Rotation–vibration state-resolved unimolecular dynamics of highly excited CH3O (X 2E). Part 3.—State-specific dissociation rates from spectroscopic line profiles and time-resolved measurements

Abstract

Vibration–rotation quantum-state resolved measurements of the unimolecular dissociation rates of highly vibrationally excited CH₃O (X ²E) have been performed over a wide range of excitation energies (7000 ⩽E/cm^–1⩽ 10 000). Single excited CH₃O (X) quantum states were prepared using the method of stimulated emission pumping (SEP). State-specific decay constants were determined from direct time-resolved measurements using laser-induced fluorescence detection (LIF) of the excited states and from SEP line profiles measured at higher resolution. In very narrow energy windows, the measured decay constants were found to vary statistically by up to two orders of magnitude. These state-specific fluctuations are in contrast with the traditional picture from unimolecular rate theory (e.g. RRKM theory). The fluctuations were analysed statistically. The average decay rates were found to increase with increasing molecular excitation energy. This general trend could be nicely described by an RRKM model on average. Indications for small deviations were observed at high energies. Viewed in connection with related data on the kinetics of intramolecular vibrational energy randomization (IVR) processes, these deviations may reflect the inherent limitations of statistical theory at high energies where dissociation and IVR compete.