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Physical Review A

Test of quasiclassical theories of redistribution in atomic collisions

Ronald James Bieniek

Published May 1, 1987• 8 citations
Publisher

Abstract

The first numerical tests, for a real system, of the quasiclassical description of redistribution of polarized radiation through atomic collisions are presented. Polarization results are reported for red-wing redistribution of Sr${(}^{1}$${\mathrm{S}}_{0}$\ensuremath{\rightarrow}${\mathrm{}}^{1}$${\mathrm{P}}_{1}$) in the presence of Ar colliders. They compare favorably to recent quantal, nonadiabatic computations. It is shown that the effects of bent trajectories are important. Modified quasiclassical formulas to account for some of these are presented and tested. The results indicate that quasiclassical analysis can be used to probe the transformation from molecular to atomic states as atoms separate in a half-collision.

DOI
10.1103/physreva.35.3663

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Journal of Physics B Atomic and Molecular Physics•Mar 14, 1981•19 citations
The first numerical tests, for a real system, of the quasiclassical description of redistribution of polarized radiation through atomic collisions are presented. Polarization results are reported for red-wing redistribution of Sr{(}^{1}$${\mathrm{S}}_{0}\ensuremath{\rightarrow}{\mathrm{}}^{1}$${\mathrm{P}}_{1}) in the presence of Ar colliders. They compare favorably to recent quantal, nonadiabatic computations. It is shown that the effects of bent trajectories are important. Modified quasiclassical formulas to account for some of these are presented and tested. The results indicate that quasiclassical analysis can be used to probe the transformation from molecular to atomic states as atoms separate in a half-collision.
The close-coupled theory of collisions in a radiation field is used to calculate the absorption profile for the Sr $^{1}\ensuremath{\leftarrow}^{1}$S resonance line broadened by collisions with Ar. The calculations predict the polarization ratios of Sr 1P^{1}\mathrm{P}1P fluorescence following line wing excitation by either linear or circular polarized light. Ab initio calculations were used to obtain the ground and excited SrAr molecular-potential curves, which were adjusted to give improved agreement with experiment. The radiative-scattering theory gives a unified description of the absorption coefficient and polarization redistribution from the small detuning impact limit region to the far spectral wings. The cross sections for elastic and inelastic depolarizing collisions of Sr 1P1^{1}\mathrm{P}_{1}1P1​+Ar were also calculated. The calculated absorption coefficient, impact-broadening rate, linear- and circular-polarization ratios, and depolarization rate coefficients are for the most part in good agreement with experiment.
Semiclassical formulations of collisional redistribution of polarized radiation are presented at several levels of approximation, from full classical path coupled equations to the locking/decoupling model. These are numerically tested against the results of a quantum mechanical coupled-channels formalism, by the comparison of polarization curves in both spectral wings of the Sr( 1 S 0 - 1 P 1 ) transition, with Ar and He as collisional perturbers. It is found that the locking/decoupling model can often produce good agreement with exact results if the effects due to trajectories and multiple Condon points are treated properly. Significant discrepancies due to the Condon approximation used by the model are seen in the near blue wing of the spectra and attributed to antistatic effects.
We have measured the alignment of Hg(6 3^{3}3P1{\mathit{P}}_{1}P1​) atoms following off-resonant excitation of the Hg 6 1S0^{1}\mathrm{S}_{0}1S0​--6 3P1^{3}\mathrm{P}_{1}3P1​ transition in the presence of Kr. We present a polarization spectrum for both red and blue wings out to detunings beyond 100 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. This represents a considerable extension to the existing results, which has been made possible by the use of a pump-and-probe excitation scheme. In the experiment a pulsed laser, detuned from the resonance, creates an alignment in the upper state. This is probed by tuning a delayed second laser to a higher transition and monitoring laser-induced fluorescence from the two-step process. The alignment of the asymptotic Hg(6 3^{3}3P1{\mathit{P}}_{1}P1​) atoms is determined by comparison of the signals obtained for different relative polarizations of the lasers. We discuss the advantages of this method and the relation of the raw data to the polarizations presented. The results are discussed within the context of a simple quasiclassical model of the process.
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.
When detuned laser light is absorbed during a collision, the subsequent fluorescence will not be 100% because of rotation of the excited molecular orbitals as the collision proceeds. A semiclassical locking/decoupling model of this successfully predicted the polarization curve in the red-wing of for Sr+Ar collisions, but was much less successful for Sr+He. The averaged over all phases. Inspired by a quantal treatment effects in photofragmentation, we introduce simple phase terms for evolutionary interference between parallel and components of the molecular orbitals, from the Condon to their decoupling as atomic orbitals. The resulting interference terms have a major effect on Sr-He depolarization, bringing it into good agreement with brute-force quantal computations.