2-COLOR (2+1') MULTIPHOTON IONIZATION THRESHOLD PHOTOELECTRON STUDY OF THE AR-NO VANDERWAALS 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(2)PI(1/2), upsilon" = 0) + 2hnu(1) --> Ar-NO* (C(2)PI, upsilon') + h-nu(2) --> Ar-NO+ (X(1)SIGMA+, upsilon+). 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 angstrom, while the O-N-Ar angle decreases by 11.3-degrees.