Short-lived Collision Complex Research Articles

Bond-forming and electron-transfer reactivity between Ar2+ and O2.

The reactivity, energetics and dynamics of the bimolecular reactions between Ar2+ and O2 have been studied using a position sensitive coincidence methodology at a collision energy of 4.4 eV. Four bimolecular reaction channels generating pairs of product ions are observed, forming: Ar+ + O2+, Ar+ + O+, ArO+ + O+ and O+ + O+. The formation of Ar+ + O2+ is a minor channel, involving forward scattering, and generates O2+ in its ground electronic state. This single electron transfer process is expected to be facile by Landau-Zener arguments, but the intensity of this channel is low because the electron transfer pathways involve multi-electron processes. The formation of Ar+ + O+ + O, is the most intense channel following interactions of Ar2+ with O2, in agreement with previous experiments. Many different combinations of Ar2+ and product electronic states contribute to the product flux in this channel. Major dissociation pathways of the nascent O2+* ion involve the ion's first and second dissociation limits. Unusually, the experimental results clearly show the involvement of a short-lived collision complex [ArO2]2+ in this channel. The formation of O+ and ArO+ involves direct abstraction of O- from O2 by Ar2+. There is scant evidence of the involvement of a collision complex in this bond forming pathway. The ArO+ product appears to be formed in the first excited electronic state (2Π). The formation of O+ + O+ results from dissociative double electron transfer via an O22+ intermediate. The exoergicity of the dissociation of the nascent O22+ intermediate is in good agreement with previous work investigating the unimolecular dissociation of this dication.

Quantum dynamical study of heavy-light-heavy reactions: application to the (Cl + CH4 → HCl + CH3) reaction

A time dependent quantum-mechanical QM study is performed on the Cl + CH4 → HCl + CH3 reaction, using a pseudotriatomic ab initio based surface. Probabilities present some clear peaks versus t, which we assign to transition state resonances where the light H atom oscillates between heavy Cl and CH3 groups. For ground-state reactants, the reactivity is of quantum origin. The reaction occurs through an abstraction mechanism, following both direct and an indirect mechanisms. The calculations show the participation of a short-lived collision complex in the microscopic reaction mechanism. This theoretical result and other oscillating properties found here could, however, be related to the existence of resonance for the production of HCl, as suggested by experimentalists.

Vibrationally enhanced charge transfer and mode/bond-specific H+ and D+ transfer in the reaction of HOD+ with N2O

The reaction of HOD(+) with N2O was studied over the collision energy (E(col)) range from 0.20 eV to 2.88 eV, for HOD(+) in its ground state and in each of its fundamental vibrational states: bend (010), OD stretch (100), and OH stretch (001). The dominant reaction at low E(col) is H(+) and D(+) transfer, but charge transfer becomes dominant for E(col) > 0.5 eV. Increasing E(col) enhances charge transfer only in the threshold region (E(col) < 1 eV), but all modes of HOD(+) vibrational excitation enhance this channel over the entire energy range, by up to a factor of three. For reaction of ground state HOD(+), the H(+) and D(+) transfer channels have similar cross sections, enhanced by increasing collision energy for E(col) < 0.3 eV, but suppressed by E(col) at higher energies. OD stretch excitation enhances D(+) transfer by over a factor of 2, but has little effect on H(+) transfer, except at low E(col) where a modest enhancement is observed. Excitation of the OH stretch enhances H(+) transfer by up to a factor of 2.5, but actually suppresses D(+) transfer over most of the E(col) range. Excitation of the bend mode results in ~60% enhancement of both H(+) and D(+) transfer at low E(col) but has little effect at higher energies. Recoil velocity distributions at high E(col) are strongly backscattered in the center-of-mass frame, indicating direct reaction dominated by large impact parameter collisions. At low E(col) the distributions are compatible with mediation by a short-lived collision complex. Ab initio calculations find several complexes that may be important in this context, and RRKM calculations predict lifetimes and decay branching that is consistent with observations. The recoil velocity distributions show that HOD(+) vibrational excitation enhances reactivity in all collisions at low E(col), while for high E(col) with enhancement comes entirely from the subset of collisions that generate strongly back-scattered product ions.

Vibrationally inelastic collisions of H+D2: A comparison of quantum mechanical, quasiclassical, and experimental results

A detailed comparison of quantum mechanical (QM) and quasiclassical trajectory (QCT) integral and differential cross sections (DCSs) as well as opacity functions is presented in this work for the vibrationally inelastic collisions of H+D(2)(v=0,j=0)-->H+D(2)(v(')=3,j(')) at 1.72 eV collision energy. These results are also compared with the experimental differential cross sections by Greaves et al. [Nature (London) 454, 88 (2008)]. The agreement between QCT and QM results is fairly good but some differences are appreciable, and it is shown that the experimental results are in a somewhat better agreement with the calculated QM DCS. The present results and their analysis confirm that the vibrational excitation takes place by elongation of the D-D bond in a "tug-of-war" mechanism, where the incoming H atom and one of the D atoms compete for the formation of a bond with the other D atom, as proposed by Greaves et al. It is also found that these collisions may give rise to the formation of short-lived collision complexes (tau(coll)=35-50 fs) that can be traced back to the presence of relatively deep wells in the potential surface when the original D-D bond is stretched. The analysis of the trajectories into v(')=3 reveals that most of them cross at least twice the reaction barrier via a recrossing mechanism.

Formation of the diphenyl molecule in the crossed beam reaction of phenyl radicals with benzene

The chemical dynamics to form the D5-diphenyl molecule, C6H5C6D5, via the neutral-neutral reaction of phenyl radicals (C6H5) with D6-benzene (C6D6), was investigated in a crossed molecular beams experiment at a collision energy of 185 kJ mol(-1). The laboratory angular distribution and time-of-flight spectra of the C6H5C6D5 product were recorded at mass to charge mz of 159. Forward-convolution fitting of our data reveals that the reaction dynamics are governed by an initial addition of the phenyl radical to the pi electron density of the D6-benzene molecule yielding a short-lived C6H5C6D6 collision complex. The latter undergoes atomic deuterium elimination via a tight exit transition state located about 30 kJ mol(-1) above the separated reactants; the overall reaction to form D5-diphenyl from phenyl and D6-benzene was found to be weakly exoergic. The explicit identification of the D5-biphenyl molecules suggests that in high temperature combustion flames, a diphenyl molecule can be formed via a single collision event between a phenyl radical and a benzene molecule.

Searching for resonances in the reaction Cl+CH4→HCl+CH3: Quantum versus quasiclassical dynamics and comparison with experiments

A quantum-mechanical (QM) and quasiclassical trajectory (QCT) study was performed on the title reaction, using a pseudotriatomic ab initio based surface. Probabilities and integral cross sections present some clear peaks versus the collision energy E(col), which we assign to Feshbach resonances of the transition state, where the light H atom oscillates between the heavy Cl and CH(3) groups. For ground-state reactants, reactivity is essentially of quantum origin (QCT observables and oscillations are smaller, or much smaller, than QM ones), and the calculated integral cross section and product distributions are in reasonable agreement with the experiment. The reaction occurs through an abstraction mechanism, following both a direct and an indirect mechanism. The quasiclassical trajectory calculations show the participation of a short-lived collision complex in the microscopic reaction mechanism. Finally, QCT differential cross sections of Cl+CH(4)-->HCl (nu(')=0 and 1)+CH(3) oscillate versus E(col), whereas experimentally this only occurs for HCl (nu(')=1). This theoretical result and other oscillating properties found here could, however, be related to the existence of a Feshbach resonance for the production of HCl (nu(')=1), as suggested by experimentalists.

Spectroscopy of interparticle interactions in ionic and molecular liquids: Novel approaches

Abstract The paper deals with several novel approaches to the studies of interactions and dynamics in liquids. A new, flexible time-correlation function is introduced. It has an analytical counterpart in the frequency domain and enables one to fit even badly overlapped vibrational line profiles obtaining dynamical information at the same time. As examples, quantitative description of the interaction potential in molten salts containing halide complexes of zinc and manganese is presented, the dynamic criterion of complex entities in a melt is introduced, and the spectroscopic features of molten alkali and alkaline earth halides are explained in terms of short-lived collision complexes. Finally, the model treatment of "anomalous" noncoincidence effects in liquids is discussed, and the contribution of repulsion and attraction forces resulting in frequency non-coincidences is separated.

Scattering of NO from a graphite surface at cryogenic temperatures

The scattering of NO molecules from a graphite surface at cryogenic temperatures ( T s<80 K) allows to study the molecular translation-rotation energy transfer without surface phonon contributions in the entrance channel. The angular distributions show that there are no diffusely scattered molecules at surface temperatures below T s=70 K, whereas the signal of forward scattered molecules remains present down to T s=20 K, the lowest temperature investigated. The rotational behavior of the scattered molecules can be described by a Boltzmann distribution characterized by a rotational temperature T rot. It is nearly constant below T s=80 K and is determined by the molecular kinetic energy in surface normal direction. The data are consistent with the formation of a short-lived collision complex (NO··C n ) between the NO molecule and a few surface atoms. The complex decomposes in a unimolecular fashion. The cryogenic surface temperatures require effective shielding of the crystal from the heat radiation of the surrounding experimental equipment. The data show that the heat radiation influences the crystal temperature, however, it has only negligible influence on the molecule–surface interaction.

A QCT study of the microscopic mechanisms proceeding via the ground PES of the O( 1D) + H 2 (X [formula omitted]g+) → OH(X [formula omitted])+H( 2S) reaction

The reaction was studied using the quasiclassical trajectory method and the surface developed by Schatz et al. Dynamical properties were calculated for the collision energy ( E C) distribution resulting from O( 1D) generated by photodissociation of O 3 at 266 nm (〈 E C〉=0.074 eV), obtaining a good agreement with experiment. A little amount of the reactivity was due to an abstraction pathway, though the well-known insertion mechanism was the dominant one. Particular attention was paid to compare the dynamics of both mechanisms and to analyze the dynamical properties of the insertion mechanism in terms of the lifetime of the short-lived collision complex involved.

Instantaneous collision complexes in molten alkali halides: Picosecond dynamics from low-frequency Raman data

The picosecond dynamics of molten alkali halides is discussed, and the low-frequency Raman spectra of molten LiCl, CsCl, and the LiCl–CsCl eutectic are fitted to the model enabling to obtain the times of vibrational dephasing, τV and vibrational frequency modulation τω. In terms of the Wilmshurst criterion [J. Chem. Phys. 39, 1779 (1963)] and using the data of NMR studies and molecular dynamics simulations, a conclusion is drawn that molten alkali halides cannot contain long-lived stable complexes with lifetimes greater than 10−8 s. The low-frequency Raman spectra of molten alkali halides and their mixtures probe the presence of instantaneous spatial configurations of MXn−n+1 type, where M+ is the alkali metal cation and X− is the halide anion existing in melts during the time intervals equal to the time of duration of collision of oppositely charged ions τd, which is less than 0.5 ps. This time is sufficient to a collision complex to execute several (at least one) vibrations. Vibrational dephasing and modulation processes elapse during this same time, thereby indicating the instantaneous nature of configurations in question. To discern between short-lived and long-lived complexes, we propose relations between the minimal damping time of the probe oscillator set equal to the half-period of vibration T/2, τV, τω, and τd, as well as the time between collisions τBC. The duration of an act resulting in the vibrational phase shift (or energy transfer) must be equal to (or longer than) the half-period of vibration of the probe oscillator, τV⩾T/2. The modulation time may vary from this same half-period of vibration or the time between collisions τBC to very long times, τω⩾T/2, τω⩾τBC. For short-lived complexes, the longest of two characteristic times describing the phase decay cannot exceed possible duration of collision, τω⩽τd, τV⩽τd. Cs-containing configurations follow this definition and therefore should be considered instantaneous short-lived collision complexes: their τV≈T/2∼0.1 ps, and τω≈τBC∼0.03 ps. Li-containing configurations appear to be relatively long-lived: their lifetimes could be associated with τω∼0.17 ps, which is several times longer than any other shortest possible characteristic time in the system (τBC∼0.026 ps or T/2∼0.05 ps). In light of these conclusions, an a priori assumption of autocomplex MX4n−4 anions and Mn+ cations as being structural elements of molten halides made in the so-called autocomplex model by Smirnov, Shabanov, and Khaimenov [Elektrohim. 2, 1240 (1966)] is discussed, and the autocomplexes are identified as instantaneous short-lived configurations detectable by the Raman method.

Theoretical Study of the Metal Oxidation Reaction Ti + O2 → TiO + O: Ab Initio Calculation of the Potential Energy Surface and Classical Trajectory Analysis

The oxidation reaction of the ground-state Ti(3F) by the O2 molecule has been studied theoretically. The 11A‘ and 13A‘ global potential energy surfaces (PESs) were calculated at the multireference configuration interaction level. The analytical PES for the 11A‘ state is constructed on the basis of about 700 ab initio energies. Quasi-classical trajectory calculations are performed to obtain the product translational, rotational, and vibrational energy distributions as well as the lifetime of the collision complex. The primary reaction mechanism is assigned to the inserted/fast dissociation mechanism because the lifetime of the collision complex is short. The relative translational and rotational energy distributions are slightly hotter than those from the statistical prediction, reflecting a short-lived collision complex.

The collision-induced [formula omitted] transition

The dependences of the Cl 2( D0 u +,v D ,J D → Cl 2( X) g + E0 g +,v E ,J E ) collisional induced transition rate constants on the vibrational v E, v D, rotational J E quantum numbers, energy gaps, and Franck–Condon factors of the combined levels have been studied in the range of v D=1–5. It has been shown that all the observed collisional induced intramolecular transitions (CIIT) are nonresonant and correspond to loss of up to 1100 cm −1 vibrational energy. The u↔g and ΔΩ=0 propensity rules are valid in the Cl 2( D → Cl 2( X) g + ion-pair states ) CIIT. The vibrational and rotational distributions of the E state levels depend on v D number. For v D=3–5 the E state vibrational level corresponding to maximum Franck–Condon factor with the initial one is mainly populated. The E, v E=0,1,2; β1 g, v β =2; and E, v E=0 levels are populated from the D, v D=2 and 1 ones, respectively. The “ ΔJ low” propensity rule is valid in the Cl 2( D0 u +,v D =1–5,J D → Cl 2( X) g + E0 g +,v E =1–5,J E and β1 g ,v β=2,J β) CIIT. Rotational distribution of the v E=0 levels populated from the v D=1, 2 can be described as the Boltzmann one, T=300 K . The nonadiabatic transitions between potential energy surface corresponding to input Cl 2(D)+Cl 2(X) and output Cl 2(E)+Cl 2(X) channels occur in relatively short-lived collision complex, and CIIT rate constant is in the (0.4–3.2)×10 −11 cm 3/ s range.

Generalized diatomics-in-molecules method for polyatomic anions

The diatomics-in-molecules (DIM) method for the construction of polyatomic potential-energy functions from the potential energies of atomic and diatomic fragments is generalized to obtain a description of potential-energy functions of both quasistationary and bound states of polyatomic anions. The formulation is based on the combination of the DIM method with the projection-operator approach of scattering theory. The proposed theory allows the construction of diabatic discrete states, electron-molecule scattering continua, and discrete-continuum coupling elements from the corresponding data of the fragments. The polyatomic projection-operator description obtained in this way provides the basis for a rigorous treatment of the nuclear dynamics in short-lived electron-molecule collision complexes and ion-molecule collisions in terms of energy-dependent, complex, and nonlocal effective potentials. More approximate local complex potential-energy surfaces of quasistationary states of polyatomic anions also can be obtained with the generalized DIM method via the determination of the poles of the multichannel electron-molecule scattering matrix in the fixed-nuclei limit. Although the focus of the present work is on anions, the proposed theory is also applicable to quasistationary states of neutral and positively charged polyatomic systems including clusters. To illustrate the concepts, the generalized DIM method is applied to obtain the potential-energy functions of the ground and first excited states of the ${\mathrm{H}}_{3}^{\ensuremath{-}}$ anion, making use of information that is available for the bound and resonance states of ${\mathrm{H}}^{\ensuremath{-}}$ and ${\mathrm{H}}_{2}^{\ensuremath{-}}.$

Influence of Collision Energy on the Nascent OH(X2Π, v‘ ‘ = 0−4) Product Energetics for the Reaction of O(1D) with Ethane. A Laser-Induced Fluorescence and Quasiclassical Trajectory Study

The full characterization of the nascent OH(X2Π, v‘ ‘, N‘ ‘, J‘ ‘, Λ‘ ‘) state distributions for the O(1D) + C2H6 → OH + C2H5 reaction has been experimentally performed using the laser-induced fluorescence technique to probe the v‘ ‘ = 0−4 levels of the OH molecules. This thorough study has been carried out for the first time using the N2O photodissociation at 193 nm to generate the O(1D) atoms (the average collision energy, 〈ET〉, is equal to 0.52 eV). By comparison with the experimental available data for 〈ET〉 = 0.27 eV (Park and Wiesenfeld, J. Chem. Phys. 1991, 95, 8166), it has been observed the existence of a small influence of ET on the dynamics of the title reaction, as only a somewhat higher rovibrational excitation of the OH product has been found on increasing the collision energy of the system. This supports the belief that the studied reaction essentially evolves through the insertion of the O(1D) atom into a C−H bond, yielding both short-lived (insertion/fast elimination microscopic mechanism) and long-lived (insertion/slow elimination microscopic mechanism) alcohol-type collision complexes. However, although not fully conclusive, some evidence has been found about the coexistence of a third reaction mode (abstraction), which may contribute to the formation of the highest vibrationally excited OH molecules with low rotational excitation. The dynamics of the insertion/fast elimination mechanism has been qualitatively reproduced using the quasiclassical trajectory method applied on two ab initio based analytical triatomic potential energy surfaces developed by us very recently for the related O(1D) + CH4 reaction, but with a mass correction.

Reactive and Nonreactive Quenching of OH(A 2Σ+) in Collisions with H Atoms

We use trajectory surface hopping methods to calculate cross sections and rate coefficients for reactive and nonreactive quenching of OH(A 2Σ+) in collisions with H atoms. All calculations are based on multireference configuration interaction potential surfaces for the 11A‘ and 21A‘ surfaces of water, using a diabatic representation to describe electronic transitions. The overall rate of quenching plus reaction is in good agreement (lower by 25−30%) with earlier estimates at 300 and 1500 K. Analysis of the trajectories shows that this rate is dominated by a capture mechanism on the 21A‘ state of water, with a nearly unity probability for hopping to the 11A‘ state following formation of a short-lived collision complex. Both reactive and nonreactive decay of the complex are significant, with branching to H + OH (nonreactive quenching), H + OH (atom exchange with quenching), and O(1D) + H2 (reaction) accounting for approximately 45%, 35%, and 20%, respectively, of the product yield. The cross section for ato...

Ab initio and quasiclassical trajectory study of the N(2D)+NO(X 2Π)→O(1D)+N2(X 1Σg+) reaction on the lowest A′1 potential energy surface

In this work we have carried out ab initio electronic structure calculations, CASSCF/CASPT2 with the Pople’s 6-311G(2d) basis set on the ground singlet potential energy surface (1 1A′ PES) involved in the title reaction. Transition states, minima and one 1 1A′/2 1A′ surface crossing have been characterized, obtaining three NNO isomers with the energy ordering: NNO (1Σ+)&amp;lt;cyclic−C2v NON(1A1)&amp;lt;NON(1Σg+). Approximately 1250 ab initio points have been used to derive an analytical PES which fits most of the stationary points, with a global root-mean-square deviation of 1.12 kcal/mol. A quasiclassical trajectory study at several temperatures (300–1500 K) was performed to determine thermal rate constants, vibrational and rovibrational distributions and angular distributions. The dynamics of this barrierless reaction presents a predominant reaction pathway (96% at 300 K) with very short-lived collision complexes around the NNO minimum, which originate backward scattering and a similar fraction of vibrational and translational energy distributed into products. At higher temperatures other reaction pathways involving NON structures become increasingly important as well as the N-exchange reaction (3.02% of the branching ratio at 1500 K), this latter in accord with experimental data. It is concluded that the physical electronic quenching of N(2D) by NO should be negligible against all possible N(2D)+NO reaction channels.

Evidence for Scattering Resonances in the H+D2 Reaction

Integral and differential cross sections for the reaction H + D2 → HD(v′=3,j′) + D at a collision energy of 1.64 eV have been measured and compared with quasiclassical trajectory calculations. The low j′ states of the HD product show strong forward scattering (see picture for j′=0), which is attributed to the formation of a short-lived quasibound collision complex.

The product rovibrational and spin–orbit state dependent dynamics of the complex reaction H+CO2→OH(2Π;ν,N,Ω, f)+CO: Memories of a lifetime

The product-state-resolved dynamics of the reaction H+CO2→OH(2Π;ν,N,Ω,f)+CO have been explored in the gas phase at 298 K and center-of-mass collision energies of 2.5 and 1.8 eV (respectively, 241 and 174 kJ mol−1), using photon initiation coupled with Doppler-resolved laser-induced fluorescence detection. A broad range of quantum-state-resolved differential cross sections (DCSs) and correlated product kinetic energy distributions have been measured to explore their sensitivity to spin–orbit, Λ-doublet, rotational and vibrational state selection in the scattered OH. The new measurements reveal a rich dynamical picture. The channels leading to OH(Ω,N∼1) are remarkably sensitive to the choice of spin–orbit state: Those accessing the lower state, Ω=3/2, display near-symmetric forward–backward DCSs consistent with the intermediacy of a short-lived, rotating HOCO (X̃ 2A′) collision complex, but those accessing the excited spin–orbit state, Ω=1/2, are strongly focused backwards at the higher collision energy, indicating an alternative, near-direct microscopic pathway proceeding via an excited potential energy surface. The new results offer a new way of reconciling the conflicting results of earlier ultrafast kinetic studies. At the higher collision energy, the state-resolved DCSs for the channels leading to OH(Ω,N∼5–11) shift from forward–backward symmetric toward sideways–forward scattering, a behavior which resembles that found for the analogous reaction of fast H atoms with N2O. The correlated product kinetic energy distributions also bear a similarity to the H/N2O reaction; on average, 40% of the available energy is concentrated in rotation and/or vibration in the scattered CO, somewhat less than predicted by a phase space theory calculation. At the lower collision energy the discrepancy is much greater, and the fraction of internal excitation in the CO falls closer to 30%. All the results are consistent with a dynamical model involving short-lived collision complexes with mean lifetimes comparable with or somewhat shorter than their mean rotational periods. The analysis suggests a potential new stereodynamical strategy, “freeze-frame imaging,” through which the “chemical shape” of the target CO2 molecule might be viewed via the measurement of product DCSs in the low temperature environment of a supersonic molecular beam.

Gaussian-2 theoretical and direct ab initio molecular dynamics study of the reaction of O(3P) with thiirane, O(3P)+C2H4S(1A1)→SO(3Σ-)+C2H4(1Ag)

Gaussian-2 theoretical and direct density-functional molecular dynamics calculations have been carried out to characterize the reaction mechanism on the triplet potential energy surface of the title reaction. The Gaussian-2 procedure is employed to determine accurately the energies of the reactants, collision complexes, transition states and products on the O(3P) + C2H4S(1A1) potential energy surface. The minimum energy pathway on the triplet potential energy surface reveals two structural isomers as collision complexes which lie approximately 49 kcal mol−1 below the reactants. Both of these complexes show cleavage of one of the S–C bonds in the three-membered ring. The transition states leading to formation of SO(3Σ−) + H2CCH2(1Ag) lie only 2.8 and 3.3 kcal mol−1, respectively, above the two diradical complexes, indicating that they are short-lived. Direct density-functional molecular dynamics study indicates that, as O(3P) approaches the S atom of C2H4S(1A1), an S–C bond in the strained thiirane ring ruptures, giving rise to one of the diradical complexes predicted by Gaussian-2 theory, followed within 20–80 fs by rupture of the second S–C bond. The diradical complex is so short-lived that the desulfurization reaction may be viewed as a direct-mode abstraction reaction. The results of the combined Gaussian-2 theory and direct dynamics study are consistent with the recent experimental findings that angular distributions are highly anisotropic (Gao et al., J. Phys. Chem. A, 1997, 101, 187) and that nascent SO(3Σ−) product vibrational state distribution is inverted due to short-lived collision complexes (Ravichandran et al., Chem. Phys. Lett., 1996, 252, 348).

Influence of the Collision Energy on the O(1D) + RH → OH(X2Π) + R (RH = CH4, C2H6, C3H8) Reaction Dynamics: A Laser-Induced Fluorescence and Quasiclassical Trajectory Study

The influence of the collision energy (ET) on the O(1D) + RH → OH(X2Π) + R (RH = CH4, C2H6, and C3H8) reaction dynamics has been studied, using the N2O photodissociation at 193 nm as O(1D) precursor (〈ET〉 = 0.403 eV) and probing the OH v‘ ‘ = 0 and 1 levels by LIF. A triatomic QCT study of the reaction with CH4 on a fully ab initio based analytical PES has also been performed, and a quite good agreement with the experimental OH rovibrational distributions has been obtained. Our experimental results are similar to those obtained when the O3 photodissociation is used to produce O(1D) (〈ET〉 = 0.212 eV), as expected on the basis of the available energy in products and also from the QCT calculations. The P(v‘ ‘=0)/P(v‘ ‘=1) populations ratio values reported for C2H6 and C3H8 in a very recent work (Wada and Obi, J. Phys. Chem. A 1998, 102, 3481), where the N2O was also used to generate O(1D), are probably largely underestimated. The rotational distributions obtained are similar to those obtained in other experiments, and a quite good agreement has been obtained for the spin−orbit and Λ-doublet populations. The reaction takes place near exclusively through the insertion of the O(1D) atom into a C−H bond below 0.6 eV, and the mechanism may be direct or nondirect (mainly through short-lived (CH3)OH collision complexes) with about the same probability. The OH vibrational distribution arising from the direct mechanism is inverted, while the nondirect one leads to a noninverted distribution. At higher ET, the abstraction mechanism also contributes appreciably to reactivity.