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 ν₃ 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.