Abstract
Femtosecond time-resolved photoelectron spectroscopy and high-level theoretical calculations were used to study the effects of methyl substitution on the electronic dynamics of the α,β-enones acrolein (2-propenal), crotonaldehyde (2-butenal), methylvinylketone (3-buten-2-one), and methacrolein (2-methyl-2-propenal) following excitation to the S 2 (ππ*) state at 209 and 200 nm. We determine that following excitation the molecules move rapidly away from the Franck-Condon region, reaching a conical intersection promoting relaxation to the S 1 (nπ*) state. Once on the Si surface, the trajectories access another conical intersection, leading them to the ground state. Only small variations between molecules are seen in their S 2 decay times. However, the position of methyl group substitution greatly affects the relaxation rate from the Si surface and the branching ratios to the products. Ab initio calculations used to compare the geometries, energies, and topographies of the S 1 /S 0 conical intersections of the molecules are not able to satisfactorily explain the variations in relaxation behavior. We propose that the Si lifetime differences are caused by specific dynamical factors that affect the efficiency of passage through the S 1 /S 0 conical intersection. © 2007 American Chemical Society.