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 (DUV) 177.3 nm laser. Highly efficient ionization of benzene is observed with a weak C4H3+ fragment formed by undergoing disproportional C−C bond dissociation. In comparison, a major C5H6+• fragment and a minor C6H6+• radical are produced in the DUV ionization of aniline pertaining to the removal of CNH* and NH* 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 (HAT) plays an important role in the cleavage of C−C or C−N bonds in benzene and aniline ions. This study helps understand the underlying mechanisms of chemical bond fracture of benzene ring and related aromatic molecules.