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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Study of collisional decay rates of population, orientation, and alignment using stimulated photon echo in SF 6 molecular gas

L. S. Vasilenko, Н. Н. Рубцова, Хворостов Евгений Борисович

Published Oct 21, 1998• 0 citations
Publisher

Abstract

Stimulated photon echo technique with specially chosen linear polarizations of pumping pulses of exciting radiation is applied to investigation of depolarizing collisions in molecular gas SF6 and its mixtures with buffer gases He and Xe. For the first time, the collisional decay rates of population, orientation and alignment of SF6 levels are determined simultaneously. Translational velocity dependence of orientation and alignment collisional decay rates was under investigation. This velocity dependence proved to be negligible within the frames of experimental accuracy both in pure SF6 and in mixtures, which is in agreement with standard theoretical approach accepted for depolarizing collisions. Decay rates of orientation and alignment in pure SF6 have proved lower than velocity changing (elastic) collisions relaxation rate, which allows to conclude, that only some part of elastic collisions participates in destruction of multipole polarization moments of resonant levels. The evidence is obtained for j-dependence of multipole moments relaxation rates SF6.

DOI
10.1117/12.328226

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Polarization effects in gases are investigated by two methods of coherent spectroscopy - stimulated photon echo in molecular gas and laser-induced fluorescence in atomic vapour. A well developed photon echo experimental technique permitted us not only to measure relaxation rates of population, orientation and alignment on degenerated resonant levels in molecular gas but also to study their dependence on particles' longitudinal velocity. The degree of polarization of laser-induced fluorescence in Yb vapour in a mixture with Xe proves a large contribution of depolarizing collisions. The complicated non-exponential fluorescence decay at higher Yb pressures is explained by the radiation trapping.

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Stimulated photon echo technique with specially chosen linear polarizations of pumping pulses of exciting radiation is applied to investigation of depolarizing collisions in molecular gas SF6 and its mixtures with buffer gases He and Xe. For the first time, the collisional decay rates of population, orientation and alignment of SF6 levels are determined simultaneously. Translational velocity dependence of orientation and alignment collisional decay rates was under investigation. This velocity dependence proved to be negligible within the frames of experimental accuracy both in pure SF6 and in mixtures, which is in agreement with standard theoretical approach accepted for depolarizing collisions. Decay rates of orientation and alignment in pure SF6 have proved lower than velocity changing (elastic) collisions relaxation rate, which allows to conclude, that only some part of elastic collisions participates in destruction of multipole polarization moments of resonant levels. The evidence is obtained for j-dependence of multipole moments relaxation rates SF6.
The method of a stimulated photon echo with specially selected linear polarizations of pumping pulses of a coherent resonant radiation is applied to a research of depolarizing collisions in molecular gas SF6 and its mixtures with buffer gases He and Xe. For the first time in one experiment the rates of a collisional decay of three lowest moments of polarization (population, orientation and alignment) are defined in an ensemble of gas particles. These rates of a relaxation are measured as functions on longitudinal velocities of resonant particles. In the frames of an experimental accuracy any noticeable dependence on velocities of particles was not detected both for orientation and alignment neither in pure resonant gas SF6 nor in mixtures. This result confirms the standard theoretical approach to depolarizing collisions. In pure SF6 decay rates of orientation and alignment have proved lesser than relaxation rate constant due to velocity changing (elastic) collisions. It means, that only the part of elastic collisions participates in destruction of multipole polarization moments of resonant levels. In comparison with our previous work the evidence is obtained that the relaxation of multipole moments seems to be j-dependent one in a medium of molecular gas SF6.
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.
The method of stimulated photon echo with specially chosen linear polarizations of the pumping pulses of coherent resonant radiation is applied for the investigation of collisional decay of orientation and alignment in the molecular gas SF6 and its mixtures with buffers He and Xe. These relaxation rates are measured as a function of longitudinal velocities. Within the experiment accuracy, no remarkable velocity dependence of the depolarizing collisions rates was detected. This result confirms conventional approach of the depolarizing collisions theoretical treatment. In pure SF6 the rates of elastic depolarizing collisions for orientation and alignment proved smaller than the known rate of velocity changing collisions. It means that only part of the elastic collisions participates in destroying of the level multipole moments.
Method of stimulated photon echo with specially chosen linear polarizations of exciting pulses of coherent resonant radiation is applied for investigation of collisional decay of orientation and alignment in molecular gas SF6 and its mixtures with buffers He and Xe. These relaxation rates are measured as a function of longitudinal velocities. Within experiment accuracy, no remarkable velocity dependence of depolarizing collisions rates was detected. This result confirms conventional approach of depolarizing collisions theoretical treatment.
Collisional relaxation in SF6 gas and its mixtures with He and Xe is studied by photon echo and stimulated photon echo methods from the standpoint of the possibility of identifying the contributions of different types of collisions. The nonexponential nature of the kinetic curve of the photon echo is clearly observed for pure SF6, it is weaker in the mixture SF6+Xe, and it is virtually completely absent for high degrees of dilution of SF6 with helium. These features can be explained on the basis of estimates, made from experimental data, of the critical delay between the exciting pulses (for which the nonexponential behavior should be most strongly manifested). In pure SF6 it is possible to distinguish the contribution of the inelastic channel (rotational relaxation) and the contribution of weak collisions. To distinguish successfully the relaxation channels in mixtures with buffer gases a heavier buffer gas and a much better time resolution must be used. It is shown that data obtained on the orientation and alignment relaxation rates by the stimulated photon echo method can serve as an upper limit for the rates of inelastic processes which cannot be measured by the photon echo method. The combined use of photon echo and stimulated photon echo methods made it possible to obtain data on the cross sections for elastic and inelastic scattering of the collisional pairs SF6–SF6, SF6–Xe, and SF6–He.
Polarization effects in gases are investigated by two methods of coherent spectroscopy - stimulated photon echo in molecular gas and laser-induced fluorescence in atomic vapour. A well developed photon echo experimental technique permitted us not only to measure relaxation rates of population, orientation and alignment on degenerated resonant levels in molecular gas but also to study their dependence on particles' longitudinal velocity. The degree of polarization of laser-induced fluorescence in Yb vapour in a mixture with Xe proves a large contribution of depolarizing collisions. The complicated non-exponential fluorescence decay at higher Yb pressures is explained by the radiation trapping.