Ultrafast dynamics of the photochemical ring opening of 1,3-cyclohexadiene studied by multiphoton ionization

Femtosecond time-resolved studies of the ring opening of 1,3-cyclohexadiene are presented. After absorption of a single UV photon in the region 250-270 nm (1A(1) --> 1B(2) transition), the reaction is investigated using time-delayed single- and multiphoton ionization with probe pulses in the region 250-415 nm. Ions are detected by a time-of-flight mass spectrometer. The parent ion is only observed during the time when the pump and probe lasers overlap. The corresponding ionizable state, which we identify as 1B(2), has a short lifetime which we estimate to be 60 fs. The only signal detected after a delay is due to C6H7+. It arises from dissociation of vibrationally hot parent ions which are produced by two-photon ionization of vibrationally excited products. The appearance rate constant of the product is as high as 1.7 +/- 0.2 ps(-1). It is assigned to the transition from the 2A(1) to the 1A(1) surface via a conical intersection, i.e. to the photochemical ring opening of 1,3-cyclohexadiene to Z-hexatriene. Measuring fragment ions generated by photoionization at suitable wavelengths may be a general method for monitoring vibrationally hot neutral molecules.