Prereactive evolution of monoalkenes excited in the 6 eV region

The time evolution of the first excited states of ethylene, and alkyl substituted ethylenes, isomers with formula C6H12, has been studied by the femtosecond 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 studied. 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 alkyl isomers. The short time evolution is characteristic of wave packet motion on a steep potential surface. The initial motion has been identified as the torsion about the CC double bond resulting from excitation of the valence state. The evolution of the valence excited state of excited state ethylene [first studied by the group of Radloff, Chem. Phys. Lett. 288, 2044 (1997)] has been taken as a reference. The extremely rapid evolution, 20 fs, without any longer temporal component is explained by the disappearance of the wave packet from the Franck-Condon region into a conical intersection leading to the ground state surface by reference to the theoretical calculations of Ohmine [J. Chem. Phys. 83, 2348 (1985)]. This motion is essentially multidimensional to reach the funnel to the ground state; it combines the torsion about the CC double bond with a pyramidalization about one of the carbon atoms and/or H atom migration from one carbon to the other. Cyclic alkenes 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 bond. The final longer decay could also be captured in the case of tetramethylethylene 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].