Ultrafast Photodissociation Dynamics and Nonadiabatic Coupling Between Excited Electronic States of Methanol Probed by Time-Resolved Photoelectron Spectroscopy

Abstract

The electronic and nuclear dynamics in methanol, following 156~nm photoexcitation, are investigated by combining a detailed analysis of time-resolved photoelectron spectroscopy experiments with electronic structure calculations. The photoexcitation pump pulse is followed by a delayed 260~nm photoionization probe pulse, to produce photoelectrons that are analyzed by velocity map imaging. The yield of mass-resolved ions, measured with similar experimental conditions, are found to exhibit the same time-dependence as specific photoelectron spectral features. Energy-resolved signal onset and decay times are extracted from the measured photoelectron spectra to achieve high temporal resolution, beyond the 20~fs pump and probe pulse durations. When combined with {\it ab initio} calculations of selected cuts through the excited state potential energy surfaces, this information allows the dynamics of the transient excited molecule, which exhibits multiple nuclear and electronic degrees of freedom, to be tracked on its intrinsic few-femtosecond timescale. Within 15~fs of photoexcitation, we observe nuclear motion on the initially bound photoexcited 2$^{1}$A$''$ (S$_2$) electronic state, through a conical intersection with the 1$^{1}$A$'$ (S$_3$) state, which reveals paths to photodissociation following C--O stretch and C--O--H angle opening.