VIBRATIONAL COHERENCE DECAY AND POPULATION RELAXATION IN WEAKLY DISORDERED IONIC MOLECULAR-CRYSTALS

Picosecond coherent Raman measurements on ionic molecular crystals with a large energy gap in the vibrational density of states (i.e., the uranyl ion) are used to investigate vibrational relaxation mechanisms. Vibrational decay from atop the vibrational bottleneck (i.e., v1 = 830 cm-1) at low temperature requires high-order anharmonic terms in the crystal potential. However, at elevated temperatures vibrational decay is found to proceed predominantly by a cubic anharmonic mechanism in which the initially populated level, v1, absorbs a thermally populated phonon and up-converts to a higher lying vibrational level. The energy bottleneck results in an extremely long vibrational lifetime (e.g., greater-than-or-equal-to 3 ns) at low temperature even though there are 13 intramolecular vibrations, as well as the lattice modes, lower in energy. Due to its long vibrational lifetime, v1 is extremely sensitive to inhomogeneous environmental effects and our picosecond CARS results indicate a number of energy splittings on the order of 0.03 cm-1 for this vibrational mode due to the unique nature of crystal dislocations in these crystals.