Femtosecond photochemical ring opening of 1,3-cyclohexadiene studied by time-resolved intense-field ionization

We found that hydrogen ion formation due to multielectron dissociative ionization by an intense-laser field is much less efficient with 1,3-cyclohexadiene than with its isomer 1,3Z,5-hexatriene (Z-HT). Moreover by suppressing the ionization barrier an intense-laser field ejects electrons most efficiently from molecular states of low ionization potential. After pumping 1,3-cyclohexadiene at 267 nm to its 1B(2) state we probe the system by intense-laser field ionization with delayed 800 nm pulses. Monitoring of the parent ion C6H8+, of the main fragment C6H7+ and of H+ allows us to follow the motion from the 1B(2) surface to the dark 2A(1) state and from there towards the 2A(1)/1A(1) conical intersection to the ground-state surface of the product. The measured 1B(2) and 2A(1) lifetimes are 43+/-3 and 77+/-7 fs, respectively, and the primary photoproduct cZc-HT is produced within 200 fs. (C) 1997 American Institute of Physics.