyzed by theory. The radicals are produced by photolysis of 2-bromoethanol at 202-215 nm, and some of them have sufficient internal energy to predissociate. D fragments are detected by 1 + 1' REMPI and their TOF distributions are determined. They can be associated with vinyl alcohol and/or acetaldehyde cofragments. From analysis of the maximum velocities and kinetic energies of the observed D fragments it is concluded that they originate from the decomposition of CD(2)CD(2)OH, but the experimental resolution is insufficient to distinguish between the two possible channels leading to D products. Theoretical analysis and RRKM calculations of microcanonical dissociation rates and branching ratios for the range of available excess energies (up to 5000-8000 cm(-1) above the OH + C(2)D(4) threshold) indicate that the D-producing channels are minor (about 1%) compared to the predominant OH + C(2)D(4) channel, and the branching ratio for D production is more favorable when the reactant radicals have low rotational energy. The vinyl alcohol channel is strongly favored over the acetaldehyde channel at all excess energies, except near the threshold of these channels.
The Journal of Physical Chemistry A 04/2010; 114(17):5453-61. · 2.95 Impact Factor
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ABSTRACT: The low lying excited electronic states of the 2-hydroxyethyl radical, CH(2)CH(2)OH, have been investigated theoretically in the range 5-7 eV by using coupled-cluster and equation-of-motion coupled-cluster methods. Both dissociation and isomerization pathways are identified. On the ground electronic potential energy surface, two stable conformers and six saddle points at energies below approximately 900 cm(-1) are characterized. Vertical excitation energies and oscillator strengths for the lowest-lying excited valence state and the 3s, 3p(x), 3p(y), and 3p(z) Rydberg states have been calculated and it is predicted that the absorption spectrum at approximately 270-200 nm should be featureless. The stable conformers and saddle points differ primarily in their two dihedral coordinates, labeled d(HOCC) (OH torsion around CO), and d(OCCH) (CH(2) torsion around CC). Vertical ionization from the ground-state conformers and saddle points leads to an unstable structure of the open-chain CH(2)CH(2)OH(+) cation. The ion isomerizes promptly either to the 1-hydroxyethyl ion, CH(3)CHOH(+), or to the cyclic oxirane ion, CH(2)(OH)CH(2) (+), and the Rydberg states are expected to display a similar behavior. The isomerization pathway depends on the d(OCCH) angle in the ground state. The lowest valence state is repulsive and its dissociation along the CC, CO, and CH bonds, which leads to CH(2)+CH(2)OH, CH(2)CH(2)+OH, and H+CH(2)CHOH, should be prompt. The branching ratio among these channels depends sensitively on the dihedral angles. Surface crossings among Rydberg and valence states and with the ground state are likely to affect dissociation as well. It is concluded that the proximity of several low-lying excited electronic states, which can either dissociate directly or via isomerization and predissociation pathways, would give rise to prompt dissociation leading to several simultaneous dissociation channels.
The Journal of chemical physics 03/2010; 132(11):114308. · 3.09 Impact Factor
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ABSTRACT: Interactions of 13CO2 guest molecules with vapor-deposited porous H2O ices have been examined using temperature-programmed desorption (TPD) and Fourier transform infrared (FTIR) techniques. Specifically, the trapping and release of 13CO2 by amorphous solid water (ASW) has been studied. The use of 13CO2 eliminates problems with background CO2. Samples were prepared by (i) depositing 13CO2 on top of ASW, (ii) depositing 13CO2 underneath ASW, and (iii) codepositing 13CO2 and H2O during ASW formation. Some of the deposited 13CO2 becomes trapped when the ice film is annealed. The amount of 13CO2 trapped in the film depends on the deposition method. The release of trapped molecules occurs in two stages. The majority of the trapped 13CO2 escapes during the ASW-to-cubic ice phase transition at 165 K, and the rest desorbs together with the cubic ice film at 185 K. We speculate that the presence of 13CO2 at temperatures up to 185 K is due to 13CO2 that is trapped in cavities within the ASW film. These cavities are similar to ones that trap the 13CO2 that is released during crystallization. The difference is that 13CO2 that remains at temperatures up to 185 K does not have access to escape pathways to the surface during crystallization.
The Journal of Physical Chemistry A 01/2008; 111(51):13365-70. · 2.95 Impact Factor
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ABSTRACT: Guest-host interactions have been examined experimentally for amorphous solid water (ASW) films doped with CO2 or N2O. The main diagnostics are Fourier transform infrared (FTIR) spectroscopy and temperature programmed desorption (TPD). ASW films deposited at 90 K are exposed to a dopant, and the first molecules that attach to a film enter its bulk until it is saturated with them. Subsequent dopant adsorption results in crystal growth atop the ASW film. There are distinct spectral signatures for these two cases: LO and TO vibrational modes for the crystal overlayer, and an easily distinguished peak for dopant molecules that reside within the ASW film. Above 105 K, the dopant surface layer desorbs fully. Some dopants residing within the ASW film remain until 155 K, at which point the ASW-to-crystalline-ice transition occurs, expelling essentially all of the dopant. No substantial differences are observed for CO2 versus N2O. It is shown that annealing an ASW film to 130 K lowers the film's capacity to include dopants by a factor of approximately 3, despite the fact that the ASW spectral feature centered at approximately 3250 cm(-1) shows no discernible change. Sandwiches were prepared: ASW-dopant-ASW etc., with the dopant layer displaying crystallinity. Raising these samples past 105 K resulted in the expulsion of essentially all of the crystalline dopant. What remained displayed the same spectral signature as the molecules that entered the bulk following adsorption at the surface. It is concluded that the adsorption sites, though prepared differently, have a lot in common. Dangling OH bonds were observed. When they interacted with a dopant, they underwent a red shift of approximately 50 cm(-1). This is in qualitative agreement with studies that have been carried out with weakly bound binary complexes. As a result of this study, a fairly complete, albeit qualitative, picture is in place for the adsorption, binding, and transport of CO2 and N2O in ASW films.
The Journal of Physical Chemistry A 03/2006; 110(6):2097-105. · 2.95 Impact Factor
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O Gessner,
A M D Lee,
J P Shaffer, H Reisler,
S V Levchenko,
A I Krylov,
Jonathan G Underwood,
H Shi,
A L L East,
D M Wardlaw,
E T H Chrysostom,
C C Hayden,
Albert Stolow
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ABSTRACT: The coupled electronic and vibrational motions governing chemical processes are best viewed from the molecule's point of view-the molecular frame. Measurements made in the laboratory frame often conceal information because of the random orientations the molecule can take. We used a combination of time-resolved photoelectron spectroscopy, multidimensional coincidence imaging spectroscopy, and ab initio computation to trace a complete reactant-to-product pathway-the photodissociation of the nitric oxide dimer-from the molecule's point of view, on the femtosecond time scale. This method revealed an elusive photochemical process involving intermediate electronic configurations.
Science 02/2006; 311(5758):219-22. · 31.20 Impact Factor
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ABSTRACT: Photofragment yield spectra and NO(X(2)Pi(1/2,3/2); v = 1, 2, 3) product vibrational, rotational, and spin-orbit state distributions were measured following NO dimer excitation in the 4000-7400 cm(-1) region in a molecular beam. Photofragment yield spectra were obtained by monitoring NO(X(2)Pi; v = 1, 2, 3) dissociation products via resonance-enhanced multiphoton ionization. New bands that include the symmetric nu(1) and asymmetric nu(5) NO stretch modes were observed and assigned as 3nu(5), 2nu(1) + nu(5), nu(1) + 3nu(5), and 3nu(1) + nu(5). Dissociation occurs primarily via Deltav = -1 processes with vibrational energy confined preferentially to one of the two NO fragments. The vibrationally excited fragments are born with less rotational energy than predicted statistically, and fragments formed via Deltav = -2 processes have a higher rotational temperature than those produced via Deltav = -1 processes. The rotational excitation likely derives from the transformation of low-lying bending and torsional vibrational levels in the dimer into product rotational states. The NO spin-orbit state distribution reveals a slight preference for the ground (2)Pi(1/2) state, and in analogy with previous results, it is suggested that the predominant channel is X(2)Pi(1/2) + X(2)Pi(3/2). It is suggested that the long-range potential in the N-N coordinate is the locus of nonadiabatic transitions to electronic states correlating with excited product spin-orbit states. No evidence of direct excitation to electronic states whose vertical energies lie in the investigated energy region is obtained.
The Journal of Physical Chemistry B 06/2005; 109(17):8407-14. · 3.70 Impact Factor
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ABSTRACT: A study of excited states of the NO dimer is carried out at 7.1-8.2 eV excitation energies. Photoexcitation is achieved by two-photon absorption at 300-345 nm followed by (NO)(2) dissociation and detection of electronically excited products, mostly in n=3 Rydberg states of NO. Photoelectron imaging is used as a tool to identify product electronic states by using non-state-selective ionization. Photofragment ion imaging is used to characterize product translational energy and angular distributions. Evidence for production of NO(A (2)Sigma(+)), NO(C (2)Pi), and NO(D (2)Sigma(+)) Rydberg states of NO, as well as the valence NO(B (2)Pi) state, is obtained. On the basis of product translational energy and angular distributions, it is possible to characterize the excited state(s) accessed in this region, which must possess a significant Rydberg character.
The Journal of Chemical Physics 01/2005; 121(24):12353-60. · 3.33 Impact Factor
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04/2002;
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ABSTRACT: The photodissociation of the chloromethyl radical, CH2Cl, to chlorine atom and methylene is examined following excitation at selected wavelengths in the region 312–214 nm. CH2Cl is produced in a molecular beam by using pulsed pyrolysis. Cl and CH2 products are detected by laser ionization and their velocity and angular distributions are determined by using the ion imaging technique. The spectrum obtained by monitoring the Cl fragment yield as function of photolysis wavelength shows that throughout this wavelength region Cl atoms are major products. With 312–247 nm photolysis, the angular distributions are typical of a perpendicular transition (β = −0.7) and the main products are CH2( 3B1)+Cl(2P3/2). The available energy is partitioned preferentially into the translational degrees of freedom. “Hot band” transitions are prominent in this region even in the molecular beam indicating that the geometries of the ground and excited states of CH2Cl must be very different. With 240–214 nm photolysis, the angular distributions are typical of a parallel transition (β ∼ 1.2), and the predominant products are Cl(2P3/2) and Cl(2P1/2), with CH2( 1A1) as the main cofragment. A large fraction of the available energy is partitioned into internal energy of CH2( 1A1). Comparison with the ab initio calculations of Levchenko and Krylov presented in the accompanying paper enables the assignment of the perpendicular and parallel transitions predominantly to 1 2A1←1 2B1 and 2 2B1←1 2B1 excitations, respectively, and both upper states are probably repulsive in the C–Cl coordinate. The electronic states of the products obtained via these two transitions are in agreement with the predictions of a simple diabatic state correlation diagram based on the calculated vertical energies of the upper states. © 2001 American Institute of Physics.
The Journal of Chemical Physics 10/2001; 115(16):7474-7484. · 3.33 Impact Factor
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ABSTRACT: Photodissociation processes of molecules and radicals involving multiple pathways and nonadiabatic crossings are studied using the photofragment imaging technique and the core-sampling version of time-of-flight spectroscopy. Capabilities and challenges are illustrated by two systems. The isocyanic acid system demonstrates how interactions among potential energy surfaces can change during dissociation. The hydroxymethyl photodecomposition system highlights Rydberg-valence interactions common in free radicals. The cross-fertilization between theory and experiment is emphasized.
Accounts of Chemical Research 09/2001; 34(8):625-32. · 21.64 Impact Factor
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ABSTRACT: The photoinitiated unimolecular decomposition of formaldehyde via the H+HCO radical channel has been examined at energies where the S0 and T1 pathways both participate. The barrierless S0 pathway has a loose transition state (which tightens somewhat with increasing energy), while the T1 pathway involves a barrier and therefore a tight transition state. The product state distributions which derive from the S0 and T1 pathways differ qualitatively, thereby providing a means of discerning the respective S0 and T1 contributions. Energies in excess of the H+HCO threshold have been examined throughout the range 1103 ⩽ E† ⩽ 2654 cm−1 by using two complementary experimental techniques; ion imaging and high-n Rydberg time-of-flight spectroscopy. It was found that S0 dominates at the low end of the energy range. Here, T1 participation is sporadic, presumably due to poor coupling between zeroth-order S1 levels and T1 reactive resonances. These T1 resonances have small decay widths because they lie below the T1 barrier. Alternatively, at the high end of the energy range, the T1 pathway dominates, though a modest S0 contribution is always present. The transition from S0 dominance to T1 dominance occurs over a broad energy range. The most reliable value for the T1 barrier (1920±210 cm−1) is given by the recent ab initio calculations of Yamaguchi et al. It lies near the center of the region where the transition from S0 dominance to T1 dominance takes place. Thus, the present results are consistent with the best theoretical calculations as well as the earlier study of Chuang et al., which bracketed the T1 barrier energy between 1020 and 2100 cm−1 above the H+HCO threshold. The main contribution of the present work is an experimental demonstration of the transition from S0 to T1 dominance, highlighting the sporadic nature of this competition. © 2000 American Institute of Physics.
The Journal of Chemical Physics 02/2000; 112(6):2752-2761. · 3.33 Impact Factor
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ABSTRACT: The decomposition of jet-cooled HNCO is investigated near the H+NCO channel threshold [D0(H+NCO)=38 370 cm−1]. Dissociation to H+NCO at energies 17–411 cm−1 above D0(H+NCO) proceeds on the ground potential energy surface (S0), apparently without a barrier. The rotational state distributions of the NCO(X 2Π3/2,0010) fragment are well described by phase space theory (PST), provided that dynamical constraints are included. These constraints are associated with long range (4–7 Å) centrifugal barriers, which are significant even near threshold because of the small reduced mass of H+NCO, and result in a fraction of energy deposited in fragment rotation much smaller than predicted by unconstrained PST. The influence of orientation averaging on the attractive, long-range part of the potential is discussed, and it is argued that angular averaging with respect to the center of mass of the rotating polyatomic fragment results in a shift in the effective potential origin, accompanied by an attenuation of the magnitude of the potential compared to its value for fixed H–N distance. Following initial S1(1A″)←S0(1A′) excitation and internal conversion to S0, HNCO(S0) decays both via unimolecular decomposition of H+NCO and intersystem crossing to the dissociative first triplet state, T1 [yielding NH(X 3Σ−)+CO products]. The competition between the two processes is interrogated by monitoring changes in the relative yields of NCO and NH(X 3Σ−) as a function of excitation energy. It is concluded that near D0(H+NCO), the S0→T1 intersystem crossing rate is several-fold faster than the H+NCO unimolecular decomposition rate. © 1999 American Institute of Physics.
The Journal of Chemical Physics 06/1999; 110(22):10774-10783. · 3.33 Impact Factor
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ABSTRACT: The photofragment ion imaging technique is used to determine product recoil anisotropy parameters, β, and correlated state distributions in the S1(1A″)←S0(1A′) photoinitiated decomposition of HNCO into three competing channels: (1) 3NH+CO, (2) H+NCO, and (3) 1NH+CO [where 3NH and 1NH denote NH(X 3Σ−) and NH(a 1Δ), respectively]. In particular, the region in the vicinity of the 1NH+CO threshold is investigated. The measured recoil anisotropies fall into two distinct groups corresponding to time scales of <1 ps (β<−0.6), and >5–10 ps (β ≅ 0.0). With 230.1 nm photolysis, CO(J = 0–14) originating in channel (3) is produced with β = −0.8±0.05 via direct dissociation on S1 above a barrier of 470±60 cm−1. CO at low J-states appears with most of the available energy in the translational degree of freedom and is correlated with 1NH in its lowest rotational states. A small contribution to channel (3) from S0 dissociation (observed mainly for J = 14,15) gives rise to an isotropic recoil distribution, and a hotter correlated 1NH rotational distribution. At the same wavelength, CO correlated with 3NH is identified by its high translational energy and exhibits an isotropic angular distribution. We propose that the pathway leading to its formation is S1→S0→T1. H-atom signals from channel (2) have isotropic angular distributions at photolysis wavelengths 243−215 nm; this places a lower limit of 8140 cm−1 on the barrier to direct dissociation on S1 to channel (2). The >5 ps time scale for the appearance of channel (2) implies dissociation on S0 following internal conversion. The mechanism described here for the one-photon decomposition of HNCO in the wavelength region 260-230 nm is in accord with other available experimental and theoretical findings. © 1999 American Institute of Physics.
The Journal of Chemical Physics 01/1999; 110(4):2059-2068. · 3.33 Impact Factor
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ABSTRACT: NO2 in high vibrational levels was prepared in a pulsed molecular beam by laser excitation of the mixed 12A1/22B2 state to energies hν below dissociation threshold D0, D0− hν = 0−500 cm-1. The beam of excited molecules was crossed with pulsed, neat molecular beams of HCl, CO2, N2O, and NH3 at relative collision energies of 2000 cm-1, and the NO produced by collision-induced dissociation (CID) was detected state-selectively. The CID yield spectra obtained by monitoring specific NO rotational levels while scanning the excitation wavelength show spectral features identical with those in the fluorescence excitation spectrum of NO2. The yield of the CID products, however, decreases exponentially (compared with the fluorescence spectrum) with the increase of the amount of energy required to reach the threshold of appearance of the monitored NO state. The average energy transferred per activating collision with polyatomic colliders is in the range 130−200 cm-1, having values similar to or lower than those for diatomic and atomic colliders. This is in contrast to deactivating collisions, in which polyatomic colliders are in general more effective. The results are discussed in terms of a mechanism in which the NO2 molecules are activated by impulsive collisions creating a distribution of molecules in quantum states above D0 whose populations diminish exponentially with energy. The collisional activation is followed by unimolecular decomposition. The differences between the activation and deactivation pathways are rationalized in terms of the number of degrees of freedom available for energy transfer in each channel.
03/1996;
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Canadian Journal of Chemistry-revue Canadienne De Chimie - CAN J CHEM. 01/1994; 72(3):737-744.
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ABSTRACT: Molecule–surface collision-induced dissociation (CID) has been studied for n-C3F7NO and i-C3F7NO molecular beams scattered from MgO(100) at incident kinetic energies (Eincident) up to 7 eV. The NO fragment was detected state selectively using two-photon, two-frequency ionization, and rotational and spin–orbit distributions are reported for several Eincident values. State and angle-resolved signals were integrated to give CID yields, which increased sharply with Eincident . In most cases, rotational excitation could be described by separate temperatures for each spin–orbit state. The upper 2∏3/2 state was underpopulated relative to statistical predictions (e.g., for n-C3F7NO at Eincident =5.0 eV, the spin–orbit temperature was ∼170 K, while Trot was ∼500 K). The CID results are compared to NO state distributions derived from the photodissociation of expansion-cooled molecules under collision-free conditions, at different energies (E°) above D0. These distributions were measured for both n-C3F7NO and i-C3F7NO up to E°∼4500 cm−1, and rotational excitation within each spin–orbit state was statistical, except at E°≥3000 cm−1. As with CID, a low [2∏3/2]/[2∏1/2] ratio was observed, and the reaction mechanism is probably unimolecular decomposition via the lowest triplet surface T1 with little or no exit channel barrier. The pronounced similarities between the CID and photodissociation results suggest that common reaction mechanisms may be operative. All of the CID results are compatible with direct inelastic scattering followed by unimolecular reaction on the S0 and/or T1 potential surfaces.
Chemical Physics - CHEM PHYS. 01/1991; 94:2330-2345.
