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-4 nu H- centres using the Dutch fre e-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 rela xation. which accounts well for the temperature dependence of the decay rat e. 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 grou nd state. A simple rate equation model gives a good account of the observed transient.