• clusters;
  • IR spectroscopy;
  • isomers;
  • molecular recognition;
  • noncovalent interactions


The S1–S0 resonant enhanced multiphoton ionization (REMPI) spectrum as well as the infrared (IR) spectra in the S0 and S1 states of 4-aminobenzonitrile (4ABN) and its van der Waals complex with Ar (4ABN–Ar) were measured by means of IR depletion spectroscopy (REMPI–IR). The IR spectrum of 4ABN–Ar in S0 shows symmetric and antisymmetric NH stretching vibrations (νs and νa) of the amino group at the same positions as those in the 4ABN monomer. This suggests that the Ar ligand locates above the benzene ring by van der Waals interactions (π-bound). The same coincidence of vibrational frequencies was found in S1, and the π-bound geometry was kept by the electronic excitation. The REMPI–IR spectrum of 4ABN+–Ar was also measured, and three major vibrational transitions were found. From the comparison to the IR dissociation spectrum with an electron impact source (EI–IR), they were assigned to νs, νa and an NH-bending overtone of the π-bound structure. It is concluded that photoionization of 4ABN+–Ar does not promote site-switching of Ar from the π-site to the H-site.