Structural dynamics of laser-ionized cis-stilbene studied by ultrafast electron diffraction

Stilbene has been a model system for studying photoisomerization upon absorption of a UV photon. Although the dynamics of the first excited state have been investigated in great detail previously, the structural dynamics of cationic states remains largely experimentally unexplored. In this study, the dynamics following ionization via absorption of two UV photons was captured using ultrafast electron diffraction. cis-stilbene was optically pumped with 267 nm ultraviolet light with two different pump fluences and probed with 3.7 MeV electrons. We compare our experimental results to ab-initio multiple spawning simulations (AIMS) for single-photon excitation and molecular dynamics simulations for ionization in order to separate the contributions from the two channels. We found that witha fluence of 170 mJ/cm2 both the single and two-photon channels are present, while with higher fluence of 280 mJ/cm2 the two-photon channel dominates. We did not observe isomerization of the cis-stilbene cation and found the main structural motion a to be a vibration of the phenyl rings about the central C-C bond.