Two-color zero kinetic energy photoelectron spectra of benzonitrile and its vanderWaals complexes with argon. Adiabatic ionization potentials and cation vibrational frequencies

Benzonitrile and its van der Waals (vdW) complexes with argon (benzonitrile-Ar and benzonitrile-Ar-2) have been studied in supersonic jets by two-color zero-kinetic-energy (ZEKE) photoelectron spectroscopy for the first time. From the (1+1') ZEKE photoelectron spectra obtained via the first electronic excited state (S-1), we have determined adiabatic ionization potentials as I-a(benzonitrile) = 78 490 +/- 2 cm(-1) (9.7315 +/- 0.0002 eV), I-a(benzonitrile-Ar) = 78 241 +/- 4 cm(-1) (9.7007 +/- 0.0005 eV), and I-a(benzonitrile-Ar-2) = 78 007 +/- 4 cm(-1) (9.6716 +/- 0.0005 eV). These values indicate that the decrease in ionization potential due to the complex formation (Delta I-a = -249 and -483 cm(-1), respectively) is very large compared with other monosubstituted benzenes. Eleven low-frequency vibrational modes of the bare benzonitrile cation below 1300 cm(-1) have been identified in the ZEKE photoelectron spectra. The in-plane C-C=N bending mode (33(+)) and the out-of-plane C-C-C bending mode (22(+)) have been clearly identified from the ZEKE photoelectron spectra obtained via the S-1 vibrational levels 33(1) and 22(2), respectively. Furthermore, in the benzonitrile-Ar and -Ar-2 spectra, we have observed well-resolved vibrational progressions with very low frequencies of 12 and 9 cm(-1), respectively, which may be assigned to the vdW bending mode (b(x)(+)) and the vdW symmetric bending mode (b(xx)(+)) along the substituent axis, respectively.