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Physical Review Letters

Theory of Collisionally Aided Radiative Excitation in Three-Level Systems: New Interference Effect.

S.C. Yeh, P. R. Berman

Published Oct 22, 1979• 1 citation
Publisher

Abstract

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)

DOI
10.1103/physrevlett.43.1274.4

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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)
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.
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.
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.
We have studied theoretically constructive interference via collision-aided radiative excitation in an open four-level system using a density matrix approach. The four-level system consists of a Λ-type three-level quantum-beat configuration driven by one laser field and a fourth level coupled by a vacuum mode. It is shown that through the incoherent process (collision), coherence between widely-separated doublets and subsequent constructive interference can be realized. We analyse the effects of the collision-induced coherent and incoherent decay rates, laser intensity and energy separation of the doublets on the interference. Meanwhile, the constructive interference between the two transition pathways 3P 1/2 -4D and 3P 3/2 -4D via equal-frequency hybrid excitation and collision-aided radiative excitation in an Na 2 -Na system is experimentally observed. A good agreement between the theoretical and experimental results is obtained.
We have studied theoretically constructive interference via collision-aided radiative excitation in an open fourlevel system using a density matrix approach.The four-leval system consists of a Λ-type three-level quantum-beat configuration driven by one laser field and a fourthe level coupled by a vacuum mode.It is shown that through the incoherent process (collision),coherence between widely-separated doublets and subsequent constructive interference can be realized.We analyse the effects of the collision-induced coherent and incoherent decay rates,laser intensity,and energy separation of the doublets on the interference.Meanwhile,the constructive interference between the two transition pathways 3P1/2-4D and 3P3/2-4D via equal-frequency hybrid excitation and collision-aided radiative excitation in an Na2-Na system is experimentally observed.A good agreement between the theoretical and experimental results is obtained.