Prereactive evolution of monoalkenes excited in the 6 eV region

The time evolution of the first excited states of ethylene, and alkyl subst ituted ethylenes, isomers with formula C6H12, has been studied by the femto second pump probe method, using mass spectrometric detection, in the region of 6 eV (200 nm). Two cyclic alkenes of the formula C6H10 have also been s tudied. These systems exhibit a multi-exponential decay characterized by a very short time decay, ranging from 20 fs(ethylene) to 100 fs (trans hex-2- ene) and a longer decay, in the picosecond range follows for most of the al kyl isomers. The short time evolution is characteristic of wave packet moti on on a steep potential surface. The initial motion has been identified as the torsion about the CC double bond resulting from excitation of the valen ce state. The evolution of the valence excited state of excited state ethyl ene [first studied by the group of Radloff, Chem. Phys. Lett. 288, 2044 (19 97)] has been taken as a reference. The extremely rapid evolution, 20 fs, w ithout any longer temporal component is explained by the disappearance of t he wave packet from the Franck-Condon region into a conical intersection le ading to the ground state surface by reference to the theoretical calculati ons of Ohmine [J. Chem. Phys. 83, 2348 (1985)]. This motion is essentially multidimensional to reach the funnel to the ground state; it combines the t orsion about the CC double bond with a pyramidalization about one of the ca rbon atoms and/or H atom migration from one carbon to the other. Cyclic alk enes exhibit a similar behavior as ethylene with a single ultrashort decay that arises from this same mechanism. Also in the other substituted alkenes the short decay has been assigned to the wave packet motion away from the Franck-Condon region under the influence of the torsion about the double bo nd. The final longer decay could also be captured in the case of tetramethy lethylene by a 800 nm probe as the internal conversion to the ground state via a funnel more difficult to reach. These measurements emphasize the role of conical intersections which could not be brought into evidence without time dependent methods. (C) 2000 American Institute of Physics. [S0021-9606 (00)01721-9].