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AIP conference proceedings

Light absorption during a resonant or near-resonant collision: Study of the cross section in the far-wing

Stefano Cavalieri, Milva Celli

Published Jan 1, 1997• 0 citations
Publisher

Abstract

We have studied the absorption of light induced by a resonant or near-resonant collision between two atoms. The calculations have been performed by taking into account also the magnetic sublevels, which makes our theoretical predictions more applicable to realistic cases. Analytical expressions for the far wing absorption cross-section have been obtained.

DOI
10.1063/1.51864

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We have studied the absorption of light induced by a resonant or near-resonant collision between two atoms. The calculations have been performed by taking into account also the magnetic sublevels, which makes our theoretical predictions more applicable to realistic cases. Analytical expressions for the far wing absorption cross-section have been obtained.
An analysis of near-resonant collisions between dissimilar atoms in the presence of a radiation field reveals their effect on the far wings of the absorption profile. Extending a model due to Cavalieri, Arimondo, and Matera, we take into account the collisional shifts of atomic levels, as well as the collisional coupling between atoms and the coupling with the radiation field. A general formula for the absorption profile is derived and expressed in terms of the universal line-shape function of the unified Franck-Condon treatment of pressure broadening. We show that in some cases collision-induced coherence can give rise to a strongly depressed red wing that exhibits transparency for some values of the laser detuning. \textcopyright{} 1996 The American Physical Society.
An absorption (and/or emission) event which takes place during a strong collision is called a 'correlated event'. It is discussed how correlated events affect the far red wing depolarization of fluorescence. Attention is given to an atomic vapor which is irradiated by linearly polarized light of a frequency on the red side of the resonance line. Two limiting cases are considered, corresponding to excitation in the impact region and in the quasi-static wing. In the quasi-static wing, absorption of a photon followed by fluorescence (rather than Rayleigh scattering), occurs mostly during a collision. Correlated events dominate the scattering process. Expressions derived for the polarization of the fluorescent light are applied to far red wing depolarization. It is found that the polarization of the fluorescent light does not go to zero in the far wing, but depends crucially on the detailed nature of the anisotropy in the long-range part of the interatomic potential.
A theoretical calculation of the collision-induced far-infrared absorption, which has been observed in rare-gas mixtures, is carried out using the quantum-mechanical expression for the absorption coefficient. Using a simple empirical form to describe the variation of the collision-induced dipole moment with internuclear separation, and using plane-wave eigenstates (i.e., assuming straight-line collision paths), an analytic expression is obtained for the spectrum which agrees well with the experiments. Comparison with a previous classical calculation based upon the same model shows that quantum effects are small at room temperature except in the high-frequency wing of the spectrum. A number of analytic integrations allow us to derive simple expressions for the correlation function, relaxation time, static dielectric constant, and other spectral invariants.