The information content of ultrafast observables: spectroscopy and scattering in excited norbornadiene.

Abstract

We assess the sensitivity of time-resolved observables for characterising ultrafast nonadiabatic reaction dynamics. Using the ultrafast (<100 fs) photoisomerisation of norbornadiene as a testbed, we compare the observables for trajectory surface hopping simulations carried out at four different levels of electronic structure theory, SA-CASSCF and XMS-CASPT2 with (2,2) and (4,4) active spaces, resulting in four different sets of dynamics with distinct lifetimes and branching ratios. For each simulation, we calculate time-resolved photoelectron spectra, and electron- and X-ray scattering signals. The observables are critically evaluated and the advantages and disadvantages of each observable for characterising the excited state dynamics are discussed, including the level of theory required to accurately predict the observables and links to recent and future experiments. Broadly, photoelectron spectroscopy tracks electronic character and internal conversion, while X-ray and electron scattering resolve structural dynamics with the potential for additional information relating to electronic structure. We conclude that multimodal detection is required to uniquely constrain the underlying molecular dynamics.

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