Resonant Secondary Emission by Impurities in Crystals

Abstract

Problems of resonant Raman scattering have been among the essential ones in light scattering studies for quite a long time. New horizons, opened by tunable lasers, stimulated remarkable growth of activities in this field (see, e.g. (1)). In the case under consideration resonance actually means that the exciting light is absorbed to some extent by the matter under study. The photons emitted by matter, after they had been captured (really absorbed) for a certain period of time, should be different from those which were not really absorbed but only scattered. These differences should be checked and studied in order to get a better understanding of the physical situation and to get more information out of experimental data. So, in comparison with nonresonant scattering the situation turns out to be more complicated and, at the same time, more informative. When matter consists of non-interacting atoms or molecules and only a pair of resonant levels is to be taken into account we have the well-known case examined by Heitler (2). The relation between the spectral widths of the excited level and that of the excitation line is decisive. If the latter is much smaller than the former, i.e. Г>>∆ω, resonant secondary emission (RSE) (light emitted by an atom) performs as scattered light; in an opposite case, RSE should be, according to Heitler, interpreted as resonance fluorescence*. In intermediate cases it is proper to speak about RSE. Note that the same classification may be formulated when comparing the duration of the excitation pulse and the lifetime of the resonant level.