Theory and observation of self-trapping of resonance radiation in gadolinium chloride

Self-trapping of resonance radiation in gadolinium chloride is experimentally and theoretically investigated at 77°K. The dependence of the self-reversed line shapes on crystal thickness is used to distinguish between different elementary line profiles. By comparing the observed dependence of the peak separation on crystal thickness with theory, it is shown that the lines have Lorentzian profile. This indicates that the elementary profile of the lines is determined by lifetime broadening due to relaxation processes. The mean free path and width of the lines are also obtained by this method. A direct observation of the line profile seems to be difficult in the present case. The Boltzmann equation for transport of photons in planar geometry is solved numerically for Lorentzian and Gaussian line profiles. The dependence of line shape on crystal thickness and on direction of observation is calculated. Results of calculations of the amplification of nonresonant lines accompanying the resonance emission, and the lengthening of the period of the fundamental decay mode of the resonance radiation in the crystal, are also presented.