Abstract

We have performed pump-probe, free induction decay and photon echo experiments on hydride and deuteride localised modes of vibration in alkaline earth fluoride crystals using the dutch free electron laser (FELIX) in Nieuwegein, the Netherlands. Using a balanced, three beam pump-probe technique we have determined the population decay times of the substitutional H^- localised vibrational mode of singly ionised hydrogen in CaF₂, SrF₂ and BaF₂ single crystals. From the temperature dependence of the decay rate, we determine that the decay of the CaF₂ :H^- mode may be accounted for by anharmonic coupling to lattice band phonon modes. However, for both SrF₂ and BaF₂ crystals anharmonic coupling can only account for the temperature dependent behaviour up to a temperature of approximately 150 K. Beyond this it is necessary to invoke elastic phonon scattering process, which are shown to contribute strongly at higher temperatures. By contrast, two phonon anharmonic coupling can account for the decay of analogous modes in both CaF₂ :D^- and BaF₂ :D^- crystals, SrF₂ :D^- being inaccessible due to the superposition of the D^- fundamental upon a three phonon TO lattice band mode. In all cases, the derived anharmonic parameters from the fits are in reasonable agreement with previous results from studies of the vibrational linewidths.

© 2000 IEEE