Abstract:

The absorption of light by a molecule is the first step in a set of dynamical processes that can yield disparate chemical outcomes, including radiative decay (flourescence, phosphoresence), nonradiative decay (internal conversion, intersystem crossing) and/or fragmentation. In this talk, I will focus on how ultrafast nonradiative decay channels are discovered using ab initio simulation techniques.  Some details of the computational and theoretical approaches for simulating these processes, which invariably involve nonadiabatic transitions between electronic states mediated by the presence of conical intersections, will be presented.  In particular, it will be shown that the 'on-the-fly' determination of the requisite potential energy surfaces using high-level quantum chemistry methods is a useful approach for uncovering precisely which molecular motions engender fast relaxation to the ground electronic state. The relationship between time-resolved experimental results and computational simulation, and the best methods to compare the two, will also be explored.  Time-resolved photoelectron spectroscopy will be shown to be a particularly sensitive probe of the type of dynamics (i.e. vibronic) of interest and it will be shown that the direct simulation of these spectra using ab initio approaches is the most unambiguous manner in which theory can complement complex time-resolved experimental measurements.