Collision Redistribution by Two-Atom Coherence in Weak Fields

J. Cooper, R. J. Ballagh

Resonant scattering (redistribution) of weak monochromatic light by atoms in the presence of collisions has been considered by Omont et al., who allowed for the broadening of both upper and lower levels of the transition. Mollow (who also has considered this problem for intense radiation) has shown that their results are applicable only if collisional repopulation of the levels (in contrast to collisional depopulation) is ignored. We here consider in detail the effect of repopulation. It leads, in the weak-field limit, to a true-coherent component ($\ensuremath{\delta}$-function spectrum) at the driving frequency, as well as a component at this frequency whose width is essentially the inelastic collisional rate of the lower state. An additional redistributed component due to repopulation of the upper level is also obtained.

W. Gough

Gerard Nienhuis, F Schuller

The authors study cooperative effects on the fluorescence spectrum of several atoms in an intense radiation field. In the secular approximation the spectrum contains three lines, as in the single-atom case. The authors evaluate the strengths of these lines for an arbitrary number of atoms without collisions. In the case of two atoms, collisional interactions with a perturber bath are included. The interatomic coherence in the absence of collisions leads to an enhancement of the line strengths, which is most pronounced for the central Rayleigh line. When collisions are present, the strengths of the two sidebands can actually be diminished by the cooperative effects.

H. J. Beyer, Karl Blum, R. Hippler

Henri Van Regemorter

Perturbation theory for the interaction of light with a binary system is applied to collisional redistribution of radiation in the weak-field limit. Within the very large domain of validity of the binary model applied to collisions between non-hydrogenic atoms and atomic perturbers at low densities, redistribution can be viewed as collisional-induced absorption followed by spontaneous decay during a second collision. The formalism can be applied from the quasimolecular domain at large detunings Delta omega 1 or Delta omega 2 up to the impact regime, but emphasis is placed here on the non-impact region, at small Delta omega 1 or Delta omega 2, where non-adiabatic effects are important in collision-induced radiative absorption or emission rates as well as in the redistribution. The impact limit of the redistribution function is easily rederived in this way.