Anharmonic vibrational relaxation dynamics for a molecular impurity mode in alkali halide crystals

High-resolution diode-laser spectroscopy and the nonequilibrium techniques of incoherent laser saturation and transient hole-burning spectroscopy have been used to measure the relaxation dynamics (specifically T1 and T2) of the 3 internal vibrational mode of ReO4- molecules as functions of temperature and host alkali halide lattice. Inhomogeneously broadened linewidths less than 0.02 cm-1 were observed in annealed crystals at low temperatures. To retrieve the homogeneous linewidths from the inhomogeneously broadened lines, hole-burning measurements of T2 were performed with the use of a CO2 laser as a saturating pump and either a Pb-salt diode laser or another CO2 laser as a tunable probe. Holes as narrow as 10 MHz (full width at half maximum) were observed in inhomogeneously broadened lines several cm-1 wide. Excited-state lifetimes T1 were measured by CO2-laser saturation measurements, which provide values for the saturation intensity Is and hence the product T1T2. Above 10 K the dominant dephasing (T2) mechanism is acousticphonon scattering, whereas below 10 K T2 achieves the fundamental upper limit of 2T1, signifying that dephasing is lifetime limited. The results of a systematic study of the alkali halide host lattice dependence of the excited-state decay rate at low temperatures show that the decay channel consists of multistep emission of lower-energy internal modes, localized phonon modes, and band phonons. © 1984 The American Physical Society.