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Optics and Spectroscopy

Change of anisotropic collisional relaxation of atomic states to quasiisotropic relaxation during selective excitation of a Doppler contour

A. G. Petrashen, V. N. Rebane, T. K. Rebane

Published Aug 1, 1986• 0 citations
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Abstract

It is shown that at a certain value of the deviation of the frequency of a single-mode laser from the center of the Doppler contour of an atomic spectral line, the basic characteristics of anisotropic relaxation: creation of alignment and change of alignment to orientation: disappear.

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Theoretical study is made of the transfer between orientation and alignment of excited-state atoms by anisotropic collisions with ground-state atoms. The anisotropy considered is due to the anisotropic velocity distribution of atoms excited by a single-mode laser beam. The transfer rate and relaxation rates for each multipole moment are calculated for the excited state with J=1J=1J=1 as a function of detuning of the laser frequency from the center of an absorption line and mass of perturbing ground-state atoms. Experiments to observe the transfer from alignment to orientation and from orientation to alignment in a weak magnetic field are considered, and corresponding signals are analyzed in terms of the transfer and relaxation rates.
The first experimental evidence of transfer from alignment to orientation of laserexcited atoms due to anisotropic collisions is reported. The experiment has been made with respect to neon atoms in the 2p42{p}_{4}2p4​ state, excited by a single-mode laser, which collide with the ground-state neon atoms, and good agreement between experiment and theory is found.
The induction of higher polarization moments-the octupole orientation and the hexadecapole alignment—in the case of two-step excitation of atoms through dipole transitions and their manifestation in the polarization of dipole emission under conditions of anisotropic collisions are studied. For all channels of the two-step excitation process J 0→J 1 J, the efficiencies of production of higher polarization moments with integer and half-integer angular momenta from J=2 to J=7 through the absorption of linearly or circularly polarized light are calculated. The rates of collisional relaxation and interconversion of higher polarization moments, for both orientation and alignment, as functions of frequency detuning of the laser line from the center of the Doppler profile are calculated. The theory is illustrated by an example of two channels of excitation of the atomic state J=2 by circularly polarized light: J 0=0→J 1=1J=2 and J 0=3→J 1=2 (or 3)→J=2. In the first case, the octupole orientation enhances or attenuates the signal of circular polarization depending on the laser frequency detuning. In the second case, it represents the only source producing orientation and circular polarization of the emitted light.
Calculations of the collisional relaxation of electronic--nuclear polarization moments of all state of the fine and hyperfine structure of the 3/ital p//sup 2//ital P/ doublet of the sodium atom (/ital J/=1/2 and 3/2, nuclear spin /ital I/=3/2) are carried out. The case of selective monochromatic excitation of the fine level /sup 2//ital P//sub 1/2/, allowing for the overlap of the Doppler shapes of the hyperfine components of the spectral line, is discussed. Such excitation gives rise in the /sup 2//ital P//sub 1/2/ state to four groups of excited atoms differing in values of the velocity projection on the direction of the light beam. The problem of collisional relaxation of the polarization moments is solved for each of these groups of atoms, account being taken of the anisotropy of the velocity distribution and intramultiplet mixing. The alignment induced in the levels of the hyperfine structure of the /sup 2//ital P//sub 3/2/ state, populated by anisotropic collisions with atoms of different inert gases, is calculated, as is the linear polarization of the light of sensitized fluorescence. The calculation results are in good agreement with the experiment.
Our anisotropic rate equation model outlines the relationship between the relaxation dynamics in a four-level solid-state laser and its anisotropic gain properties. Anisotropic pump rates and stimulated emission cross-sections were included to account for specific atom orientations in the gain material. The model is compared with experimental measurements of two relaxation oscillation frequencies which are related to the anisotropic atom-laser interaction in orthogonally polarized dual-mode lasers. The model predicts that crystal orientation and pump polarization affect the laser operation characteristics, as found experimentally. The gain anisotropy influences the fast laser dynamics, as in single-mode relaxation oscillations.
Optical pumping performed with a quasi monochromatic laser beam in an atomic gas introduces a redistribution of Zeeman populations selectively in a single velocity class. In the limit of low pumping intensity the rate theory is valid for a description of velocity-selective optical pumping (V.S.O.P.). Well-defined atomic observables (population, orientation, alignment) and their velocity-correlated collisional relaxation can be studied. Also, a modified scheme for the detection of second order effects of optical pumping allows one to observe light shift induced signals. In particular, V.S.O.P. and its combination with modulated pumping, magnetic depolarization (level crossing), and magnetic resonance techniques provide information about the atomic collision characteristics such as collision rates and kernels for velocity-changing collisions.