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Journal of Modern Optics

Non-stationary Two-photon Doppler-free Coherent Spectroscopy in an Atomic Gas in the Presence of Depolarizing Collisions

A. I. Alekseev, V. N. Beloborodov

Published May 1, 1994• 0 citations
Publisher

Abstract

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.

DOI
10.1080/09500349414550951

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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.
A theoretical investigation of the non-stationary Raman scattering (RS) of light in gas atoms with no nuclear spin has been made. Account was taken of the level degeneracy due to the different orientation of the total angular momenta, the dependence of the RS signal on the polarisations of the light pulses and of the elastic depolarising collisions. Interesting RS polarisation properties have been found and assumed as a basis method for the identification of optically forbidden transitions and for the determination of parameters of the cubic susceptibility tensor. It has been proved that the polarisation properties and damping law of the non-stationary RS signal permit multipole relaxation rates of forbidden and allowed atomic transitions to be determined.
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.
A great variety of coherent transient processes in atomic and molecular gases is investigated. The degeneracy with respect to magnetic sublevels, which is typical of working levels of atomic and molecular transitions in gases, leads to new phenomena. In particular, the non-Faraday rotation of the coherent-response polarization vector at the 0 ↔ 1 transition is investigated in the presence of the longitudinal magnetic field in the 174 Yb vapor for both photon echo and stimulated photon echo. A specific relaxation channel that involves the depolarizing collisions emerges in a gas due to the degeneracy of working levels. Such collisions are especially important for atoms and are also observed in molecules. The anisotropy of the depolarizing collisions leads to a new phenomenon of the collision photon echo, which is experimentally demonstrated for the 0 ↔ 1 transition in the 174 Yb mixtures with atomic buffers. The velocity-dependent relaxation rates are observed for various coherent phenomena in molecular gases. The coherent control of a few coherent transient processes is implemented.
A theory of the influence of binary collisions on the line shapes associated with Doppler-free spectroscopy is presented. The specific calculation is for two-photon absorption through a real or virtual intermediate state, but the extension of the results to alternative level schemes is included. Collisions are treated quite generally at first, but, ultimately, a simple but reasonable collision model is adopted to properly account for both the "phase-interrupting" and "velocity-changing" aspects of collisions. It is shown that collisional processes can be used to distinguish between the "two-quantum" and "stepwise" contributions to the two-photon excitation rate. Moreover, it is demonstrated that systematic experimental line-shape investigations, in addition to providing tests of current collision theories, can lead to values for collision broadening and shift parameters, excitation transfer rates, magnetic substate relaxation rates, velocity-changing collision rates, and collision kernels. Consequently, Doppler-free spectroscopy can provide a new and important probe of collision effects in atomic and molecular systems.
Polarization nondegenerate nonstationary coherent anti-Stokes Raman spectroscopy was used in measurements of pulse responses for separate electronic multipole moments. The 6P(1/2)-6P(3/2) transition of Tl atoms was investigated. Doppler dephasing conditions, quantum beats of the hyperfine components of this transition are clearly seen when pure anisotropic scattering is registered, but they are small in the case of purely antisymmetric scattering. Under collisional dephasing conditions the relaxation rates of anisotropic and antisymmetric scattering agree with existing theory within +/-5%.