A THEORETICAL INVESTIGATION OF PROSPECTS FOR MODE SELECTIVE OVERTONE PHOTOCHEMISTRY (QUANTUM DYNAMICS)

Abstract

We present several related studies which probe the energy flow dynamics of overtone initiated photochemistry using theoretical methods. The nature of the classical dynamics for two model isomerization systems has been investigated with an assumed initially localized excitation to a high overtone vibration state of a bond oscillator. For these systems, the classical equations of motion are propagated and analyzed using nonlinear resonance theory. We find energy flows along resonance enhanced pathways leading to nonstatistical chemical reactivity. There exist, however, profound perturbations for both models due to nonresonant energy flow. It is concluded that this perturbation leads to significant complications which render mode selective photochemistry unobservable. Since most experimental investigations have been unable to identify nonstatistical behavior, these theoretical results are in agreement with the majority of previous observations. We identify therefore, the primary difficulty which prevents the observation of mode selectivity to be nonresonant energy flow. We also investigate the nature of the excitation dynamics of the overtone excitation process itself. Completely classical descriptions for this process are highly inaccurate, showing no excitation of overtone levels. Quantum mechanically, the dynamics for excitation are allowed. The overtone excitation process is thereby identified as an example of dynamic tunneling, which is slow relative to intramolecular vibrational relaxation, resulting in completely delocalized excitations at all times. It is not accurate therefore to assume an initially localized excitation, but interpretations which make such an assumption may still be reasonable if there is no competing dynamic process such as dissociation. These delocalized excitations are very eigenstate specific, which leads to yet another hypothesis. Multiple laser excitation, each laser tuned to a different excited eigenstate, will lead to coherent superpositions of these states, which may lead to destructive interference in chemically significant portions of coordinate space. As examples, we calculate that such a coherent multi-color (CMC) excitation can lead to long time localized bond excitations of high overtones for a polyatomic model and show that phosphorescence can be eliminated as a radiative channel for diatomic carbon monosulfide. Finally, overall implications of these separate studies on the nature of overtone photochemistry are discussed.