Photodissociation of the CH2Br radical: A theoretical study

Bromine atom (Br) reactions lead to ozone depletion in the troposphere and stratosphere. Photodegradation of bromocarbons is one of the main sources of bromine atoms in the atmosphere. Here, we use high-level ab initio methods, including spin-orbit effects, to study the photodissociation of the CH2Br radical. All possible fragmentation pathways, namely CH2Br + hν → CH2 + Br, HCBr + H, and CBr + H2, have been analyzed. Potential-energy curves of the ground and several excited electronic states along the corresponding dissociating bond distance of each pathway have been calculated. Considering the actinic fluxes of solar irradiation in the troposphere and in the stratosphere in the relevant range of frequencies, it is found that the first five excited states of CH2Br can be accessed from the ground state. Analysis of the potential curves shows that the pathways producing CH2 + Br and HCBr + H can proceed through a fast direct dissociation mechanism, while the pathway leading to CBr + H2 involves much slower dissociation mechanisms like internal conversion between electronic states, predissociation, or tunneling through exit barriers. The main implications are that the two faster channels are predicted to be dominant, and the slower pathway is expected to be less relevant. The tropospheric and stratospheric solar actinic fluxes also allow for further dissociation of the HCBr and CBr fragments, generating additional Br atoms, provided that they survive possible collisions with other atmospheric reagents. Finally, we discuss the possible effect of each of the three CH2Br dissociation pathways on the depletion of atmospheric ozone.

An ab initio study of the photodissociation of CH2I and CH2I. Phys Chem Chem Phys 2023. [PMID: 37465906 DOI: 10.1039/d3cp01460f]

Photodissociation of the CH₂I radical and the CH₂I⁺ cation is studied by means of high-level ab initio calculations, including spin-orbit effects. Potential-energy curves (PEC) along the dissociating bond distances involved in some fragmentation pathways of these species are computed for the ground and several excited electronic states. Based on the PECs obtained, the possible photodissociation mechanisms are analyzed and suggested. Significant differences are found between the fragmentation dynamics of the neutral radical and that of the cation. While a relatively simple dissociation dynamics is predicted for CH₂I, more complex fragmentation mechanisms involving internal conversion and couplings between different excited electronic states are expected for CH₂I⁺. The species studied here are relevant to atmospheric chemistry, and the present work can help to understand better how their photodissociation may affect chemical processes in the atmosphere.

An ab initio study of the photodissociation of the vinyl radical

Photodissociation of the vinyl radical through pathways CH₂CH → CH₂C + H, CH₂CH → CHCH + H, and CH₂CH → CH₂ + CH is investigated by means of high-level ab initio calculations. Potential-energy curves (PECs) along the corresponding dissociating bond distance associated with the ground and several excited electronic states involved in the above fragmentation pathways, as well as the nonadiabatic couplings connecting the different states, are obtained. The findings of several experiments on vinyl photodissociation performed at different excitation wavelengths are analyzed and explained qualitatively in the light of the present PECs. A two-dimensional representation (consisting of radial and angular coordinates to represent one of the H atoms of the CH₂ group) is also used to calculate the electronic states. The surfaces obtained reflect a rich variety of conical intersections, exit barriers, and nonadiabatic couplings leading to predissociation in different regions of energy and of the two coordinates, suggesting a complex photodissociation dynamics of the CH₂CH → CHCH + H pathway, with rather different fragmentation mechanisms involved. The two-dimensional results also provide interesting information on the mechanism of in-plane hydrogen migration from the CH₂ group to the CH one through a high-lying transition state.

Weak-field coherent control of photodissociation in polyatomic molecules

A coherent control scheme is suggested to modify the output of photodissociation in a polyatomic system. The performance of the scheme is illustrated by applying it to the ultrafast photodissociation of CH₃I in the A-band. The control scheme uses a pump laser weak field that combines two pulses of a few femtoseconds delayed in time. By varying the time delay between the pulses, the shape of the laser field spectral profile is modulated, which causes a change in the initial relative populations excited by the pump laser to the different electronic states involved in the photodissociation. Such a change in the relative populations produces different photodissociation outputs, which is the basis of the control achieved. The degree of control obtained over different photodissociation observables, like the branching ratio between the two dissociation channels of CH₃I yielding I(²P_3/2) and I*(²P_1/2) and the fragment angular distributions associated with each channel, is investigated. These magnitudes are found to oscillate strongly with the time delay, with the branching ratio changing by factors between two and three. Substantial variations of the angular distributions also indicate that the scheme provides a high degree of control. Experimental application of the scheme to general polyatomic photodissociation processes should be straightforward.

Coulomb Explosion Imaging for the Visualization of a Conical Intersection

Coulomb explosion imaging is proposed as a method to directly map the presence of conical intersections encountered by a propagating wave packet in a molecular system. The case of choice is the nonadiabatic coupling between two dissociative surfaces in the methyl iodide molecule, probed by Coulomb explosion with short, intense near-infrared pulses causing multiple ionization. On-the-fly multidimensional trajectory calculations with surface hopping using perturbation theory and including spin-orbit coupling are performed to visualize the dynamics through the conical intersection and compare with experimental results. The possibilities and limitations of the technique are examined and discussed.

Observation of middle-sized metal clusters in femtosecond laser ablation plasmas through nonlinear optics

Femtosecond laser ablation of solids is known to produce ejection of material to a large extent composed of particles of nanometer dimensions for a broad variety of targets. This work explores the ultrashort laser ablation of metal targets (Ag, Cu, Al, Mn) through non-conventional diagnostics based on the nonlinear response of the transient medium created upon ablation. The full temporal mapping of the nonlinear response constitutes a useful flag that signals the abundance of clusters and nanoparticles in the plume. The use of this method for diagnosis has allowed us to perform direct observation of middle-sized aggregates that are extremely elusive with other techniques. Additionally, one crucial and seldom explored parameter in this context has been identified: the ablation laser spot size. Optimum conditions for overall nanoparticle generation as well as relative nanoparticle/cluster/atom ratios have been found.

XUV/X-ray light and fast ions for ultrafast chemistry

The deposition of large amounts of energy in a molecule by XUV/X-ray photon absorption or fast-ion collision, triggers a set of complex ultrafast electronic and nuclear dynamics that allow a deep understanding and control of chemical reactivity. This themed issue showcases the research performed in the understanding, monitoring and control of these processes.

An ab initio study of the ground and excited electronic states of the methyl radical

The ground and some excited electronic states of the methyl radical have been characterized by means of highly correlated ab intio techniques. The specific excited states investigated are those involved in the dissociation of the radical, namely the 3s and 3p_z Rydberg states, and the A₁ and B₁ valence states crossing them, respectively. The C-H dissociative coordinate and the HCH bending angle were considered in order to generate the first two-dimensional ab initio representation of the potential surfaces of the above electronic states of CH₃, along with the nonadiabatic couplings between them. Spectroscopic constants and frequencies calculated for the ground and bound excited states agree well with most of the available experimental data. Implications of the shape of the excited potential surfaces and couplings for the dissociation pathways of CH₃ are discussed in the light of recent experimental results for dissociation from low-lying vibrational states of CH₃. Based on the ab initio data some predictions are made regarding methyl photodissociation from higher initial vibrational states.

Photodissociation of the CH3O and CH3S radical molecules: an ab initio electronic structure study

The electronic states and the spin-orbit couplings between them involved in the photodissociation process of the radical molecules CH₃X, CH₃X → CH₃ + X (X = O, S), taking place after the Ã(²A₁) ← X[combining tilde](²E) transition, have been investigated using highly correlated ab initio techniques. A two-dimensional representation of both the potential-energy surfaces (PESs) and the couplings is generated. This description includes the C-X dissociative mode and the CH₃ umbrella mode. Spin-orbit effects are found to play a relevant role in the shape of the excited state potential-energy surfaces, particularly in the CH₃S case where the spin-orbit couplings are more than twice more intense than in CH₃O. The potential surfaces and couplings reported here for the present set of electronic states allow for the first complete description of the above photodissociation process. The different photodissociation mechanisms are analyzed and discussed in light of the results obtained.

Slice imaging of the UV photodissociation of CH2BrCl from the maximum of the first absorption band

The photodissociation dynamics of bromochloromethane (CH₂BrCl) have been investigated at the maximum of the first absorption band, at the excitation wavelengths 203 and 210 nm, using the slice imaging technique in combination with a probe detection of bromine-atom fragments, Br(²P_3/2) and Br*(²P_1/2), via (2 + 1) resonance enhanced multiphoton ionization. Translational energy distributions and angular distributions reported for both Br(²P_3/2) and Br*(²P_1/2) fragments show two contributions for the Br(²P_3/2) channel and a single contribution for the Br*(²P_1/2) channel. High level ab initio calculations have been performed in order to elucidate the dissociation mechanisms taking place. The computed absorption spectrum and potential energy curves indicate the main contribution of the populated 4A″, 5A', and 6A' excited states leading to a C-Br cleavage. Consistently with the results, the single contribution for the Br*(²P_1/2) channel has been attributed to direct dissociation through the 6A' state as well as an indirect dissociation of the 5A' state requiring a 5A' → 4A' reverse non-adiabatic crossing. Similarly, a faster contribution for the Br(²P_3/2) channel characterized by a similar energy partitioning and anisotropy than those for the Br*(²P_1/2) channel is assigned to a direct dissociation through the 5A' state, while the slower component appears to be due to the direct dissociation on the 4A″ state.

A velocity map imaging study of the photodissociation of the methyl iodide cation

The photodissociation dynamics of the methyl iodide cation has been studied using velocity map imaging and ab initio theory to disentangle the dissociation mechanisms.

Femtosecond Time-Resolved Photofragment Rotational Angular Momentum Alignment in Electronic Predissociation Dynamics

This Letter presents an experimental and theoretical study of femtosecond time-resolved vector correlations in methyl iodide (CH₃I) electronic predissociation via the second absorption B-band at 201.2 nm. The time evolution of the phenomenological anisotropy parameters β_l was determined from time-resolved photofragment angular distributions obtained by means of the femtosecond laser pump-probe technique coupled with velocity map imaging detection of vibrational ground-state CH₃(ν = 0) fragments and spin-orbit excited I*(²P_1/2) atoms. Theoretical interpretation of the experimental results was performed on the basis of a fitting procedure using quasiclassical theory, which elucidates vector correlations in photodissociation of symmetric top molecules. The results of the fitting are in very good agreement with the experimental data and demonstrate the important role of molecular excited-state lifetimes, parent molecule and methyl fragment rotations, and methyl fragment angular momentum alignment on the time-dependent electronic predissociation dynamics.

Stereodynamics of the photodissociation of nitromethane at 193 nm: unravelling the dissociation mechanism

The photodissociation of nitromethane at 193 nm is reviewed in terms of new stereodynamical information provided by the measurement of the first four Dixon's bipolar moments, β0(2)(20), β0(0)(22), β0(2)(02), and β0(2)(22), using slice imaging. The measured speed-dependent β0(2)(20) (directly related with the spatial anisotropy parameter β) indicates that after one-photon absorption to the S3(2 (1)A″) state by an allowed perpendicular transition, two reaction pathways can compete with similar probability, a direct dissociation process yielding ground-state CH3 and NO2(1 (2)A2) radicals and a indirect dissociation through conical intersections in which NO2 radicals are formed in lower-lying electronic states. A particularly important result from our measurements is that the low recoil energy part of the methyl fragment translational energy distribution presents a contribution with parallel character, irrespective of the experimental conditions employed, that we attribute to parent cluster dissociation. Moreover, the positive values found for the β0(0)(22) bipolar moment indicates some propensity for the fragment's recoil velocity and angular momentum vectors to be parallel.

Wave packet calculations on nonadiabatic effects for the O(3P)+HF(1Σ+) reaction under hyperthermal conditions

We present wave packet calculations of total and state-to-state reaction probabilities and integral cross sections for the nonadiabatic dynamics of the O((3)P)+HF → F((2)P)+OH((2)Π) reaction at hyperthermal collision energies ranging from 1.2 to 2.4 eV. The validity of the centrifugal sudden approximation is discussed for the title reaction and a comprehensive investigation of the influence of nonadiabatic effects on the dynamics of this reactive system at high (hyperthermal) collision energies is presented. In general, nonadiabatic effects are negligible for averaged observables, such as total reaction probabilities and integral cross sections, but they are clearly observed in detailed observables such as rotationally state-resolved reaction probabilities. A critical discussion of nonadiabatic effects on the dynamics of the title reaction is carried out by comparing with the reverse reaction and the characteristics of the adiabatic and diabatic potential energy surfaces involved.

Photodissociation of pyrrole-ammonia clusters below 218 nm: quenching of statistical decomposition pathways under clustering conditions

The excited state hydrogen transfer (ESHT) reaction in pyrrole-ammonia clusters (PyH·(NH(3))(n), n = 2-5) at excitation wavelengths below 218 nm down to 199 nm, has been studied using a combination of velocity map imaging and non-resonant detection of the NH(4)(NH(3))(n-1) products. Special care has been taken to avoid evaporation of solvent molecules from the excited clusters by controlling the intensity of both the excitation and probing lasers. The high resolution translational energy distributions obtained are analyzed on the base of an impulsive mechanism for the hydrogen transfer, which mimics the direct N-H bond dissociation of the bare pyrrole. In spite of the low dissociation wavelengths attained (~200 nm) no evidence of hydrogen-loss statistical dynamics has been observed. The effects of clustering of pyrrole with ammonia molecules on the possible statistical decomposition channels of the bare pyrrole are discussed.

Imaging the molecular channel in acetaldehyde photodissociation: roaming and transition state mechanisms

The roaming dynamics in the photodissociation of acetaldehyde is studied through the first absorption band, in the wavelength interval ranging from 230 nm to 325 nm. Using a combination of the velocity-map imaging technique and rotational resonance enhanced multiphoton ionization (REMPI) spectroscopy of the CO fragment, the branching ratio between the canonical transition state and roaming dissociation mechanisms is obtained at each of the photolysis wavelengths studied. Upon one photon absorption, the molecule is excited to the first singlet excited S(1) state, which, depending on the excitation wavelength, either converts back to highly vibrationally excited ground S(0) state or undergoes intersystem crossing to the first excited triplet T(1) state, from where the molecule can dissociate over two main channels: the radical (CH(3) + HCO) and the molecular (CO + CH(4)) channels. Three dynamical regions are characterized: in the red edge of the absorption band, at excitation energies below the T(1) barrier, the ratio of the roaming dissociation channel increases, largely surpassing the transition state contribution. As the excitation wavelength is increased, the roaming propensity decreases reaching a minimum at wavelengths ∼308 nm. Towards the blue edge, at 230 nm, an upper limit of ∼50% has been estimated for the contribution of the roaming channel. The experimental results are interpreted in terms of the interaction between the different potential energy surfaces involved by means of ab initio stationary points and intrinsic reaction coordinate paths calculations.

A 4D wave packet study of the CH3I photodissociation in the A-band. Comparison with femtosecond velocity map imaging experiments

The time-resolved photodissociation dynamics of CH(3)I in the A-band has been studied theoretically using a wave packet model including four degrees of freedom, namely the C-I dissociation coordinate, the I-CH(3) bending mode, the CH(3) umbrella mode, and the C-H symmetric stretch mode. Clocking times and final product state distributions of the different dissociation (nonadiabatic) channels yielding spin-orbit ground and excited states of the I fragment and vibrationless and vibrationally excited (symmetric stretch ν(1) and umbrella ν(2) modes) CH(3) fragments have been obtained and compared with the results of femtosecond velocity map imaging experiments. The wave packet calculations are able to reproduce with very good agreement the experimental reaction times for the CH(3)(ν(1), ν(2))+I*((2)P(1/2)) dissociation channels with ν(1) = 0 and ν(2) = 0,1,2, and also for the channel CH(3)(ν(1) = 0, ν(2) = 0)+I((2)P(3/2)). However, the model fails to predict the experimental clocking times for the CH(3)(ν(1), ν(2))+I((2)P(3/2)) channels with (ν(1), ν(2)) = (0, 1), (0, 2), and (1, 0), that is, when the CH(3) fragment produced along with spin-orbit ground state I atoms is vibrationally excited. These results are similar to those previously obtained with a three-dimensional wave packet model, whose validity is discussed in the light of the results of the four-dimensional treatment. Possible explanations for the disagreements found between theory and experiment are also discussed.

Slice imaging and wave packet study of the photodissociation of CH3I in the blue edge of the A-band: evidence of reverse 3Q0 ← 1Q1 non-adiabatic dynamics

The photodissociation of CH(3)I in the blue edge (217-230 nm) of the A-band has been studied using a combination of slice imaging and resonance enhanced multiphoton ionization (REMPI) detection of the CH(3) fragment in the vibrational ground state (ν = 0). The profiles of the CH(3) (ν = 0) kinetic energy distributions and the photofragment anisotropies are interpreted in terms of the contribution of the excited surfaces involved in the photodissociation process, as well as the probability of non-adiabatic curve crossing between the (3)Q(0) and (1)Q(1) states. In the studied region, unlike in the central part of the A-band where absorption to the (3)Q(0) state dominates, the I((2)P(J)), with J = 1/2, 3/2, in correlation with CH(3) (ν = 0) kinetic energy distributions show clearly two contributions of different anisotropy, signature of the competing adiabatic and non-adiabatic dynamics, whose ratio strongly depends on the photolysis wavelength. The experimental results are compared with multisurface wave packet calculations carried out using the available ab initio potential energy surfaces, transition moments, and non-adiabatic couplings, employing a reduced dimensionality model. A good qualitative agreement is found between experiment and theory and both show evidence of reverse (3)Q(0)←(1)Q(1) non-adiabatic dynamics at the bluest excitation wavelengths both in the fragment kinetic energy and angular distributions.

Femtosecond spectral pulse shaping with holographic gratings recorded in photopolymerizable glasses

The majority of the applications of ultrashort laser pulses require a control of its spectral bandwidth. In this paper we show the capability of volume phase holographic gratings recorded in photopolymerizable glasses for spectral pulse reshaping of ultrashort laser pulses originated in an Amplified Ti: Sapphire laser system and its second harmonic. Gratings with high laser induce damage threshold (LIDT) allowing wide spectral bandwidth operability satisfy these demands. We have performed LIDT testing in the photopolymerizable glass showing that the sample remains unaltered after more than 10 million pulses with 0,75 TW/cm2 at 1 KHz repetition rate. Furthermore, it has been developed a theoretical model, as an extension of the Kogelnik's theory, providing key gratings design for bandwidth operability. The main features of the diffracted beams are in agreement with the model, showing that non-linear effects are negligible in this material up to the fluence threshold for laser induced damage. The high versatility of the grating design along with the excellent LIDT indicates that this material is a promising candidate for ultrashort laser pulses manipulations.

Generation of femtosecond paraxial beams with arbitrary spatial distribution

We present an approach to generate paraxial laser beams with arbitrary spatial distribution in the femtosecond time regime. The proposed technique is based upon a pair of volume phase holographic gratings working in parallel arrangement. It exploits the spatial coherence properties of the incoming laser beam in a compact and robust setup that mitigates angular and spatial chirp. The gratings were recorded in a photopolymerizable glass with a high optical damage threshold and a large optical throughput. Setup performance is studied and experimentally demonstrated by generating Laguerre-Gaussian femtosecond pulses.

Dynamics of insertion reactions of H2 molecules with excited atoms

Recent progress in the study of insertion reactions of hydrogen molecules with excited atoms is reviewed in this article. In particular, the dynamics of the reaction of O(1D), N(2D), C(1D), and S(1D) with H2 and its isotopomers, which have received a great deal of attention over the past decade, are examined in detail. All of these systems have in common the existence of several potential energy surfaces (PES) correlating with the reagents' states, and consequently, they can give rise to reaction following different adiabatic and nonadiabatic pathways. The main contribution, however, arises from their ground singlet PESs which feature the existence of deep wells with small or null barriers for insertion. Accordingly, these reactions proceed mainly via formation of relatively long-lived collision complexes and display an overall nearly statistical behavior. In spite of their similarities, the various reactions have peculiar characteristics caused by important differences of their respective PESs. The contribution of excited PES to the global reactivity, which has also become an important issue and a challenge both for theory and experiment, is also examined. The different theoretical approaches are discussed in the text, along with the experimental results obtained by a variety of techniques. The recent exact quantum treatments of these reactive systems together with the development of a rigorous statistical model have contributed to a very accurate description which in many cases matches very well the detailed measurements. The quasi-classical trajectory (QCT) method has also provided a fairly accurate description of the reaction dynamics for these systems. In particular, the analysis in terms of collision times has yielded interesting clues about the reaction mechanisms.

Imaging the photodissociation of CH3SH in the first and second absorption bands: The CH3(X̃A12)+SH(XΠ2) channel

The CH3(X2A1)+SH(X2Pi) channel of the photodissociation of CH3SH has been investigated at several wavelengths in the first 1 1A"<--X 1A' and second 2 1A"<--X1A' absorption bands by means of velocity map imaging of the CH3 fragment. A fast highly anisotropic (beta=-1+/-0.1) CH3(X2A1) signal has been observed in the images at all the photolysis wavelengths studied, which is consistent with a direct dissociation process from an electronically excited state by cleavage of the C-S bond in the parent molecule. From the analysis of the CH3 images, vibrational populations of the SH(X2Pi) counterfragment have been extracted. In the second absorption band, the SH fragment is formed with an inverted vibrational distribution as a consequence of the forces acting in the crossing from the bound 2 1A" second excited state to the unbound 1 1A" first excited state. The internal energy of the SH radical increases as the photolysis wavelength decreases. In the case of photodissociation via the first excited state, the direct production of CH3 leaves the SH counterfragment with little internal excitation. Moreover, at the longer photolysis wavelengths corresponding to excitation to the 1 1A" state, a slower anisotropic CH3 channel has been observed (beta=-0.8+/-0.1) consistent with a two step photodissociation process, where the first step corresponds to the production of CH3S(X2E) radicals via cleavage of the S-H bond in CH3SH, followed by photodissociation of the nascent CH3S radicals yielding CH3(X2A1)+S(X3P0,1,2).

Femtosecond multichannel photodissociation dynamics of CH3I from the A band by velocity map imaging

The reaction times of several well-defined channels of the C-I bond rupture of methyl iodide from the A band, which involves nonadiabatic dynamics yielding ground state I(2P3/2) and spin-orbit excited I*(2P1/2) and ground and vibrationally excited CH3 fragments, have been measured by a combination of a femtosecond laser pump-probe scheme and velocity map imaging techniques using resonant detection of ground state CH3 fragments. The reaction times found for the different channels studied are directly related with the nonadiabatic nature of this multidimensional photodissociation reaction.

Quasiclassical trajectory study of the Cl+CH4 reaction dynamics on a quadratic configuration interaction with single and double excitation interpolated potential energy surface

An ab initio interpolated potential energy surface (PES) for the Cl+CH(4) reactive system has been constructed using the interpolation method of Collins and co-workers [J. Chem. Phys. 102, 5647 (1995); 108, 8302 (1998); 111, 816 (1999); Theor. Chem. Acc. 108, 313 (2002)]. The ab initio calculations have been performed using quadratic configuration interaction with single and double excitation theory to build the PES. A simple scaling all correlation technique has been used to obtain a PES which yields a barrier height and reaction energy in good agreement with high level ab initio calculations and experimental measurements. Using these interpolated PESs, a detailed quasiclassical trajectory study of integral and differential cross sections, product rovibrational populations, and internal energy distributions has been carried out for the Cl+CH(4) and Cl+CD(4) reactions, and the theoretical results have been compared with the available experimental data. It has been shown that the calculated total reaction cross sections versus collision energy for the Cl+CH(4) and Cl+CD(4) reactions is very sensitive to the barrier height. Besides, due to the zero-point energy (ZPE) leakage of the CH(4) molecule to the reaction coordinate in the quasiclassical trajectory (QCT) calculations, the reaction threshold falls below the barrier height of the PES. The ZPE leakage leads to CH(3) and HCl coproducts with internal energy below its corresponding ZPEs. We have shown that a Gaussian binning (GB) analysis of the trajectories yields excitation functions in somehow better agreement with the experimental determinations. The HCl(v'=0) and DCl(v'=0) rotational distributions are as well very sensitive to the ZPE problem. The GB correction narrows and shifts the rotational distributions to lower values of the rotational quantum numbers. However, the present QCT rotational distributions are still hotter than the experimental distributions. In both reactions the angular distributions shift from backward peaked to sideways peaked as collision energy increases, as seen in the experiments and other theoretical calculations.

Quantum mechanical and quasiclassical trajectory scattering calculations for the C(D1)+H2 reaction on the second excited 1A″1 potential energy surface

Time-independent quantum mechanical (QM) and quasiclassical trajectory (QCT) scattering calculations have been carried out for the C(1D) + H2 --> CH + H reaction at a collision energy of 80 meV on a newly developed ab initio potential energy surface [B. Bussery-Honvault et al., Phys. Chem. Chem. Phys. 7, 1476 (2005)] of 1 1A" symmetry, corresponding to the second singlet state 1 1B1 of CH2. A general good agreement has been found between the QM and QCT rotational distributions and differential cross sections (DCSs). In both cases, DCSs are strongly peaked in the forward direction with a small contribution in the backward direction in contrast with those obtained on the 1 1A' surface, which are nearly symmetric. Rotational distributions obtained on the 1 1A" surface are somewhat colder than those calculated on the 1 1A' surface. The specific dynamics and the contribution of the 1 1A" surface to the overall reactivity of this system are discussed.

Influence of rotation and isotope effects on the dynamics of the N(D2)+H2 reactive system and of its deuterated variants

Integral cross sections and thermal rate constants have been calculated for the N((2)D)+H(2) reaction and its isotopic variants N((2)D)+D(2) and the two-channel N((2)D)+HD by means of quasiclassical trajectory and statistical quantum-mechanical model methods on the latest ab initio potential-energy surface [T.-S. Ho et al., J. Chem. Phys. 119, 3063 (2003)]. The effect of rotational excitation of the diatom on the dynamics of these reactions has been investigated and interesting discrepancies between the classical and statistical model calculations have been found. Whereas a net effect of reagent rotation on reactivity is always observed in the classical calculations, only a very slight effect is observed in the case of the asymmetric N((2)D)+HD reaction for the statistical quantum-mechanical method. The thermal rate constants calculated on this Potential-Energy Surface using quasiclassical trajectory and statistical model methods are in good agreement with the experimental determinations, although the latter are somewhat larger. A reevaluation of the collinear barrier of the potential surface used in the present study seems timely. Further theoretical and experimental studies are needed for a full understanding of the dynamics of the title reaction.

Low-Temperature Rotational Relaxation of CO in Self-Collisions and in Collisions with Ne and He

The low-temperature rotational relaxation of CO in self-collisions and in collisions with the rare-gas atoms Ne and He has been investigated in supersonic expansions with a combination of resonance-enhanced multiphoton ionization (REMPI) spectroscopy and time-of-flight techniques. For the REMPI detection of CO, a novel 2 + 1' scheme has been employed through the A(1)Pi state of CO. From the measured data, average cross sections for rotational relaxation have been derived as a function of temperature in the range 5-100 K. For CO-Ne and CO-He, the relaxation cross sections grow, respectively, from values of approximately 20 and 7 A(2) at 100 K to values of approximately 65-70 and approximately 20 A(2) in the 5-20 K temperature range. The cross section for the relaxation of CO-CO grows from a value close to 40 A(2) at 100 K to a maximum of 60 A(2) at 20 K and then decreases again to 40 A(2) at 5 K. These results are qualitatively similar to those obtained previously with the same technique for N(2)-N(2), N(2)-Ne, and N(2)-He collisions, although in the low-temperature range (T < 20 K) the CO relaxation cross sections are significantly larger than those for N(2). Some discrepancies have been found between the present relaxation cross sections for CO-CO and CO-He and the values derived from electron-induced fluorescence experiments.

Quasiclassical Trajectory Study of the F + CH4 Reaction Dynamics on a Dual-Level Interpolated Potential Energy Surface

An ab initio interpolated potential energy surface (PES) for the F + CH4 reactive system has been constructed using the interpolation method of Collins and co-workers. The ab initio calculations have been performed using second-order Möller-Plesset (MP2) perturbation theory to build the initial PES. Scaling all correlation (SAC) methodology has been employed to improve the ab initio calculations and to construct a dual-level PES. Using this PES, a detailed quasiclassical trajectory study of integral and differential cross sections, product rovibrational populations and internal energy distributions has been carried out for the F + CH4 and F + CD4 reactions and the theoretical results have been compared with the available experimental data.

Rovibrational product state distribution for inelastic H+D2 collisions

Experimental measurements of rovibrational product state distributions for the inelastic scattering process H + D2(nu=0,j)-->H + D2(nu' = 1,2,j') are presented and compared with the results of quasiclassical and quantum mechanical calculations. Agreement between theory and experiment is almost quantitative. Two subtle trends are found: the relative amount of energy in product rotational excitation decreases slightly with increasing collision energy and increases slightly with increasing product vibrational excitation. These trends are the reverse of what has been found for reactive scattering in which the opposite trends are much more pronounced.