Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n'arrivez pas à les repérer, communiquez avec nous à PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. Questions? Contact the NRC Publications Archive team atPublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. If you wish to email the authors directly, please see the first page of the publication for their contact information. NRC Publications Archive Archives des publications du CNRCThis publication could be one of several versions: author's original, accepted manuscript or the publisher's version. / La version de cette publication peut être l'une des suivantes : la version prépublication de l'auteur, la version acceptée du manuscrit ou la version de l'éditeur. June 14, 2007; In Final Form: August 21, 2007 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 R, -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 S 1 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 S 1 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 S 1 lifetime differences are caused by specific dynamical factors that affect the efficiency of passage through the S 1 /S 0 conical intersection.