Excess Energy Dependence of Vibrational Relaxation and PhotoPhysical Branching Ratios in Isolated Aromatic Molecules: Relevance to Vacuum Ultraviolet Photochemistry

Abstract

During the last decade there has been a great deal of activity in the study of electronic relaxation in gaseous organic molecules under collision-free conditions. Early experimental investigations were primarily concerned with radiationless transitions in bulk gas at ambient and elevated temperatures [1], Although these studies have yielded some useful information concerning vibronic level dependence of electronic relaxation processes, the interpretation of some of these results has been hampered by the fact that true photo selective excitation of well-defined initial quantum states cannot, in many cases, be accomplished because of the severe spectral congestion arising from thermal inhomogeneous broadening (vibrational sequence congestion and rotational broadening). Most of the recent experimental efforts, therefore, have been directed at the study of electronic relaxation in molecules which are cooled by supersonic expansion of seeded molecular beams, and which are excited by narrow-bandwidth dye lasers. Strong vibrational and rotational cooling (resulting from the supersonic expansion) significantly reduces thermal inhomogeneous broadening, thus providing greatly simplified and resolved spectra, the dynamical properties of which may be probed by laser- induced processes (e.g., fluorescence).