Non-adiabatic Dynamics of Molecules Studied Using Vacuum-Ultraviolet Ultrafast Photoelectron Spectroscopy

Abstract

Quantitative prediction of branching ratios and photoproduct quantum yields when an excited electronic state has multiple conical intersections (CIs) with other electronic states remains a challenging problem for theoretical chemists. Experimental benchmarking of these observables is thus highly desirable for evaluating the appropriateness of various approximations made in theoretical calculations. In this chapter, we present time-resolved photoelectron spectroscopy (TRPES) using a vacuum-ultraviolet (VUV) laser as a means to observe ultrafast dynamics through CIs in real time. We describe the details of our apparatus, and discuss some representative examples from our recent studies. In the UV photochemistry of furan following ππ* photoexcitation, the ring-puckering CI dominates the dynamics, safely returning more than 90% of the excited molecules to the original ground state. The remaining 10% undergo irreversible isomerization after passing through the puckering CI. In the ultrafast photodissociation of nitromethane, we find that ππ* electronic excitation leads to ultrafast cascading internal conversion (S3 → S2 → S1 + S0) prior to dissociation into CH3 + NO2 fragments, and that the dissociation predominantly proceeds on the S1 surface, leading to comparable production of NO2(A) and NO2(X).