Two-color (2+1′) multiphoton ionization threshold photoelectron study of the Ar-NO van der Waals complex: Observation of intermolecular vibrational progressions of the Ar-NO⁺ cation

Two-color (2 + 1′) multiphoton ionization high-resolution threshold photoelectron spectroscopy has been applied to the Ar-NO van der Waals complex in a supersonic free jet. The resonant ionization process studied may be expressed by Ar-NO(X ²Π_1/2,υ″ = 0) + 2hν₁ → Ar-NO*(C ²Π,υ′) + hν₂ → Ar-NO⁺ (X ¹Σ ⁺,υ⁺). Two strong anharmonic vibrational progressions have been observed in the threshold photoelectron spectra having frequencies of 79 and 94 cm^-1. These vibrations have been assigned to the intermolecular bending and stretching motions of the (Ar-NO) ⁺ complex cation. From the threshold photoelectron spectra, the adiabatic ionization potential of Ar-NO has been determined as 73 869 ± 6 cm ^-1 (86 cm^-1 higher than previously reported), and the dissociation energy of the (Ar-NO)⁺ cation has been calculated as 951 cm^-1. The structural change of Ar-NO that occurs on photoionization has been calculated with the aid of simple Franck-Condon calculations, which suggest that the intermolecular bond distance of Ar-NO decreases by 1.03 Å, while the O-N-Ar angle decreases by 11.3°.