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Soviet Journal of Quantum Electronics

Influence of collisions on four-photon parametric processes in gases

Э. Г. Сапрыкин, Gennady I. Smirnov

Published Mar 31, 1977• 0 citations
Publisher

Abstract

The influence of isotropic depolarizing collisions on four-photon parametric processes is considered under resonance conditions. It is shown that, in particular, there is no parametric oscillation for J = 0→1 and 1→1 transitions between levels with total angular momenta J and waves with perpendicular linear polarizations if the line-up and orientation constants are equal. A study is made of the possibility of using resonance four-photon parametric processes in the determination of the relaxation constants of multipole moments of electron shells.

DOI
10.1070/qe1977v007n03abeh009570

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Sep 26, 1997•0 citations
The influence of isotropic depolarizing collisions on four-photon parametric processes is considered under resonance conditions. It is shown that, in particular, there is no parametric oscillation for J = 0→1 and 1→1 transitions between levels with total angular momenta J and waves with perpendicular linear polarizations if the line-up and orientation constants are equal. A study is made of the possibility of using resonance four-photon parametric processes in the determination of the relaxation constants of multipole moments of electron shells.
Non-stationary two-photon absorption coherent spectroscopy free of Doppler broadening in an atomic gas has been considered in detail. Correct account has been taken of the level degeneracy and depolarizing collisions. This two-photon Doppler-free spectroscopy has been developed for determining the relaxation rates of the atomic multipole moments in gas. A criterion to test the adopted model of the depolarizing collisions has been developed. A possible method for the identification of the atomic transitions in the angular momentum has been formulated. A technique of precisely determining the separation between the energy levels of the two-photon transition has been proposed. The conditions under which the Stokes wave is conjugated with respect to the probe pulse in amplitude, phase and polarization have been determined for the atomic gas in the presence of depolarizing collisions.
A detailed analysis is made of transient coherent Doppler-free spectroscopy of two-photon absorption in an atomic gas. The hyperfine structure of the levels and elastic depolarizing collisions are allowed for. The analysis is used to develop a method for the determination of the time constant of relaxation of the multipole moments of atoms when such relaxation is due to radiative decay and collisions with a buffer gas. A way of determining the separation between the hyperfine structure components is demonstrated. It is shown that under conditions of excitation of the atoms the hyperfine structure of the levels can give rise to quantum beats of the density of a conjugate wave.
Effects of electric dipole reorienting collisions on photon echoes are investigated. In an extension of the author's earlier work, a general expression is derived, including terms from binary collisions, for the photonecho dipole moment amplitude. The explicit results are worked out for simple cases such as ${j}_{1}=\frac{1}{2}\ensuremath{\leftrightarrow}{j}_{2}=\frac{1}{2}$, ${j}_{1}=1\ensuremath{\leftrightarrow}{j}_{2}=0$, ${j}_{1}=1\ensuremath{\leftrightarrow}{j}_{2}=1$, and ${j}_{1}=\frac{3}{2}\ensuremath{\leftrightarrow}{j}_{2}=\frac{1}{2}$ transitions. While it is shown that echoes resulting from the above simple transitions will not be depolarized by atomic collisions, photon echoes induced by any other transitions are necessarily depolarized by collisions. In addition, it is possible to show that the general expression for the echo dipole moment can be written as a sum of multipole moments. Only the odd moments contribute to the echoes. The importance of this picture is that collision-induced relaxation of each moment is described by a single time constant---as in the model commonly used for Hanle-effect experiments. As a result, for an echo arising from a ${j}_{1}\ensuremath{\leftrightarrow}{j}_{2}$ transition (${j}_{1}\ensuremath{\ge}{j}_{2}$), there must be 12j1(j1+1)\frac{1}{2}{j}_{1}({j}_{1}+1)21​j1​(j1​+1) time constants to describe the collision damping of the echo intensity, j1{j}_{1}j1​ being an integer. The corresponding formula for the half-integer case is 12(j2+12)(j2+32)\frac{1}{2}({j}_{2}+\frac{1}{2})({j}_{2}+\frac{3}{2})21​(j2​+21​)(j2​+23​).
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.