Vibrational Relaxation of Diatomic Molecules in Ionic Crystals

This thesis addresses the fundamental experimental question of how an excited vibrational level of a molecule couples to an ordered solid-state environment. The study focuses on diatomic molecules, eliminating intramolecular relaxation to other vibrational levels.

Cyanide has long been believed to be a rather weakly coupled defect, exhibiting free-rotor sidebands at high temperatures. However, the spectrum cannot be described using the gas-phase value of the rotational constant. New isotope measurements are performed here which establish that the rotations are coupled to elastic lattice distortions resulting from the asphericity of the molecule. In contrast, no rotor branches are apparent in the IR spectra of silver-halide-doped CN. These systems are discovered to have two translational local modes. Laser-induced fluorescence experiments establish that the decay of cyanide in AgCl and AgBr occurs on a microsecond timescale, 1000 times faster than in the potassium, rubidium, and cesium halides. It is found that this enhancement can be described by an energy gap law, with the highest-frequency local phonon constituting the accepting mode. The same coupling parameters also fit the alkali halide bulk-phonon decay. A local mode is discovered for the sodium halides, explaining their short lifetimes.

Alkali-halide doped chalcogen hydrides are found to have vibrational linewidths as narrow as 0.015 cm at 1.7 K. A high-frequency librational sideband is identified for SH, SeH, and TeH, analogous to a previously-observed line for OH. Unexpectedly, it is found to melt away with a characteristic temperature of ~90 K. Hydrosulfide is persistently hole burned for the first time, by double-doping the host. The burning occurs by defect reorientations during vibrational relaxation. Incoherent laser saturation experiments are performed on these non-fluorescent molecules, using a difference-frequency system. The vibrational lifetimes turn out to be extremely short, 0.3-3 ns, and correlate to the librational modes of the impurities. No significant dependence is found on internal strain fields and the rate decreases with increasing temperature. The final results point to the need to develop more sophisticated theories of microscopic coupling before any defect-lattice system can be described accurately.

Dissertation Abstracts International 55, 4920-B (1995).
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