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Physical Review A

Intensity effects in two-photon collisional redistribution of radiation

G. Alber, J. Cooper

Published Oct 1, 1985• 2 citations
Publisher

Abstract

We study laser-intensity effects in two-photon collisional redistribution of radiation. Thereby we concentrate on a special case of recent experimental interest, where the transition from the excited to the final state is saturated by a strong laser field. Both exciting lasers are, however, assumed weak in the sense that the dynamics of a collision between the radiator and a perturber is not significantly influenced by the laser fields. In particular, our treatment explains the intensity independence of the polarization dependence of total fluorescence from the final state which has been measured previously even if the transition from the excited to the final state is saturated.

DOI
10.1103/physreva.32.2186

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We study laser-intensity effects in two-photon collisional redistribution of radiation. Thereby we concentrate on a special case of recent experimental interest, where the transition from the excited to the final state is saturated by a strong laser field. Both exciting lasers are, however, assumed weak in the sense that the dynamics of a collision between the radiator and a perturber is not significantly influenced by the laser fields. In particular, our treatment explains the intensity independence of the polarization dependence of total fluorescence from the final state which has been measured previously even if the transition from the excited to the final state is saturated.
We study collisional redistribution in the presence of two weakly exciting laser fields, including the effects due to degeneracy of the radiator states. A general expression for the total redistributed intensity is derived valid for arbitrary detunings and polarizations of the exciting laser fields. In particular, this expression contains all single- and sequential-collisional contributions, which are equally important under certain circumstances. We also point out the similarities and differences between the redistributed intensity as calculated in this paper and collisionally aided radiative excitation cross sections, which have been studied previously.
The formal structure of the time dependence of the state of an atom in a perturber bath in an intense classical monochromatic radiation field is studied. The authors allow the cross sections of binary collisions to be affected by the radiation field. The result is used to derive an expression for the collision-broadened saturated fluorescence excitation spectrum and absorption rate in terms of a collisional relaxation operator. This operator can be related to the rate of collisionally aided radiative excitation.
We extend the theory of light scattering from an atom undergoing collisions to the case of strong laser fields, where the expansion in powers of $\ensuremath{\Omega}{\ensuremath{\tau}}_{c}$ that was used formerly in a series of papers by Burnett et al. breaks down. In particular we consider in detail the conditions necessary to relate the observable quantities to intense-field collisional rate constants in a dressed-state basis (such as those calculated by Light and Sz\"oke). We also show (following Rabin and Ben-Reuven) that by studying the spectrum emitted by the atoms in the presence of a strong field one may measure the collisional rates for transfer between the dressed states of atom plus radiation field.
A theory is given of radiative collisions, i.e., of elementary events in which the emission of a photon occurs in the course of a collision. Special attention is paid to radiative collisions stimulated by a laser field. Simple but clear approaches formulated in the forties or fifties are described briefly. This theory applies, in particular, to collisions in a strong electromagnetic field which affect the collision dynamics. Chemical radiative collisions, i.e., chemical reactions stimulated by photons absorbed in the course of a collision, are reviewed. An analysis is made of several recent experiments in which the theoretically predicted effects were observed: these include the radiative-collisional transfer of excitation from atom to atom, nonlinear effects due to the influence of the field on the collision dynamics, and photosubstitution of one of the atoms in a halogen molecule with an inert gas atom.
The author derives an expression for the spectrum of the radiation scattered by an atom that suffers binary collisions in the weak-field limit. The frequency of the incident radiation is in near-resonance with a multiplet transition. The authors accounts for strong atom-perturber correlations that are induced when the incident or the detected frequency is in the far wing of an absorption line. For an isolated transition between two degenerate atomic levels the model picture of quasistatic absorption followed by adiabatic reorientation of the excited state predicts a redistributed fluorescence line with a polarisation that is determined by geometrical factors.
Resonance fluorescence due to intense laser fields from a Zeeman degenerate atom being perturbed by collisional interactions is calculated in the impact regime by using the quantum-fluctuation-regression theorem. Various interesting effects are found. For example, the scattered intensity spectrum for a J=0J=0J=0 to J=1J=1J=1 transition for polarization parallel to the laser polarization is essentially an asymmetric triplet, whereas for a perpendicular polarization due to collisions the spectrum is essentially a doublet (whose frequencies do not correspond with any of those of the triplet). Further, the width of the fluorescent component (whose frequency is close to the unperturbed frequency) actually decreases with increasing laser power.