Intramolecular Vibrational Redistribution (IVR) of Jet-Cooled Alkylphenols and Alkylanilines as Revealed by Stimulated Emission Ion Dip Spectroscopy

Abstract

Intramolecular vibrational redistribution (IVR) is an important nonradiative process in an isolated large molecule, and it is being extensively studied experimentally and theoretically. Especially, IVR in an electronically excited state has been studied by various experimental means such as fluorescence excitation, dispersed fluorescence spectroscopies and the measurement of fluorescence life-time [1]. As a result, our information on IVR for electronically excited states is now considerably accumulated. On the other hand, the study on IVR in electronically ground-state is very few. This is due to lack of suitable experimental means. The study by infrared radiation is an orthodox way. However, because of poor time response of the infrared detection and of severe selection rule for infrared absorption, the IVR study of a ground state molecule by infrared light is greatly restricted. Instead of direct vibrational excitation by infrared light, the ground-state vibrational level of an isolated molecule can be populated by stimulated emission from an electronically excited state, say, S1 state with UV/visible laser light (v 2). One of the stimulated emission methods is “stimulated emission pumping” in which the stimulated emission from S1 to a ground-state vibrational level is monitored by the dip of the fluorescence from the S1 level pumped with v 1[2,3]. The depth of the fluorescence dip represents the decrease of the S1 state population which is determined by the loss of the S1 state population by the stimulated emission with v 2 and the gain of the population by the reabsorption with same v 2 from the ground-state vibrational level. When the life-time of the ground-state vibrational level to which the stimulated emission occurs is long, the molecules in the vibrational level have a great chance to come back to S1, resulting in a small fluorescence dip. Conversely, when the life-time (decay rate) is short (large), we have a large dip. Since the life-time of the ground-state vibrational level of an isolated molecule is in most cases determined by IVR process, we can obtain the IVR rate from the observed depth of the fluorescence dip. Therefore, the depth of the fluorescence dip measured in percentage relative to the fluorescence signal in absence of the stimulated emission provides us with very useful information on the IVR rate of ground-state vibrational level. However, the quantitative determination of the percentage fluorescence dip depth is very difficult because it requires complete spatial matching of the two laser beams v 1 and v 2. The observed percentage dip depth sensitively varies by mismatching of the two beams whose elimination is practically impossible.