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Physical Review A

Theory of collisionally aided radiative excitation in three-level systems

S. Yeh, P. R. Berman

Published Oct 1, 1980• 15 citations
Publisher

Abstract

A theory of collisionally aided radiative excitation (CARE) for three-level systems in the weak-field limit is presented. Cross sections for the excitation of three-level atoms by two off-resonant pulsed radiation fields in the presence of collisions with structureless perturbers are calculated. Analytic expressions for the cross sections as functions of atom-field detunings are obtained under usual classical-trajectory and rotating-wave approximations using perturbation theory for various regions of detunings. Examples for the resulting excitation line shapes are given mostly for van der Waals potentials. A dressed-atom picture of the CARE processes is discussed. Emphasis is put on an interesting effect arising from the interference between the "stepwise" and the "direct" channel of excitation. Such an interference effect manifests itself as modulations in the total excitation cross section as a function of relative interatomic speed in some cases.

DOI
10.1103/physreva.22.1403

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A theory of collisionally aided radiative excitation (CARE) for three-level systems in the weak-field limit is presented. Cross sections for the excitation of three-level atoms by two off-resonant pulsed radiation fields in the presence of collisions with structureless perturbers are calculated. Analytic expressions for the cross sections as functions of atom-field detunings are obtained under usual classical-trajectory and rotating-wave approximations using perturbation theory for various regions of detunings. Examples for the resulting excitation line shapes are given mostly for van der Waals potentials. A dressed-atom picture of the CARE processes is discussed. Emphasis is put on an interesting effect arising from the interference between the "stepwise" and the "direct" channel of excitation. Such an interference effect manifests itself as modulations in the total excitation cross section as a function of relative interatomic speed in some cases.
Collisionally aided radiative excitation in a three-level atomic system is investigated with use of a stationary-phase method for the case of large atom-field detunings and weak incident fields. It is found that collision-induced coherent phase-interference effects can give rise to oscillatory structure in the total absorption cross section as a function of relative speed (energy). An example is given for a specific interatomic potential, indicating that experimental observation of such an effect is feasible.
Collisionally Aided Radiative Excitation in a three-level atomic system is investigated using a stationary phase method for the case of large atom-field detunings and weak incident fields. It is found that collision-induced coherent phase interference effects can give rise to oscillatory structure in the total absorption cross section as a function of relative speed (energy). An example is given for a specific interatomic potential, indicating that experimental observation of such an effect is feasible. (Author)
The excitation of a two-level active atom by a radiation field in the presence of a low-density perturber bath is studied. In particular, the collisional enhancement of the absorption cross section is investigated. Features of the absorption-line profiles are predicted on physical grounds using a dressed-atom approach and verified by detailed numerical calculations for attractive, repulsive, and Lennard-Jones-type interatomic potentials. Complete spectra of the absorption cross section both as a function of detuning and as a function of field strength are given. In the weak-field limit, the results are compared with those of traditional pressure-broadening theory and to the recent experimental results of Carlsten et al. In the strong-field limit, the results display a saturation behavior consistent with the predictions of Lisitsa and Yakovlenko.
Using both a bare-atom and dressed-atom picture of atom-field interactions, we present a theory of the pressure-induced resonances that can arise when four radiation fields interact with a three-level quantum system. The four fields drive two coupled atomic transitions which share a common level, the levels being arranged in a ``ladder'' scheme. As the field frequencies are varied, certain resonant structures in the excitation spectrum of the upper state population appear only in the presence of collisions. The resonant structures result from processes that have been referred to in the past as collisionally aided radiative excitation (CARE) and pressure-induced extra resonances in four-wave mixing (PIER4). We show that both CARE- and PIER4-type resonances have a common origin, which can be linked to conservation of energy. In contrast to conventional PIER4 signals, line shapes calculated in this work result from a ``single atom'' rather than a collective emission. Moreover, the signals vanish unless the applied fields are relatively coherent, again in contrast to conventional PIER4.
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 have studied the problem of a collisionally perturbed two-level atom in an intense driving field by developing a method which allows us to calculate the field-dependent collision operator in terms of the low intensity field-independent line-broadening function arising in the usual unified theory of pressure broadening. Our relations are exact to all orders in the field in the limiting case of weak collisions, but they are exact also in the complementary case of strong collisions and weak fields. We apply our results to derive explicit expressions for the rate of optical collisions, the fluorescence spectrum in the case of well-separated components, and the absorption during an adiabatic square pulse.