The à 2Σ+ state of Ar⋅NO studied using resonance-enhanced multiphoton and zero-kinetic-energy pulsed-field ionization spectroscopy

Abstract

Resonance-enhanced multiphoton ionization (REMPI) and zero-kinetic-energy pulsed-field ionization (ZEKE-PFI) techniques have been used to study the spectroscopy of the à 2Σ+ state of Ar⋅NO. à 2Σ+–X̃ 2Π REMPI spectra of Ar⋅NO have been interpreted using two different models that have allowed assignment of the observed rovibronic transitions to be made and an estimate of the strength of the à state intermolecular potential to be determined. Simulations of the spectra have indicated that the ground vibrationless level of the à state has a linear geometry while some higher vibrational levels have a skewed T-shaped structure. The analysis has also confirmed that the à state of Ar⋅NO is very weakly bound and is non-Rydberg in character. Reported for the first time are ZEKE-PFI spectra observed via the intermediate à 2Σ+ state. No structure was detected close to the ionization threshold, although peaks observed at higher excitation energy have been assigned to transitions to high-lying vibrational levels of the Ar⋅NO+ ion. ZEKE-PFI spectra recorded via the ground vibrational level of the intermediate à state, which has a linear structure, show progressions which correlate with transitions to highly excited van der Waals stretching and bending levels; however, spectra recorded via an intermediate level with a T-shaped geometry show progressions which correlate principally with the van der Waals stretching mode. These assignments have been interpreted in terms of the large changes in intermolecular bond length and bond angle upon ionization and fully support the interpretation of the REMPI spectra. The results of the REMPI and ZEKE-PFI spectra have also been used to investigate the character of the à state and the geometry that the complex adopts in each of the vibronic levels of this state has been rationalized in terms of the possible interactions involved.