Spectroscopy and femtosecond dynamics of the ring opening reaction of 1,3-cyclohexadiene

The early stages of the ring opening reaction of 1,3-cyclohexadiene to form its isomer 1,3,5-hexatriene, upon excitation to the ultrashort-lived 1 B-1(2) state, were explored. A series of one-color two-photon ionization/photoelectron spectra reveal a prominent vibrational progression with a frequency of 1350 cm(-1), which is interpreted in a dynamical picture as resulting from the ultrafast wave packet dynamics associated with the ring opening reaction. Photoionization in two-color three-photon and one-color four-photon ionization schemes show an ionization pathway via the same ultrashort-lived 1 B-1(2) state, and in addition, a series of Rydberg states with quantum defects of 0.93, 0.76, and 0.15, respectively. Using those Rydberg states as probes for the reaction dynamics in a time-resolved pump-probe experiment provides a direct observation of the elusive 2 (1)A(1) state that has been implicated as an intermediate step between the initially excited 1 B-1(2) state and the ground electronic state. The rise and decay times for the 2 (1)A(1) state were found to be 55 and 84 fs, respectively. (c) 2006 American Institute of Physics.