Ionization and Dissociation of Benzene and Aniline under Deep Ultraviolet Laser Irradiation^†

We report a study on photo-ionization of benzene and aniline with incidental subsequent dissociation by the customized reflection time-of-flight mass spectrometer utilizing a deep ultraviolet 177.3 nm laser. Highly efficient ionization of benzene is observed with a weak C _4 H _3 ^+ fragment formed by undergoing disproportional C - C bond dissociation. In comparison, a major C _5 H _6 ^+\cdot fragment and a minor C _6 H _6 ^+\cdot radical are produced in the ionization of aniline pertaining to the removal of CNH ^\cdot and NH ^\cdot radicals, respectively. First-principles calculation is employed to reveal the photo-dissociation pathways of these two molecules having a structural difference of just an amino group. It is demonstrated that hydrogen atom transfer plays an important role in the cleavage of C - C or C - N bonds in benzene and aniline ions. This study is helpful to understand the underlying mechanisms of chemical bond fracture of benzene ring and related aromatic molecules.