a Picosecond Laser Study of the Vibrational Quasicontinuum of Polyatomic Molecules.

A systematic study of the collisionless, unimolecular absorption of infrared radiation in the dense vibrational quasicontinuum (QC) of polyatomic molecules is presented. Previous studies of QC absorption had been plagued by interference due to initial discrete level excitation. The dual-pulse, pump-probe experiments reported in this thesis enable the measurement of absorption by molecules unambiguously populated into the QC. The three polyatomic molecules studied were SF(,6), C(,2)F(,5)Cl and C(,3)F(,7)I. Small-signal absorption measurements of collisionless absorption in the QC of polyatomic molecules revealed an increase in the small-signal absorption cross-section ((sigma)(,o)) with decreasing optical free induction decay picosecond laser pulse duration. Molecules with lower vibrational degrees of freedom exhibited a greater enhancement. The breakdown of the energy fluence scaling law is interpreted in terms of intramolecular energy nonequilibration on a timescale of the order of tens of picoseconds. The broad spectral features in the QC of the three molecules studied have also been systematically analyzed with reference to their variation with laser-molecule interaction time. In all the cases, a spectral narrowing with decreasing picosecond interaction time was observed. The implication of this observation in terms of a dynamical intramolecular coupling of energy is discussed. An empirical model yields the time constant for such a coupling to be >20 ps for all the molecules studied, confirming nonrandomization of deposited energy within the molecule on such timescales. A higher density of available states demonstrated increased initial spectral widths and faster coupling rates. A comparative study of collisional and collisionless QC spectra revealed a collision-induced spectral red-shift and broadening. A higher density of states exhibited increased collision-induced broadening. All the experimental observations in this thesis are consistently explainable based on a dynamic multi-tier classification of energy levels in polyatomic quasicontinua. A detailed, two-level Bloch equation, based on such a scheme, to model small-signal absorption, is used to corroborate calculationally the observed dependence of (sigma)(,o) on the picosecond pulse duration. The possibility of nonstatistical photochemistry, due to intramolecular energy nonrandomization for tens of picoseconds as evidenced in this work, is envisaged as an interesting possibility.