Product State Distributions Research Articles

An empirical potential energy surface for He–Cl2(B 3Πu) based on a multiproperty fit

An empirical interaction surface for the He–Cl2(B 3Πu) complex based on additive pairwise potentials is reported. A novelty of the present surface with respect to previous empirical potentials is that a dependence on the Cl–Cl separation is introduced in some of the potential parameters, which makes more flexible the analytic form used. The parameters of the surface are adjusted in order to reproduce the available data for several properties such like spectral blueshifts, predissociation lifetimes, and Cl2 product state distributions. The fitted surface yields very good agreement with experiment for most of the properties measured. As compared with previous surfaces, the present potential is found to improve significantly on the description of the vibrational dependence of the lifetime, in all the range of vibrational excitations probed by the experiment. In previous works this property has revealed difficult to describe accurately for He–Cl2(B). For the remaining properties, the current surface provides a level of accuracy as good (or better in some cases) as the best one achieved by earlier interaction potentials. The features of the proposed potential surface and its range of validity are discussed. It is found that validity of the potential is essentially limited to the range of vibrational levels probed experimentally.

Infrared action spectroscopy and time-resolved dynamics of the OD–CO reactant complex

The infrared action spectrum of the linear OD–CO reactant complex has been recorded in the OD overtone region near 1.9 μm using an infrared pump-ultraviolet probe technique. The pure overtone band of OD–CO (2νOD) is observed at 5148.6 cm−1 and combination bands involving the simultaneous excitation of OD stretch and D-atom bend are identified 160.0 and 191.2 cm−1 to higher energy. Band assignments and spectroscopic constants are derived from the rotationally resolved structure of the spectra. The change in the ground state rotational constant upon deuteration demonstrates that the H/D-atom of the hydroxyl radical points toward CO in the OH/D-CO complex. Direct time-domain measurements yield a lifetime of 37(4) ns for OD–CO (2νOD) prior to decay via inelastic scattering or chemical reaction. This is significantly longer than the laser-limited lifetime of ⩽5 ns observed for OH–CO (2νOH), and is attributed in part to the closing of a near-resonant vibration to vibration energy transfer channel upon deuteration. Vibrational predissociation of OD–CO (2νOD) proceeds by a vibration to rotation and/or translation mechanism that yields highly rotationally excited OD (v=1) fragments. Intermolecular D-atom bend excitation, which drives the structural transformation from the reactant complex to the transition state for reaction, results in a dramatic shortening of the lifetime to ⩽6 ns (laser-limited). Excitation of the D-atom bend also supplies sufficient energy to reopen the near-resonant vibrational energy transfer channel, resulting in minimal rotational excitation of the OD (v=1) fragments. Finally, a ground state binding energy for OD–CO of D0⩽456 cm−1 is established from the OD (v=1) product state distribution.

Spectrum and vibrational predissociation of the HF dimer. II. Photodissociation cross sections and product state distributions

We study vibrational predissociation of the HF dimer both by a full coupled channels treatment as well as in the Fermi golden rule approximation. Photodissociation cross sections, linewidths, and rotational state distributions are computed for excitations from the ground state with rotational quantum numbers J=1, K=0 to monomer stretch excited states with J=K=0, both for even and odd permutation symmetry. The resonances investigated include excitation of the hydrogen bond donor and acceptor stretches, as well as combinations of one of these modes with the dimer stretch and dimer geared-bending modes. We find that dissociation is sufficiently slow for the Fermi golden rule approximation to be valid. The resonance positions and line strengths are compared with quasibound state calculations. The agreement with experimental data is fairly good for the photofragment angular distributions that were determined from the rotational state distributions, less good for some of the linewidths. Since we carefully checked that the results are converged with respect to the number of vibrational and rotational channels included, the remaining discrepancies are almost certainly due to small deficiencies in the SO-3 potential used in the calculations.

Nuclear Hyperfine Populations for D Atoms Generated by the 266 nm Photolysis of DI

Results are presented on the photolysis of DI at 266 nm. An experimental approach is used that combines narrow-band, velocity-aligned Doppler spectroscopy (VADS) and a constrained detection geometry. Atomic deuterium is detected, and transitions out of the individual nuclear hyperfine states of the ground electronic state (the F states; 0.010 92 cm-1 splitting) are resolved via the Lyman-α transition (∼82 280 cm-1). Consistent with previous work on the hydrogen halides, the D-atom product state distributions are statistical. However, the methodological improvements required to resolve the states bode well for studying other chemical systems with this approach.

Multiple-Configuration Quantum/Classical Studies of the Photodissociation Dynamics of H2O

The photodissociation dynamics of water is investigated, using a quantum/classical treatment. Here, the H + OH dissociation coordinate is treated classically, whereas the dynamics of the remaining OH molecule is propagated quantum-mechanically. It is found that this approach does not provide an accurate description of the dynamics of this system, because of the strong coupling between the quantum and classical coordinates near the transition state. To remedy this situation, a multiple-configuration quantum/classical approach is investigated in which the quantum wave packet is divided into several contributions, each of which is coupled to a separate classical trajectory. This approach is found to provide an accurate representation of the total and partial cross sections for the photodissociation of water on the à state, as well as the OH product state distributions.

Modeling of the non-adiabatic E0 +g→D0 +u transitions induced by Ar in molecular iodine: a first attempt

The non-adiabatic transitions between the first-tier E and D ion-pair states of iodine molecule induced by collisions with Ar atom are investigated using first-order semiclassical perturbation theory. PES’s and diabatic couplings between electronic states are obtained in analytical form by the first-order perturbation theory-diatomics-in-molecule method. Electronic and vibrationally inelastic rate constants and product state distributions are calculated and compared with available experimental data.

Eley–Rideal reactions of H atoms with Cl adsorbed on Au(111): Quantum and quasiclassical studies

The Eley–Rideal reactions of H atoms with Cl adsorbed on Au(111) surfaces are examined. Electronic structure calculations based on density functional theory are used to construct a model potential energy surface. Both quantum and quasiclassical methods are used to compute reaction cross sections and product state distributions. Steering of the incident H atom towards the adsorbed Cl leads to relatively large reaction cross sections of 2–3 Å2. The product HCl in this strongly exothermic reaction has over an eV of energy in vibrational motion, and a bit less than one eV each in rotation and translation.

Role of isomerization channel in unimolecular dissociation reaction H2CO→H2+CO: Ab initio global potential energy surface and classical trajectory analysis

We constructed a full dimensional potential energy function of H2CO that can describe both the dissociation and isomerization channels by the modified Shepard interpolation method. Ab initio calculations at the MP2/cc-pVTZ level were carried out to obtain the local potential functions at about 4700 points. The interpolant points were sampled by classical trajectory calculations and by the grid searches in the internal coordinate space. Classical trajectory calculations were performed to examine the intramolecular dynamics associated with the dissociation as well as the product state distributions. The time scale of intramolecular vibrational energy randomization was much faster than that of the dissociation reaction. The dissociation rate was obtained from the classical trajectory results and the effect of the isomerization channel on the dissociation was estimated. The calculated rate constants were compared with those by Rice–Ramsperger–Kassel–Marcus theory.

Stereodynamics of O(1D) and O(3P) Reactions Studied via Doppler-Resolved Polarization Spectroscopy

Abstract Studies on stereodynamics of the gas-phase elementary reactions of atomic oxygen, O(1D) and O(3P), with simple molecules are reviewed. The angular correlations among the relative velocity vectors of reactants (k) and products (k′), and the product rotational angular momentum vector (j′) have been investigated via Doppler-resolved polarization spectroscopy. In the O(1D) reactions with hydrocarbons and water, OH products are scattered into wide angular range and rotate mainly in the k–k′ scattering plane. These angular distributions suggest the formation of an H–O–X (X = C or O) bond in reactive intermediates. By contrast, the NO products of the O(1D) reaction with N2O have isotropic angular distribution of j′, which indicates the participation of the out-of-plane motion in addition to the in-plane motions within the complex. These trends of O(1D) reactions are consistent with the feature of product-state distribution of the individual reactions. Similar analyses for the reactions of O(3P) atom with hydrocarbons have revealed that these reactions are not always dominated by the simple rebound abstraction mechanism based on the triatomic picture, which has long been assumed. As a background of the titled study, this account reviews the progress in the studies on gas-phase chemical reaction dynamics and presents the theory and techniques to investigate the stereodynamics of bimolecular reactions.

Calculation of Resonances and Product State Distributions for the Unimolecular Dissociation of H2S

AbstractFor Abstract see ChemInform Abstract in Full Text.

Effect of a single quantum rotational excitation on state-to-state dynamics of the O(1D)+H2-->OH+H reaction.

Crossed molecular beams scattering experiments on the O(1D)+H2 reaction have been carried out in order to study the effect of the reagent (H2) rotational excitation on the detailed dynamics of this benchmark insertion reation. Experimental results indicate that a single quantum rotational excitation of H2 has a significant impact on the product state distributions at the forward and backward scattering directions, while very little effect has been found in the sideway scattering direction. No clear patterns of this effect are found in the OH-product state distributions, indicating that the single quantum excitation on the dynamics is rather complicated.

Theoretical studies of product polarization and state distributions of the H + HCl reaction

The angular momentum polarization and rotational state distributions of the H 2 and HCl products from the H + HCl reaction are calculated at a relative translational energy of 1.6 eV by using quasiclassical trajectories on two potential energy surfaces, one from G3 surface [T.C. Allison et al., J. Phys. Chem. 100 (1996) 13575], and the other from BW2 surface [W. Bian, H.-J. Werner, J. Chem. Phys. 112 (2000) 220]. Product rotational distributions obtained on the G3 potential energy surface (PES) are much closer to the experimental results (P.M. Aker et al., J. Chem. Phys. 90 (1989) 4795; J. Chem. Phys. 90 (1989) 4809) than the distributions calculated on the BW2 PES. The distributions of P( φ r) for the H 2 and HCl products obtained on the G3 PES are similar, whereas the rotational alignment effect of the H 2 product is stronger than that of the HCl product. In contrast to the polarization distributions obtained on the G3 PES, the rotational alignment effect of the two products calculated on the BW2 PES is similar. However, the abstraction reaction is dominated by out-of-plane mechanisms, while the exchange reaction is dominated by in-plane mechanisms. The significant difference of the product rotational polarization obtained on the G3 and BW2 PESs implies that the studies of the dynamical stereochemistry can provide a sensitive test for the accuracy of the PES.

Chebyshev real wave packet propagation: H+O2 (J=0) state-to-state reactive scattering calculations

In this paper we explore the relative performance of two recently developed wave packet methodologies for reactive scattering, namely the real wave packet Chebyshev domain propagation of Gray and Balint-Kurti [J. Chem. Phys. 108, 950 (1998)] and the Lanczos subspace wave packet approach of Smith et al. [J. Chem. Phys. 116, 2354 (2002); Chem. Phys. Lett. 336, 149 (2001)]. In the former method, a modified Schrödinger equation is employed to propagate the real part of the wave packet via the well-known Chebyshev iteration. While the time-dependent wave packet from the modified Schrödinger equation is different from that obtained using the standard Schrödinger equation, time-to-energy Fourier transformation yields wave functions which differ only trivially by normalization. In the Lanczos subspace approach the linear system of equations defining the action of the Green operator may be solved via either time-dependent or time-independent methods, both of which are extremely efficient due to the simple tridiagonal structure of the Hamiltonian in the Lanczos representation. The two different wave packet methods are applied to three dimensional reactive scattering of H+O2 (total J=0). State-to-state reaction probabilities, product state distributions, as well as initial-state-resolved cumulative reaction probabilities are examined.

Effects of interchannel coupling in associative detachment: electron spectra for H+Cl- and H+Br- collisions.

We present experimental and theoretical energy spectra of the electrons detached in collisions of slow Cl- and Br- ions with atomic hydrogen. Nonlocal resonance theory predicts two kinds of features in the spectra: steplike structures associated with rovibrational onsets and steep rises associated with interchannel coupling, the latter being absent in a calculation using the simpler local-complex potential theory. Our experimental spectra confirm the presence of both types of structures and thus the necessity of including interchannel coupling to properly describe the product-state distribution.

Calculation of Resonances and Product State Distributions for the Unimolecular Dissociation of H2S

Resonance phenomena associated with the unimolecular dissociation of H2S → SH + H have been investigated quantum mechanically by the Lanczos homogeneous filter diagonalization method using a newly developed potential energy surface (J. Chem. Phys. 2001, 114, 320). Resonance energies, widths (rates), and product state distributions have been obtained. Both dissociation rates and product state distributions of SH show strong fluctuations, indicating that the dissociation of H2S is essentially irregular. Statistical analysis of neighboring level spacing and width distributions also confirms this behavior. The dissociation rates and product state distributions are compared to the predictions of quantum phase space theory.

Lanczos Subspace Time-Independent Wave Packet Calculations of S (1D) + H2 Reactive Scattering

In this paper. we present the results of quantum dynamical simulations of the S (D-1) + H-2 insertion reaction on a newly developed potential energy surface (J. Chem. Phys. 2001, 114, 320). State-to-state reaction probabilities. product state distributions, and initial-state resolved cumulative reaction probabilities from a given incoming reactant channel are obtained from a time-independent wave packet analysis, performed within a single Lanczos subspace. Integral reaction cross sections are then estimated by J-shifting method and compared with the results from molecular beam experiment and QCT calculations.

Double-resonance overtone photofragment spectroscopy of trans-HONO. II. State- and time-resolved dissociation and OH-product state distributions

Linewidths, unimolecular dissociation rates and product state distributions (PSDs) have been measured for single rovibratational states of the ν1=5–7 levels of gas-phase trans-nitrous acid (HONO) by double-resonance overtone photofragment spectroscopy (DROPS). The linewidth measurements, together with the unimolecular dissociation rates in 5ν1, suggest that the intramolecular dynamics are not statistical but rather depend sensitively upon specific intramolecular couplings and the vibrational character of the initial state. Comparison with calculated rate constants from statistical unimolecular rate theory reveals that intramolecular vibrational energy redistribution (IVR) is the rate determining step in the dissociation of HONO subsequent to vibrational overtone excitation. Despite this, we find the measured product state distributions to be close to the predictions of statistical theory. We explain these observations in terms of a simple tier model incorporating hierarchical IVR. The experimental findings underscore the importance of the preparation technique, and hence the nature of the initially excited state, in determining the subsequent intramolecular dynamics.

Near Threshold Photochemistry of Propanal. Barrier Height, Transition State Structure, and Product State Distributions for the HCO Channel

The photodissociation dynamics of propanal have been investigated at photolysis wavelengths between 300 and 327 nm. The threshold for production of HCO fragments was found to be 326.26 nm, which corresponds to 30645 cm-1 (366.6 kJ mol-1) above the zero-point of the S0 state. From known thermochemical data, this threshold lies 25.0 ± 3.6 kJ mol-1 above the bond dissociation energy. The nascent HCO rotational and translational energy distributions were determined following dissociation at threshold. The rotational population was measured as a function of N, Ka, Kc, and S. The distribution of rotational states followed a Gaussian function with an average rotational energy of 2.5 ± 0.5 kJ mol-1. The population of the near-degenerate spin-rotation states was equal, while the population in the asymmetry doublets favored the upper energy component by about 3:1. Careful measurement of the Doppler profiles of individual Ka = 0 lines in the LIF spectrum revealed that the translational energy also shows a Gaussian-l...

The dynamics of the H + H2O reaction.

This article reviews the history and recent progress in the study of the dynamics of the H + H2O reaction, which has become a benchmark for experimental research in the field of gas-phase reaction dynamics. The dynamics of H + H2O is discussed in terms of the different observable properties: integral cross-sections, rate coefficients, product state distributions, differential cross-sections, and vector correlations. It is shown how experimental measurements and first-principle theoretical calculations have revealed the interesting microscopic aspects of this elementary chemical reaction.

On product state distributions in triatomic unimolecular reactions

We develop an analytical theory of state distributions in the products of three-atom unimolecular reactions involving a barrier and a dissociation path defined by bent geometries. The approach combines transition state theory (TST), kinematic rotations (KR) and the linear transformation model (LTM), published recently. Its predictions are compared with quasiclassical trajectory calculations on a model potential energy surface.