Brigitte Pouilly

Spin–orbit branching in the photofragmentation of HCl

The dynamics of the photofragmentation of HCl and DCl, subsequent to A 1Π←X 1Σ+ electronic excitation, is treated exactly based on new multireference, configuration‐interaction ab initio calculations of the relevant electronic potential energy curves and off‐diagonal matrix elements. The calculated total cross section agrees well with both earlier calculations and experiment. By contrast, the relative cross sections for formation of the two accessible fine‐structure channels [Cl(2P1/2) and Cl(2P3/2)] disagree with the most recent experimental results, and, more dramatically, with the results of prior theoretical predictions. Analysis of the redistribution of the photofragment flux, as a function of the H–Cl separation, reveals that the product branching is determined at relatively large HCl distances, considerably beyond the Franck–Condon region, and is governed by the spin–orbit coupling between the initially excited A 1Π state and the Ω=1 components of the a 3Π and 1 3Σ+ states.

Spin–orbit branching in the photodissociation of HBr: Time-independent, time-dependent, and semiclassical calculations

The dynamics of the photofragmentation of HBr is treated within time-independent, time-dependent, and semiclassical methods. The calculated relative cross sections for formation of the two accessible fine-structure channels [Br(2P1/2) and Br(2P3/2)] agree well with the experimental results, both in magnitude and in dependence on photon excitation wavelength. For relatively small photon wavelength (λ=193 nm), vertical excitation in the Franck–Condon region populates preferentially the A 1Π state, and only three states (A 1Π, the Ω=1 components of the a 3Π and 1 3Σ+), coupled by the spin–orbit interaction, are invoved in the dissociation process. For larger photon wavelength (λ=243 nm), the product branching is governed by initial excitation in both the A 1Π state and the a 3Π(Ω=0) component. Comparison of the redistribution of the time-independent photofragment fluxes as a function of the H–Br separation with the temporal evolution of the populations within a time-dependent framework shows that the two met...

A new look at the retention of orbital alignment in collisions involving atoms in 1P electronic states: Ca(4sSp 1P) + He

Abstract The retention of orbital alignment in collisions of Ca(4s5p 1 P) atoms with He targets is examined anew, by inward propagation of the full wavefunction of the system, corresponding to a specifically prepared orientation of the p orbital, followed by projection of the incoming flux onto the Hunds case (a) Σ and Π states appropriate to the description of the molecular region. A previous error in the description of the initial state by Alexander and Pouilly has been corrected here. The results of the quantum flux redistribution calculations are virtually identical to the predictions of more conventional semiclassical impact parameter calculations, and, furthermore, are consistent with the concept of orbital locking, in which the initial space frame orientation of the p orbital is preserved until the collision partners attain the “locking radius”. However, locking is found to occur at values of the internuclear separation far smaller than would be predicted by somesimple models. Analysis of the flux redistribution and of the partial cross sections for the inelastic process Ca (4s5p 1 P) → Ca(4s5p 3 P) collisions are completely consistent with a model in which the transition probability is proportional solely to the flux in the 1 Π state at the point of crossing with the 3 Σ curve.

New investigation of the photodissociation of the HBr molecule: total cross-section, anisotropy parameter and dependence of the spin–orbit branching on the ground state vibrational level

Abstract We report new calculations on the photodissociation of the HBr molecule within a time-independent framework. This work extends our previous study to the dependence on excitation wavelength of both the total cross-section and the anisotropy parameter which characterizes the contribution of the parallel and the perpendicular transitions in the fragmentation process. In addition, we study the dependence of the spin–orbit branching ratio σ( 2 P 1/2 )/σ( 2 P 3/2 ) on the initial vibrational level ( v ″=1 and 2) of the ground X 1 Σ + state.

Analytical semiclassical calculation of photodissociation of the HCl molecule

This paper describes an analytical method to the solution of semiclassical first‐order, time‐dependent coupled equations in the case of a three states process. The method is applied to the study of the photodissociation of the HCl molecule. The results of the semiclassical instantaneous probabilities as function of the interparticle distance are compared with quantum–quantum flux redistribution calculations [M. H. Alexander, B. Pouilly, and T. Duhoo, J. Chem. Phys. 99, 1752 (1993)].

Theoretical study of redistribution of light in Ca–He collisions

The quantum theory of atomic collisions in the presence of a weak radiation field is used to describe the process of redistribution of light in Ca–He collisions. This work is devoted to the simulation of the recent experiment of Lin and co‐workers [J. Chem. Phys. 89, 4771 (1988)] in which, the absorption profile of the (4s2 1S→4s5p 1P) transition and the dependence on the detuning of the spin‐changing (4s5p 1P→4s5p 3P) transition were determined. Close‐coupled calculations are carried out, based on model interaction potentials for the ground and the excited states of the Ca–He system. The calculated absorption profile for the production of Ca atom in the (4s5p 1,3P) states is in qualitative agreement with the experimental results. The blue wing/red wing asymmetry in the relative transfer to the 3P state, is interpreted in terms of flux transfer among the molecular states, which occurs when the Ca atom is approached by the closed‐shell partner.

Electron-gas interaction potentials for collisions of CaCl(X 2Σ+) and KCl(X 1Σ+) with Ar

Abstract Interaction potentials for CaCl(X 2 Σ + )-Ar and KCl(X 1 Σ + )-Ar have been determined. They include a Gordon-Kim electron-gas repulsive part smoothly joined to the long-range van der Waals potential. The van der waals potential for KClAr was taken from Meyer and Toennies. For CaClAr, the necessary molecular parameter were estimated from the Rittner model, which predicts both the dipole and quadrupole moments fairly accurately. The CaClAr interaction potential is quite different from that of KClAr. Due to the outer 4s electron on the Ca + ion. the CaClAr potential exhibits a deep minimum in the odd-order Legendre terms which is expected to have a large effect on the cross sections for collisional rotational excitation. The KClAr potential determined here also shows significant differences in the repulsive and well regions from that predicted by Meyer and Toennies using a site-site model for the repulsive contribution.

Theoretical study of Ca(4s5p 1P)→Ca(4s5p 3P) energy transfer in collisions with He. Initial and final state alignment

In this paper we present the results of close‐coupling calculations of the cross sections for Ca(4s5p 1P)→Ca(4s5p 3P) energy transfer in collisions with He, based on new potential energy curves. Particular attention is devoted to the simulation of the recent experiment of Smith and co‐workers [J. Chem. Phys. 96, 8212 (1992)], in which, for the 1P→3P2 transfer both initial and final alignment are controlled with respect to the initial relative velocity vector of the two partners Vrel. The calculated polarization ratios (σ⊥/σ∥), defined as the ratio of the cross sections for the 1P→3P transfer summed over final levels for initial alignment of the 5p orbital perpendicular and parallel to Vrel are in good agreement with the experimental results. The theoretical cross sections for the 1P→3P2 transfer determined in the so‐called collision frame (σj1m1m1→j2m2m2), where the axis of quantization is taken along Vrel are in good agreement with the experiment in the case of initial perpendicular excitation, but show ...

Ground state analytical ab initio intermolecular potential for the Cl2-water system

The chlorine/water interface is of crucial importance in the context of atmospheric chemistry. Modeling the structure and dynamics at this interface requires an accurate description of the interaction potential energy surfaces. We propose here an analytical intermolecular potential that reproduces the interaction between the Cl2 molecule and a water molecule. Our functional form is fitted to a set of high level ab initio data using the coupled-cluster single double (triple)/aug-cc-p-VTZ level of electronic structure theory for the Cl2 - H2O complex. The potential fitted to reproduce the three minima structures of 1:1 complex is validated by the comparison of ab initio results of Cl2 interacting with an increasing number of water molecules. Finally, the model potential is used to study the physisorption of Cl2 on a perfectly ordered hexagonal ice slab. The calculated adsorption energy, in the range 0.27 eV, shows a good agreement with previous experimental results.