Free-electron-laser studies of the relaxation of H- and D- local modes in CaF2, SrF2, and BaF2 - art. no. 144303

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 l ifetimes of the H- local modes were measured as 43 +/-5, 170 +/- 15, and 21 2 +/- 20 ps for CaF2, SrF2, and BaF2, respectively. For D-, the correspondi ng 10-K lifetimes are 3.8 +/-0.5, 10.5 +/-0.5, and 11.6 +/-0.5 ps. Three-ph onon anharmonic decay accounts for the temperature dependence of the H- lif etimes, while two-phonon anharmonic decay is expected to apply for the D- l ifetimes. However, the D- in BaF2 lifetime is anomalous in requiring the th ree-phonon decay occasioned by a close resonance of this D- mode with a thr ee-phonon lattice peak. From a combination of lifetime and linewidth measur ements, the pure dephasing rates of these hydrogenic local modes have been determined and well accounted for by elastic band-phonon scattering process es.