Free-electron-laser studies of the relaxation of H- and D- local modes in CaF2, SrF2, and BaF2

We report a three-pulse, pump-probe experiment on H- and D- local modes in CaF2, SrF2, and BaF2 using the Dutch free electron laser, FELIX. The 10-K lifetimes of the H- local modes were measured as 43 +/-5, 170 +/- 15, and 212 +/- 20 ps for CaF2, SrF2, and BaF2, respectively. For D-, the corresponding 10-K lifetimes are 3.8 +/-0.5, 10.5 +/-0.5, and 11.6 +/-0.5 ps. Three-phonon anharmonic decay accounts for the temperature dependence of the H- lifetimes, while two-phonon anharmonic decay is expected to apply for the D- lifetimes. However, the D- in BaF2 lifetime is anomalous in requiring the three-phonon decay occasioned by a close resonance of this D- mode with a three-phonon lattice peak. From a combination of lifetime and linewidth measurements, the pure dephasing rates of these hydrogenic local modes have been determined and well accounted for by elastic band-phonon scattering processes.