Vibrational spectra of aniline-Ar_n van der Waals cations (n = 1 and 2) observed by two-color "threshold photoelectron" [zero kinetic energy (ZEKE)-photoelectron] spectroscopy

Measurements of mass-selected ion-current and threshold photoelectron spectra of jet-cooled aniline-Ar_n van der Waals complexes (n = 1 and 2) have been carried out with a two-color resonantly enhanced multiphoton ionization (REMPI) technique using a high-resolution threshold photoelectron analyzer developed in this laboratory. From our (1 + 1′) REMPI experiments via the respective excited S₁ states, we have obtained photoelectron spectra with well-resolved vibrational progressions due to "low-frequency van der Waals modes" of the cations; v_vdW = 16 cm^-1 (n = 1) and v_vdW = 11 cm^-1 (n = 2). From Franck-Condon calculations, we have assigned these low-frequency vibrations to the "van der Waals bending" of the cations. We have also found that the angles of the van der Waals bonds in the cations are changed by 8.2 (n = 1) and 8.8 (n = 2) degrees with respect to the S₁ states. The adiabatic ionization potentials (I_a) of aniline and the aniline-Ar_n complexes (n = 1 and 2) have been determined as 62 268 ± 4 cm^-1 (aniline), 62 157 ± 4 cm^-1 (n = 1), and 62 049 ± 4 cm^-1 (n = 2). Their shifts ΔI_a are 111 cm ^-1 (n = 1) and 219 cm^-1 (n = 2) with respect to aniline. Spectral shifts due to complex formation have been observed for a total of 13 ring modes of the cations.