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 τ i (30, 60, 52, 800 fs, and 92 ps). τ 1−τ 3 represent traveling times through observation windows on excited surfaces, whereas τ 4 and τ 5 reflect processes in the hot ground state. We assign τ 1 to departure from the Franck−Condon region of the ππ* state populated at 200 nm, and τ 2 to traveling along the ππ* surface. To explain the subsequent window (τ 3), we suggest that the ππ* 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 τ 4 and τ 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.