Ultrafast dynamics of cyclohexene and cyclohexene-d(10) excited at 200 nm

By exciting cyclohexene in the gas phase at 200 nm and probing it by nonres onant multiphoton ionization with mass-selective detection of the ion yield s, we found four time constants tau (i) (20, 47, 43, 350 fs). Whereas deute ration lengthens tau (2) by a factor of 1.4, the other constants do not cha nge. tau (1)-tau (3) represent traveling times through observation windows on excited surfaces, whereas tau (4) reflects a process in the hot ground s tate. We assign tau (1) (20 fs) to departure from the Franck-Condon regions of the Rydberg and pi pi* states, which are both populated at 200 nm, and tau (2) (47 fs) to traveling along the pi pi* surface and suggest that a [1 ,3]-sigmatropic H shift begins in this state. This rationalizes the deuteri um effect on 2. To explain why this window is followed by a process not sub ject to a D effect, we postulate that the pi pi* surface is crossed late (i .e., at low energy) by the zwitterionic state Z and that formation of a car bene (the known photochemical product, cyclopentylcarbene) begins there. Th e corresponding 1,2-shift of a CC bond is then (within tau (4) = 350 fs) la rgely reversed on the ground-state surface, while a smaller part of the car bene forms products such as methylenecyclopentane within the same time. Car bene formation is probably accompanied by some cis-trans isomerization. The wavelength dependence of carbene formation is attributed to a memory for t he initially excited state, based on momentum conservation. The processes a re most likely typical of simple olefins. The fragmentation pattern showed that butadiene is not formed until at least 500 ps. The retro-Diels-Alder r eaction, known to take place in the ground state, thus only occurs later.