Ultrafast Ring Closure Reaction of Gaseous -Stilbene from S(ππ*).

-Stilbene (-St) is a well-known benchmark system for - photoisomerization. -St also produces 4a,4b-dihydrophenanthrene (DHP) in solution with a quantum yield of less than 0.19. The ring closure reaction, however, has never been identified for gaseous -St, and a recent computational simulation predicted the quantum yield of DHP to be only 0.04. In the present study, we identified an ultrafast ring closure reaction of gaseous -St for the first time using extreme ultraviolet time-resolved photoelectron spectroscopy. Surface hopping trajectory calculations at the SA3-XMS-CASPT2(2,2) level of theory reproduce the features of the observed time-resolved photoelectron spectra and predict the -St:DHP:-St branching ratio to be 0.55:0.41:0.04, in contrast with previous estimates. The results indicate that photoexcited -St favors ring closure over - isomerization under the isolated condition.