Ultrafast rearrangement of norbornene excited at 200 nm

By exciting norbornene (bicyclo[2,2,1]heptene) in the gas phase at 200 nm and probing it by nonresonant multiphoton ionization with mass-selective detection of the ion yields, we found five time constants tau (i) (30, 60, 52, 800 fs, and 92 ps). tau (1)-tau (3) represent traveling times through observation windows on excited surfaces, whereas tau (4) and tau (5) reflect processes in the hot ground state. We assign tau (1) to departure from the Franck-Condon region of the pi pi* state populated at 200 mn, and tau (2) to traveling along the pi pi* surface. To explain the subsequent window (tau (3)), we suggest that the pi pi* surface is crossed by the zwitterionic state Z and that the two known carbene products form from this state. The carbenes then stabilize on the ground-state surface within tau (4) and tau (5) to form the two known photoproducts. - The fragmentation pattern showed that cyclopentadiene was not detected until at least 600 ps. Hence the retro-Diels-Alder reaction, known to be allowed in the ground state, probably takes longer.