Resonant Scattering and Trapping of 29 cm⁻¹ Acoustic Phonons in Ruby Crystals

Abstract

The phenomenon of resonant radiation trapping caused by multiple scattering is of general physical interest. In an optically dense resonant medium with the photon mean free path small compared to the size of the system, photons escape from the medium after several reabsorption events resulting in an increase of the photon residence time in the medium. This phenomenon has been well known to exist in gases, the mechanism of trapping having been considered by Holstein [1] and Biberman [2]. A distinctive feature of radiation trapping in gases consists in the existence of both Doppler and collisional broadening of spectral lines. As a result, photon frequency may vary over the line profile in each absorption and reemission event. Photons reemitted at line wings are absorbed only weakly in the medium and escape freely from the bulk, while those at line center undergo repeated absorption. Because of such a frequency transformation, radiation emerges from the bulk primarily at the wings of the line, the emission line revealing a dip at the center (self-absorption). This illustration demonstrates the importance of the nature of the scattering event and of the character of line broadening for the trapping process. In gases, because of the interaction of atoms with the bath the secondary radiation spectrum does not depend on the excitation event and corresponds to thermalized luminescence.