Effect of Atomic Collisions

F. W. Sohon

M. Pinard, Franck Laloë

In optical pumping experiments, metastability exchange collisions are used to transfer orientation, alignment, ... between different atomic levels. This article studies the effect of such collisions on the atom internal variables density operator by a method used in a previous publication for spin exchange collisions. The calculations are valid when the nuclei of the two atoms are distinguishable as well as when they are identical particles, which allows a detailed discussion of nuclear identity effects (apparent magnetic field, etc...). Two cases are successively studied : no depolarization of the electronic angular momentum (He*-He collisions) and partial depolarization (Ne*-Ne collisions for example). The nuclear identity effects should be observable in low temperature optical pumping experiments with noble gases. In an Appendix, another particle identity effect is studied, which can be observed in atomic beam experiments : during the collision of two 3He atoms, both in the ground state, the spin state of the atoms scattered in a particular direction can be changed by nuclear indistinguishability effects.

A. Ben‐Reuven

General expressions are derived for the spectral line shapes of resonance absorption and scattering of coherent radiation in collision-broadened gases, taking into account effects of coherent excitation of two or more atoms (or molecules), as steady-state solutions of a hierarchy of master equations described in a previous publication (paper I). Coupling between the coherent motions of the atoms, provided by a Bethe-Salpeter---type effective interaction, in the binary-collision approximation, forms the essential mechanism for introducing cooperative coherent effects into the steady-state spectra. Explicit expressions are given for the effects of two-atom coherence in the binary-collision approximation, in which the Bloch-type dressed-atom self-energy superoperator is modified by the presence of collisions in which both atoms retain memory of their coherent propagation before the collision. The self-energies include the effects of resonance exchange symmetrization in self-broadening, and are renormalized by the coincidence of radiative transitions during the collisions. The impact (near-resonance) and the quasistatic (line-wing) limits of the applied-frequency detunings are discussed. In the quasistatic limit, coherent many-atom excitations become irrelevant; however, interactions of both collision partners with the radiation during the collision accounts for such phenomena as collision-induced absorption or radiative collisions. In the impact limit, the inclusion of the Bethe-Salpeter interactions allows for the appearance of two-atom resonances. Magnitude estimates of these effects are discussed. Effects of higher-rank (many-body) coherences are formally discussed with the help of a diagrammatic method, leading into implicit bootstrap equations that can be solved by iterative or other procedures.

Joachim Burgdörfer

Борис М. Смирнов

Igor V. Yevseyev, Valery M. Yermachenko, Самарцев Виталий Владимирович

The effect of atomic collisions on the optical characteristics of an ensemble of excited atoms is well known [ 1 , 2 ]. The evolution of laser techniques and the resulting interest in various nonlinear-spectroscopy methods have suggested that atomic collisions have a significant effect on the lasing mode of gas lasers and the shape and width of various nonlinear signals. For a long time attention has been focused on the important role of elastic collisions, among other things, and various methods have been proposed for describing such collisions [ 3 ].