title: Hexamethylcyclopentadiene: time-resolved photoelectron spectroscopy and ab initio multiple spawning simulations.
abstract: Progress in our understanding of ultrafast light-induced processes in molecules is best achieved through a close combination of experimental and theoretical approaches. Direct comparison is obtained if theory is able to directly reproduce experimental observables. Here, we present a joint approach comparing time-resolved photoelectron spectroscopy (TRPES) with ab initio multiple spawning (AIMS) simulations on the MS-MR-CASPT2 level of theory. We disentangle the relationship between two phenomena that dominate the immediate molecular response upon light absorption: a spectrally dependent delay of the photoelectron signal and an induction time prior to excited state depopulation in dynamics simulations. As a benchmark molecule, we have chosen hexamethylcyclopentadiene, which shows an unprecedentedly large spectral delay of (310 ± 20) fs in TRPES experiments. For the dynamics simulations, methyl groups were replaced by "hydrogen atoms" having mass 15 and TRPES spectra were calculated. These showed an induction time of (108 ± 10) fs which could directly be assigned to progress along a torsional mode leading to the intersection seam with the molecular ground state. In a stepladder-type approach, the close connection between the two phenomena could be elucidated, allowing for a comparison with other polyenes and supporting the general validity of this finding for their excited state dynamics. Thus, the combination of TRPES and AIMS proves to be a powerful tool for a thorough understanding of ultrafast excited state dynamics in polyenes.
authors.affiliation: Institut für Physikalische Chemie and Center for Functional Nanostructures (CFN), Karlsruhe Institute of Technology (KIT), Fritz-Haber-Weg 2, 76131 Karlsruhe, Germany. andreas.unterreiner@kit.edu.
authors.forename: T J A
authors.initials: TJ
authors.lastname: Wolf
authors.forename: T S
authors.initials: TS
authors.lastname: Kuhlman
authors.forename: O
authors.initials: O
authors.lastname: Schalk
authors.forename: T J
authors.initials: TJ
authors.lastname: Martínez
authors.forename: K B
authors.initials: KB
authors.lastname: Møller
authors.forename: A
authors.initials: A
authors.lastname: Stolow
authors.forename: A-N
authors.initials: AN
authors.lastname: Unterreiner