Photofragmentation spectroscopy of cold protonated aromatic amines in the gas phase

The electronic spectra of cold protonated aromatic amines: anilineH(+) C6H5-NH3+, benzylamineH(+) C6H5-CH2-NH3+ and phenylethylamineH(+) C6H5-(CH2)(2)-NH3+ have been investigated experimentally in a large spectral domain and are compared to those of their hydroxyl homologues. In the low energy region, the electronic spectra are similar to their neutral analogues, which reveals the pi pi* character of their first excited state. A second transition is observed from 0.4 to 1 eV above the origin band, which is assigned to the excitation of the pi sigma* state. In these protonated amine molecules, there is a competition between different fragmentation channels, some being specific to UV excitation i.e., not observed in low-energy collision induced dissociation experiments. Besides, for one amine a drastic change in the fragmentation branching ratio is observed within a very short energy range that reveals the complex excited state dynamics and fragmentation processes. The experimental observations can be rationalized using a simple qualitative model, the pi pi*-pi sigma* model [A. L. Sobolewski, W. Domcke, C. Dedonder-Lardeux and C. Jouvet, Phys. Chem. Chem. Phys., 2002, 4, 1093-1100], which predicts that the excited state dynamics is controlled by the crossing between the pi pi* excited state and a pi sigma* state repulsive along the XH (X being O or N) coordinate.