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Physical Review A

Influence of resonance exchange during velocity-selective excitation of a gas

Michael A. Vaksman

Published Sep 1, 1991• 1 citation
Publisher

Abstract

Resonance exchange of excitation (REE) between gas molecules can reduce the magnitude of various macroscopic kinetic phenomena produced in the gas by velocity-selective excitation. Considerable reduction will occur whenever the rate of REE exceeds both the rate of elastic velocity-randomizing collisions and the rate of the spontaneous decay of the excited state. Expressions given here are necessary for the evaluation of the change in magnitude of the kinetic effects as a result of REE.

DOI
10.1103/physreva.44.3125

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Resonance exchange of excitation (REE) between gas molecules can reduce the magnitude of various macroscopic kinetic phenomena produced in the gas by velocity-selective excitation. Considerable reduction will occur whenever the rate of REE exceeds both the rate of elastic velocity-randomizing collisions and the rate of the spontaneous decay of the excited state. Expressions given here are necessary for the evaluation of the change in magnitude of the kinetic effects as a result of REE.
We consider a mixture of resonant two-level molecules under velocity-selective excitation and a buffer gas. The corresponding kinetic equations are augmented by a strong-collision model with free parameters. We obtain the equilibrium velocity distribution functions of the resonant and buffer components. We then use the Boltzmann kinetic equations to derive the H-theorem analog for this mixture (valid for arbitrary values of the collision rates \ensuremath{\nu} and the spontaneous decay rate \ensuremath{\gamma}) and discuss the implications for the stability of the stationary solutions. \textcopyright{} 1996 The American Physical Society.
Populations of excited atoms that all have the same z component of velocity can be produced by pumping a vapor with a narrow-band laser. This velocity-selected population is then thermalized by velocity-changing collisions. However, in a pure vapor, even at low densities where velocity-changing collisions can be ignored, the mechanisms of radiation trapping and resonance exchange collisions can still lead to substantial thermalization of excited-atom velocity distributions. In this paper, we present data demonstrating these effects. We compare our data with results from a simple model of these processes. The model is in qualitative agreement with the experimental results.
The velocity changing collisions (vcc) play an important role in the description of velocity redistribution of particles caused by nearly resonant interaction with the laser light and collisions (1). Collision relaxation rates are conveniently described by collision kernels which, in turn, depend on interatomic potential and scattering amplitudes and is proportional to the collision cross section. The population kernel gives the probability density per unit time that an active particle in state 1 i> undergoes collisions with perturbers (usually a noble gas atom or impurity), which changes the active particle velocity from v′ to v. Due to the fact that perturber-active particle interaction is state dependent the collisions cause different changes in the ground and excited states of active particle. Although the shape of the populations distributions resembles those from well-known Bennett hole theory, the distribution of the total population has two peaks and obviously is asymmetrical (2). Vcc together with velocity selective laser excitation lead to the asymmetry of the ground and excited state population. The flux of excited particles has properties which are different from that of ground state because of interaction collision potentials for both states. The intensity of the laser light strongly influences the velocity distributions of the ground and excited state populations.
An experimental study of cesium resonance exchange collision, Cs(6P(3/2), v) + Cs(6S(1/2), v')-->Cs(6S(1/2), v) + Cs (6P(3/2), v'), was carried out. Populations of excited atoms that all have the same z component of velocity were produced by pumping a vapor with a narrow-band laser. A counterpropagating single-mode diode laser was used to probe the excited atom velocity distribution in the 6P(3/2)-->8S(1/2) transition. Fluorescence was monitored in the 8S(1/2)-->6P(3/2) transition. The magnitude of the thermalizinng effect of resonance exchange collisions was estimated by measuring the ratio of the intensity in the narrow features to that associated with the Doppler pedestals. The rate coefficient of 9.62 x 10(-7) cm3 x s(-1) for the resonance exchange collisions was yielded. This work demonstrates that, in a pure metal vapor, the thermalization of velocity-selected excited-atom distribution by the mechanism of resonance exchange can be three orders of magnitude greater than that from velocity-changing collisions.
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.
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.