Triplet vs πσ state mediated N-H dissociation of aniline.

UV-excited aromatic molecules with N-H/O-H moieties often possess an important nonradiative relaxation pathway, from an optically bright ππ state to a dark dissociative πσ state. We apply a new time-selected photofragment translational spectroscopy method to disclose a previously unknown triplet-mediated N-H dissociation of aniline prevented by the multiphoton dissociative ionization in conventional methods. We further determined the branching fractions of aniline dissociated in the πσ, triplet, and ground states at 248 nm.

Communication: Mode-dependent excited-state lifetime of phenol under the S/S conical intersection.

Phenol can serve as a model for examining the deactivation of the aromatic amino acid tyrosine following UV excitation, which mainly occurs through a repulsive πσ state along the O-H bond. The reaction barrier formed by the conical intersection between the optically bright S (ππ) state and the dissociative S (πσ) state does not inhibit O-H bond rupture even though the excitation energy is below the barrier height. To examine the O-H bond-rupture dynamics in association with the initial excited vibrational modes, we used a picosecond laser to investigate the vibrational-mode-dependent excited-state lifetime of phenol under the S/S conical intersection.