Population dynamics of H^- local modes in CaF₂: Lu³⁺ crystals studied using a free-electron laser

We report on pump-probe measurements of vibrational decay times for the H^- (X, Y) and Z local modes of CaF₂:Lu³⁺ C_4v H^- centres using the Dutch free-electron laser. For excitation of the (X, Y) mode, a 10 K lifetime of 43 picoseconds is measured and is attributable to three-phonon anharmonic relaxation, which accounts well for the temperature dependence of the decay rate. The 10 K lifetime of the higher-energy Z mode results from a combination of one-phonon decay to the (X, Y) modes and three-phonon decay to the ground state. A simple rate equation model gives a good account of the observed transient.