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Journal of Physics B Atomic and Molecular Physics

Experimental evidence of coherences produced by a radiative collision process

Anne Débarre

Published Jun 14, 1982• 27 citations
Publisher

Abstract

Recently, a theoretical calculation has been presented assuming that coherences can be produced in a radiatively assisted inelastic collision. The author reports the observation of final-state coherences in the simple case of dipole-dipole interaction in a strontium-europium mixture illuminated by a pi -polarised laser field. The author has measured the polarisation ratio P of the fluorescence emitted from the final degenerate Zeeman states of strontium. The result P=0.43+or-0.05 is in good agreement with theoretical predictions P=0.45.

DOI
10.1088/0022-3700/15/11/015

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Recently, a theoretical calculation has been presented assuming that coherences can be produced in a radiatively assisted inelastic collision. The author reports the observation of final-state coherences in the simple case of dipole-dipole interaction in a strontium-europium mixture illuminated by a pi -polarised laser field. The author has measured the polarisation ratio P of the fluorescence emitted from the final degenerate Zeeman states of strontium. The result P=0.43+or-0.05 is in good agreement with theoretical predictions P=0.45.
A theoretical calculation of the final-state coherences produced by a radiatively assisted inelastic collision (RAIC) is presented. Two atoms, AAA and ${A}^{\ensuremath{'}}$, collide in the presence of an external radiation field to produce the RAIC reaction ${A}_{i}+{A}_{{i}^{\ensuremath{'}}}^{\ensuremath{'}}+\ensuremath{\hbar}\ensuremath{\Omega}\ensuremath{\rightarrow}{A}_{f}+{A}_{f}^{\ensuremath{'}}$, where $|{\mathrm{ii}}^{\ensuremath{'}}〉$ is the initial state, $|{\mathrm{ff}}^{\ensuremath{'}}〉$ is the final state, and $\ensuremath{\Omega}$ is the frequency of the external field. It is assumed that the final states consist of a number of nearly degenerate levels and the coherences produced in these levels by the RAIC reaction is calculated. These final-state coherences can be monitored by standard techniques (polarization of fluorescence, quantum beats) enabling one to use the final-state coherences as a probe of the RAIC reaction. The calculation is limited to the weak-field (perturbation-theory) limit and is valid only in the impact core of the RAIC profile.
A radiatively assisted inelastic collision (RAIC) is one in which two atoms collide in the presence of a radiation field to produce a reaction of the form ${A}^{*}+B+\ensuremath{\hbar}\ensuremath{\Omega}\ensuremath{\rightarrow}A+{B}^{*}$. In this paper, a general theory of RAIC is developed with special attention given to the final-state coherences produced by RAIC. These final-state coherences can be monitored by standard experimental techniques (polarization of fluorescence, quantum beats), enabling one to use such studies to gain information on the interatomic potentials that are relevant to the RAIC under consideration.
In a previous paper [P. R. Berman and E. Giacobino, Phys. Rev. A 28, 2900 (1983)], a method was proposed for producing electronic-state coherences using radiatively-aided inelastic collisions (RAIC). Two atoms collide in the presence of two simultaneously applied laser pulses and the combined action of the fields and the collision leads to a final-state coherence ${\ensuremath{\rho}}_{\mathrm{ij}}$ between levels iii and jjj in one of the atoms. In this paper a detailed calculation of the RAIC cross section is given, including effects of magnetic degeneracy and an average over all collision orientations. With an isotropic distribution of collision orientations and with one photon absorbed from each laser field in a single RAIC reaction, a final-state coherence ${\ensuremath{\rho}}_{\mathrm{ij}}$ is created only if states iii and jjj have the same parity. In order to produce a final-state coherence between states of opposite parity, an anisotropic distribution of collision orientations is needed. Estimates of RAIC cross sections are obtained for both isotropic and axially symmetric distributions of collision orientations. For final states having opposite parity, the RAIC reaction can lead to the production of coherent radiation at the sum or difference frequency of the applied laser fields.
An experimental study of laser-induced collisional energy transfer in a mixture of europium and strontium vapors is reported. The profile of the excitation spectrum in the static wing has been measured with high accuracy and compared with available calculations, demonstrating that simple two-level models are inadequate to describe the transfer process. A comparison with the predictions of a three-level model assuming dipole-dipole interaction, recently developed by Bambini and Berman, shows for the first time agreement between theory and experiment.
The polarisation of atomic line fluorescence resulting from the excitation of a collisional complex is calculated on the basis of a simple molecular picture for the pair of colliding atoms. The red-wing region is treated in detail as arising from excitation of both Pi and Sigma molecular states at different internuclear distances, and the blue wing is identified as predominantly due to the Sigma state. A calculation for the strontium 4607 AA line gives good qualitative agreement with the experiment.