Vibrational Relaxation Dynamics of AN Infrared Laser-Excited Molecular Impurity Mode in Alkali Halide Lattices.

The vibrational relaxation dynamics of spherical top ReO(,4)('-) molecules have been investigated under conditions of nonequilibrium laser excitation. Previous studies of relaxation mechanisms for molecular impurity modes in crystalline solids have chiefly utilized the techniques of low-power, linear spectroscopy. In this work, the nonequilibrium techniques of incoherent laser saturation and high resolution hole-burning spectroscopy have been used to measure the relaxation times T(,1) and T(,2) for the inhomogeneously broadened (nu)(,3) internal mode of ReO(,4)('-) as functions of temperature and host lattice. In addition, long-lived non-photochemical holes (with lifetimes greater than 10 minutes at 1.4 K) have been observed, indicating for the first time the presence of ground state optical pumping processes for a high symmetry impurity in a cubic crystal. The CO(,2) laser saturation measurements yielded values for the saturation intensity, I(,s), and hence the T(,1) T(,2) product in various hosts. The hole-burning measurements of T(,2) utilized a CO(,2) laser as a saturating pump and either a Pb salt diode laser or another CO(,2) laser as the tunable probe. Holes as narrow as 10 MHz (FWHM) were observed in inhomogeneously broadened lines extending over a frequency interval of several cm('-1). Above 10K, the dominant dephasing (T(,2)) mechanism is acoustic phonon scattering, while below 10K, T(,2) achieves the fundamental upper limit of 2T(,1) signifying that dephasing is lifetime-limited. Study of the alkali halide dependence of the (nu)(,3) excited state decay rate suggests that a combination of molecular internal modes, local modes of the lattice-impurity complex, and band phonons represents the dominant decay channel, rather than the previously expected multiphonon decay. The discovery of long-lived holes for a spherical top molecule at a cubic substitutional site suggests that the molecular ground state consists of at least two quasi-equivalent orientations, which are coupled only by excitation to the (nu)(,3) mode excited state. These studies of the vibrational relaxation dynamics of the ReO(,4)('-) molecule in alkali halides demonstrate the power of nonequilibrium laser techniques and provide a basis for future investigations of molecular impurity modes in solids.