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. Additionally, we selectively captured the population changes in the singlet and triplet states with ionization from different laser wavelengths, 355 or 266 nm, in time-resolved photoion yields. The combination of experimental data enables us to uniquely determine the relative ionization cross sections of the singlet and triplet states at an ionization laser wavelength of 266 nm and allows us to extensively measure the rate constants of intersystem crossing and the branching fractions at various excitation wavelengths.

Communication: Mode-dependent excited-state lifetime of phenol under the S1/S2 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 S1 (ππ*) state and the dissociative S2 (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 S1/S2 conical intersection. Unexpectedly short lifetimes were observed in the S1 state for a″ symmetric vibrational modes (including v4, v16a, τOH, and v5). These results clarify recent theoretical calculations showing that the relaxation from S1 to S2 either occurs via symmetry-allowed non-adiabatic transitions or is topographically linked to a lower energy minimum on the multidimensional potential energy surface.