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 Π,υ″ = 0) + 2hν → Ar-NO*(C Π,υ′) + hν → Ar-NO (X Σ ,υ). Two strong anharmonic vibrational progressions have been observed in the threshold photoelectron spectra having frequencies of 79 and 94 cm. 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 (86 cm higher than previously reported), and the dissociation energy of the (Ar-NO) cation has been calculated as 951 cm. 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°. © 1992 American Institute of Physics.