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Soviet Journal of Quantum Electronics

Inelastic scattering of light and transitions of colliding atoms to bound molecular states

A. M. Bonch-Bruevich, Sergey G. Przhibel'skii, V. A. Khodovoǐ, V. V. Khromov

Published Apr 30, 1976• 2 citations
Publisher

Abstract

A study is made of the influence of photon absorption (emission) processes on the dynamics of collisions between two atoms. Under certain conditions the radiation field can transfer colliding atoms to a bound molecular state. The main contribution to these processes is made by atoms colliding with relative velocities and impact parameters which ensure that these atoms spend a long time near each other (quasifinite motion). The appearance of quasifinite states in the absorption spectra of Rb and Cs vapors is reported and a determination is described of the matrix elements of the transitions from these states to a bound molecular state.

DOI
10.1070/qe1976v006n04abeh011137

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A study is made of the influence of photon absorption (emission) processes on the dynamics of collisions between two atoms. Under certain conditions the radiation field can transfer colliding atoms to a bound molecular state. The main contribution to these processes is made by atoms colliding with relative velocities and impact parameters which ensure that these atoms spend a long time near each other (quasifinite motion). The appearance of quasifinite states in the absorption spectra of Rb and Cs vapors is reported and a determination is described of the matrix elements of the transitions from these states to a bound molecular state.
An investigation was made of the mechanisms of quenching of excited states and of excitation of atomic levels as a result of absorption of laser radiation by alkali metal (Na, K, Cs) vapors. It was found that several processes participated in the excitation of atomic levels: collisional mechanism of excitation energy transfer from molecules to atoms, photodissociation, predissociation, and nonresonant excitation of atomic levels (radiative collisions). The partial contribution of each of these processes (dependent on the vapor density, intensity of laser radiation, and its frequency) should be allowed for in the balance of the filling and deactivation of molecular and atomic states. Qualitative characteristics were obtained for these processes and an analysis was made of their likely role in the emission of radiation from diatomic molecules in alkali metal vapors.
Investigates theoretically a new aspect of radiative collisions on two-photon ionisation of colliding atoms of different species A and B. The authors derive the main characteristics of the resonance appearing in the two-photon ionisation cross section of atom B when the laser frequency is tuned on a resonance transition of atom A which cannot itself be ionised by two-photon absorption. Under some realistic conditions and for reasonable values of the light energy density flux F ( approximately 1 MW cm -2 ), the cross section of the process considered varies as F 2/5 while its width varies as F 1/2 . These predictions are in reasonable agreement with a recent observation on the mixture Sr(A)-Cs(B) by Brechignac et al. (1980).
The spectrum for absorption and emission of radiation by colliding atoms or a diatomic molecule is calculated for spectral regions dominated by an avoided level crossing. Example processes are absorption during an atom-atom collision with separation to either crossing atomic state, total absorption to both crossing states, and spontaneous emission during a collision with either state initially populated. Absorption and emission by bound diatomic molecules (including photodissociation) is described by the theory, and as an example it is applied to the Cs2{\mathrm{Cs}}_{2}Cs2​ A-X band. Spectra due to single-velocity and impact-parameter collisions, as well as thermal and impact-parameter averages, are given. The shapes of the resulting spectra are parametrized by the interaction strength (or excitation transfer probability) between the crossing levels and the slopes of the ground- and excited-state potentials (assumed constant) in the region of the level crossing. The theory is applied to two observations of such regions that have been reported. We conclude that measurements of spectra in the region of level crossing is a very powerful diagnostic of the potentials and transfer probability in the level-crossing region, which is responsible for most inelastic atom-atom energy-transfer processes.
Abstract The formation of metastable molecules (Feshbach resonances) at the collision of two atoms and subsequent stimulated transition to a lower unbound electronic molecular state, with emission of a photon of the laser radiation has been investigated. This can develop, in particular, for Rb 2 molecules due to resonance scattering of two Rb atoms. This process is a basis for the creation of excimer lasers. Expressions have been obtained for the cross sections of elastic and inelastic resonance scattering and the intensity of the stimulated emission of the photons.
The first analysis is made of stimulated Raman scattering and two-photon absorption (emission) processes resulting in a change in the states of two colliding particles. These processes are caused the by interaction between dipole moments induced as a result of polarization of the particles by the radiation. Equations are derived to describe multiphoton transitions during a collision between two particles in a radiation field. Using the approximation of rectilinear trajectories, expressions are derived and analyzed for the cross section of collisional two-photon transitions in weak and strong fields. It is shown that in fields of ≈103 cgs esu, the cross section of these transitions may reach 10–4 cm2
An analysis is made of the general aspects of the physics of processes occurring in a system of colliding atoms in the presence of a radiation field. The attention is concentrated on the relationship between the spectral characteristics and various photon absorption (emission) processes in a system of interacting atoms (regarded as a quasimolecule). Nonlinear optical effects in the far wings of atomic lines are considered and the effects are interpreted. A relationship between these effects and the change in the atomic collision dynamics due to the presence of a radiation field is considered.