Structural dynamics of photochemical reactions probed by time-resolved photoelectron spectroscopy using high harmonic pulses

Femtosecond ring-opening dynamics of 1,3-cyclohexadiene (CHD) in gas phase upon two-photon excitation at 400 nm (=3.1 eV) was investigated by time-resolved photoelectron spectroscopy using 42 nm (=29.5 eV) high harmonic photons probing the dynamics of the lower-lying occupied molecular orbitals (MOs), which are the fingerprints of the molecular structure. After 500 fs, the photoelectron intensity of the MO constituting the C=C sigma bond (sigma(C=C)) of CHD was enhanced, while that of the MO forming the C-C sigma bond (sigma(CC)) of CHD was decreased. The changes in the photoelectron spectra suggest that the ring of CHD opens to form a 1,3,5-hexatriene (HT) after 500 fs. The dynamics of the sigma(C=C) and sigma(CC) bands between 200 and 500 fs reflects the ring deformation to a conical intersection between the 2(1)A and 1(1)A potential energy surfaces prior to the ring-opening reaction.