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Bulletin of the Russian Academy of Sciences Physics

Effect of the translational velocity of active particles on the properties of a photon echo

Н. Н. Рубцова, Гольдорт Вениамин Гершевич, Igor V. Yevseyev, В. Н. Ищенко, Sergei A. Kochubei, Ледовских Дмитрий Васильевич, Решетов Владимир Александрович, Хворостов Евгений Борисович

Published Jul 1, 2010• 0 citations
Publisher

Abstract

The photon echo method was used to detect an increase in the relaxation rate in the vibrational-rotational transition of fluoromethane in 13CH3F gas and a 13CH3F-argon mixture upon an increase in the velocity of active particles. For a photon echo in the 0–1 transition in a Yb + Xe mixture, the dependence of the relaxation parameters on the direction of the translational velocity of Yb leads to qualitative changes and a collision photon echo arises.

DOI
10.3103/s1062873810070075

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Journal of Experimental and Theoretical Physics Letters•Mar 1, 2008•14 citations
The photon echo method was used to detect an increase in the relaxation rate in the vibrational-rotational transition of fluoromethane in 13CH3F gas and a 13CH3F-argon mixture upon an increase in the velocity of active particles. For a photon echo in the 0–1 transition in a Yb + Xe mixture, the dependence of the relaxation parameters on the direction of the translational velocity of Yb leads to qualitative changes and a collision photon echo arises.
The effect of translational velocity of active atoms and molecules on the properties of photon echo is investigated using the technique of coherent transient processes. A variation in the photon-echo decay with a frequency detuning of the excitation radiation relative to the center of the vibrational-rotational transition 0 ↔ 1 ν3 R(4, 3) is observed in a mixture of 13CH3F with atomic buffers. The results are interpreted using the dependence of the echo decay rate on the magnitude of the translational velocity of active particles. The dependence of the relaxation matrix on the direction of the velocity of active atoms results in a new phenomenon of the collision-induced echo, which is investigated at the transition 0 ↔ 1 174Yb in mixtures with atomic buffers.
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.
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.
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.
Isotropic depolarizing collisions are studied using a stimulated photon echo with a specific polarization of the excitation radiation pulses in a mixture of ytterbium with krypton for the J = 1 ⇄ J = 0 transition of 174Yb. The difference between the relaxation rates of orientation and alignment γ (2) − γ (1) of the 3 P 1(6s6p) 174Yb level is measured as a function of the krypton pressure. The collision photon echo at the J = 1 ⇄ J = 0 transition induced by the anisotropic relaxation is studied for the Yb + Xe mixture. The power of the collision echo increases from zero with the addition of a buffer gas to ytterbium, reaches an optimal level, and decreases with an increase in the buffer gas pressure. The polarization of this collision-induced echo differs from the polarization of the conventional echo. The experimental results are in qualitative agreement with theoretical predictions.
A photon echo induced exclusively by collisions of ytterbium atoms with buffer gas atoms has been observed at a 0 ⟷ 1-type1 S 0(6s 2)-3 P 1(6s6p) 174Yb transition. The polarization properties of a collision echo and the buffer gas density dependence of its intensity agree with theoretical predictions of a model of depolarizing collisions that takes into account the dependence of a relaxation matrix on the velocity of active particles. Thus, direct experimental evidence of the relaxation anisotropy due to depolarizing collisions has been obtained.