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The Journal of Chemical Physics

Collisional redistribution in Sr–He spin-changing energy transfer collisions: Final-state alignment

Sharath Ananthamurthy, Paul D. Kleiber

Published Feb 1, 1995• 1 citation
Publisher

Abstract

We have measured the product alignment resulting from the collisional redistribution of polarized light in Sr–He inelastic spin-changing energy transfer collisions. The experimental results are in good agreement with the predictions of an analytic theoretical model, based on a standard orbital-locking and following approximation, but generalized to this inelastic collision process. The good agreement indicates a clear understanding of the energy transfer dynamics in this case, and suggests that this simple analytic approach may be generalized to a much broader range of collisional phenomena.

DOI
10.1063/1.468757

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We have measured the product alignment resulting from the collisional redistribution of polarized light in Sr–He inelastic spin-changing energy transfer collisions. The experimental results are in good agreement with the predictions of an analytic theoretical model, based on a standard orbital-locking and following approximation, but generalized to this inelastic collision process. The good agreement indicates a clear understanding of the energy transfer dynamics in this case, and suggests that this simple analytic approach may be generalized to a much broader range of collisional phenomena.
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.
Alignment and orientation effects in the energy pooling process, Sr(5s5p 3 P 1 ) + Sr(5s5p 3 P 1 ) → Sr(5s5p 1 P 1 ) + Sr(5s 2 1 S 0 ), are studied in a single atomic beam. The Sr atoms are prepared in an aligned initial state with a polarized laser, and alignment information is extracted by observing the fluorescence from the final Sr(5s5p 1 P 1 ) state as the atoms precess in an applied magnetic field. This allows the dependence of the total energy pooling cross section (integrated over the final state alignment) on the initial state alignment to be almost completely described. A prominent alignment effect is observed. If m j represents the component of total electronic angular momentum of a Sr(5s5p 3 P 1 ) atom along the relative velocity vector of the collision, then the total energy transfer cross section for a particular initial state alignment can be expressed as a sum of the so-called fundamental cross sections, σ m 1 m 2 and σ m 1 m 2; m 1 ‘ m 2 ‘, that describe collisions between the various m j states. Here σ m 1 m 2 represents the cross section for energy transfer when an atom in state m 1 collides with an atom in state m 2 . The cross section σ m 1 m 2; m 1 ‘ m 2 ‘ represents the contribution to the total cross section from interference when the colliding system is in a superposition of the state m 1 colliding with m 2 and the state m 1 ‘ colliding with m 2 ‘. It is found that the cross sections σ 1-1, σ 00, and σ 10 as well as the interference terms Re( σ 00;1-1 ) and σ 01;10 have relatively large values while σ 11 and σ 1-1;-11 are small. Coupled with future theoretical work, these results may provide new insights into the dynamics of the curve crossings that lead to energy transfer.
Theory shows that to explain the polarization of light collisionally redistributed from the far line wings of an atomic transition, one must consider correlated events in which absorption during a collision, and propagation to the final Zeeman-state superposition at the end of the collision is important. Polarizations of up to about 40% have been measured in the far line wings, substantially confirming this prediction, and showing that scattering experiments cannot just be characterized by simple absorption or emission profiles.
We present a detailed study of the mechanism of spin–changing transitions between 1P and 3P electronic states corresponding to the nsn′p Rydberg state of an alkaline earth atom in collision with a structureless, spherical partner. This type of process takes place on four potential curves and involves collisional coupling between the electronic angular momentum L of the electronically excited atom, the spin S of the atom in the final 3P state, and the orbital angular momentum l associated with the relative motion of the collision partners. We review the use of Hund’s case coupling schemes in describing the collision and present a careful analysis of the initial state preparation under conditions of laser excitation in a crossed beam experiment. This underlies the development of a new statistical model for the dependence of the spin–changing cross section on the alignment of the pump laser. Finally, we describe a new technique to follow the redistribution of flux onto locally adiabatic states as the initially excited atom is approached by the closed-shell partner. This is used to show that dynamical corrections to our simple statistical model are small.
A four-state model considering the relative velocity distribution function for calculating the cross section of laser-induced collisional energy transfer in a Sr—Li system is presented and profiles of laser-induced collision cross section are obtained. The resulting spectra obtained from different intermediate states are strongly asymmetrical in an opposite asymmetry. Both of the two intermediate states have contributions to the final state, and none of the intermediate states should be neglected. The peak of the laser-induced collisional energy transfer (LICET) profile shifts toward the red and the FWHM becomes narrower obviously with laser field intensity increasing. A cross section of 1.2 × 10−12 cm2 at a laser field intensity of 2.17 × 107 V/m is obtained, which indicates that this collision process can be an effective way to transfer energy selectively from a storage state to a target state. The existence of saturation for cross section with the increase of the laser intensity shows that the high-intensity redistribution of transition probabilities is an important feature of this process, which is not accounted for in a two-state treatment.