Time-resolved dissociative intense-laser field ionization for probing dynamics: Femtosecond photochemical ring opening of 1,3-cyclohexadiene

The concerted photochemical ring opening of 1,3-cyclohexadiene was investig ated in the gas phase by low-intensity pumping at 267 nm and subsequent pro bing by high-intensity photoionization at 800 nm and mass-selective detecti on of the ion yields. We found five different time constants which can be a ssigned to traveling times along consecutive parts of the potential energy surfaces. The molecule is first accelerated in the spectroscopic state 1B a long Franck-Condon active coordinates, then alters direction before changin g over to the dark state 2A. All constants including that for leaving the 2 A surface are below 100 fs. These times are shorter than appropriate vibrat ional periods. Such a maximum speed is evidence that the pathway is continu ous leading from surface to surface via real crossings (conical intersectio ns) and that the molecule is accelerated right into the outlet of the 2A/1A funnel. On the ground state it arrives as a compact wave packet, indicatin g a certain degree of coherence. The experimental method promises a high po tential for investigating dynamics, since many consecutive phases of the pr ocess can be detected. This is because the fragmentation pattern depends on the location on the potential energy surface, so that monitoring several d ifferent ions permits to conclude on the population flow through these loca tions. Ionization at the intensities used is normally considered to be an e ffect of the electric field of the radiation. But in our case it is enhance d by resonances in the neutral molecule and in particular in the singly pos itive ion, and it is not sensitive for the length of the molecule (differen t conformers of the product hexatriene). The ionic resonances explain why h exatriene has a much richer fragmentation pattern than cyclohexadiene. Coul omb explosion is observed from an excited state of a doubly positive ion. I ts mechanism is discussed. (C) 2000 American Institute of Physics. [S0021-9 606(00)00819-9].