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AIP conference proceedings

Molecular coherences in collisional redistribution and photo-dissociation

Keith R. Burnett, D. M. Segal

Published Jan 1, 1990• 0 citations
Publisher

Abstract

In this paper we shall decribe work that links the study of orientation and alignment in photo‐dissociation1 and collisional redistribution.2 We shall show how simple models of these processes can be used with various species of molecular coherence and account for recent experimental results. We believe that these studies offer excellent probes of the locking radius models used in the analysis of orbitally selective collision processes.

DOI
10.1063/1.39967

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In this paper we shall decribe work that links the study of orientation and alignment in photo‐dissociation1 and collisional redistribution.2 We shall show how simple models of these processes can be used with various species of molecular coherence and account for recent experimental results. We believe that these studies offer excellent probes of the locking radius models used in the analysis of orbitally selective collision processes.
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.
Recent experimental results of Havey, Copeland, and Wang on the collisional redistribution of light by Na in Ar showed a dependence of the relative intensities of the Na D1{D}_{1}D1​ and D2{D}_{2}D2​ resonance lines on laser detuning. Calculations of these intensity ratios are reported here. The agreement between theory and experiment obtained here shows the importance of fine-structure transitions occurring during the dissociation of the excited-state molecule.
We have measured the polarization of collisionally redistributed light as a function of detuning from line center for the Hg3P1–1S0 transition in the presence of Ar. We present a polarization profile for the red wing between 5 and 18 cm−1 and polarizations for two detunings in the blue wing. We compare this type of experiment to the complementary method of photodissociation and discuss the issue of coherence between excitation path-ways. The data are analyzed using a simple molecular reorientation model suggested by others which includes coherent excitation. The model is shown to be unable to completely account for the high polarizations measured in this experiment. We suggest reasons for the failure of this model.
The rotational coherence of an ensemble of photodissociation fragments is studied in the absence of collisions. Formulas for quantum orientation correlation functions are obtained for the case when parent molecules and their fragments are asymmetric tops and the predissociation time is much shorter than the characteristic time of rotational reorientation. It is shown that photodissociation does not affect periods of a rotational echo, but the structure of the echo pulses (their shape and width) depends substantially on the features of rotational excitation of photofragments upon photodissociation.
A stimulated photon echo technique with specially selected linear polarizations of the coherent resonant driver pulses is used to study depolarizing collisions in the molecular gas SF 6 and in mixtures of it with buffer He and Xe. The collisional decay rates of the population, orientation, and alignment in an ensemble of gas particles are determined for the first time in a single experiment. These relaxation rates are measured as a function of the longitudinal translational velocities of the resonant particles. To within the experimental accuracy, no significant dependence of the collisional decay rates on the translational velocities of the particles was observed. This result confirms the conventional theoretical approach to depolarizing collisions. In pure SF 6 the decay rates for the orientation and alignment were lower than the relaxation constant for collisions involving a change in the longitudinal velocity (elastic collisions) that is known from experimental observations of the ordinary photon echo. This means that only some of the elastic collisions participate in destroying the multipole moments of the levels. Evidence is found that the relaxation of the multipole moments created by polarized radiation in a resonant medium of molecular SF 6 gas depends on j , the total angular momentum of the level.