The Ar–NO van der Waals complex studied by resonant multiphoton ionization spectroscopy involving photoion and photoelectron measurements

Using a 5% mixture of NO in Ar in a supersonic free jet, in the present work we have carried out measurements of the total ion current in the 380–385 nm laser wavelength region. We have also measured photoelectron kinetic energy spectra at individual ion current peaks. In the ion–current spectrum we have observed a new vibrational progression which consists of four peaks in the wavelength region longer than the peak of the two-photon transition of the free NO molecule NO(X, v″=0) →2hν NO(C,v′=0). It has been concluded that the new ion–current peaks are attributed to bound-to-bound transitions of the Ar–NO van der Waals complex from its ground state to the two-photon resonant state expressed by Ar–NO*(C 2Π, v′=0), in which the NO component is in the 3p Rydberg state. The whole resonant ionization process studied may be expressed by Ar–NO(X, v″=0) →2hνAr–NO*(C, v′=0) →hν Ar–NO+(X, v+=0). Each ion–current peak separation is about 50 cm−1, which may correspond to the frequency of the Ar–NO intermolecular stretching vibration, showing a strong anharmonicity. The dissociation energy (D0) of the Ar–NO*(C 2Π) state has been found to be 0.055±0.001 eV. From the photoelectron spectra, we also conclude that the adiabatic ionization energy of Ar–NO is Ia =9.148±0.005 eV and the dissociation energy of the Ar–NO+(X 1Σ) ion is D0=0.129±0.005 eV.