Redistribution - Why half a collision is better than a whole one. [spectra of scattered light from perturbed atomic system]

M. G. Raymer

Jan Fiutak, S Kryszewski, S. Zielinska

J. Cooper

The study of spectral line shapes has traditionally been mainly concerned with the measurement and interpretation of absorption or emission profiles. Often only the line widths are studied. The present investigation has the objective to evaluate the additional information which can be obtained by scattering light (usually from a laser) from an atomic system which is being perturbed by collisions. A scattering experiment is discussed. The scattered light consists of two components, a (coherent) Rayleigh component and a redistributed (fluorescent) component. In order to obtain the absorption spectrum, questions regarding the probability of photon absorption are considered. By observing the fluorescence subsequent to absorption during a collision it is found possible to obtain information on the evolution of the system from the point of absorption to the completion of the collision. The information on the intracollisional evolution is the justification for the title of the study, namely 'Why half a collision is better than a whole one'.

Michael Scott Belsley

Keith R. Burnett, J. Cooper

We apply the analysis of paper I to degenerate atomic systems and calculate the effect of correlations between an atom (the absorber) and perturbers in the binary-collision approximation. The result of our calculation is a generalized absorption profile that specifies the final state of the atom an absorption event. This profile is then related to the total intensities of Rayleigh scattering and fluorescence (redistributed radiation) from the atom. The profile is expressed in terms of collision operators that depend on the index of the multipole of the atom that is created. Some of the operators represent correlations between radiative and collisional events and cannot, except in certain limits, be expressed in terms of the usual line-broadening operators. This, of course, implies that one needs extra dynamical information, on top of that obtainable from ordinary absorption experiments, to be able to describe redistribution. Conversely, redistribution offers a probe of collisional dynamics, not given by absorption experiments, via the polarization of fluorescent light. We give practical expressions for the calculation of these operators and discuss their implications.

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.