IR-REMPI double resonance spectroscopy: the near-IR spectrum of NO-Ar revisited.

We describe a new approach to IR-UV double resonance spectroscopy of NO-containing van der Waals complexes. The basic idea combines REMPI detection through a hot band transition with a simultaneous frequency scan of the IR and UV lasers in such a way that the combined photon energy is kept constant throughout the scan, matching a UV resonance transition in the system. As a result, the two-dimensional frequency problem is reduced to a fixed number of one-dimensional frequency scans, each defined by a particular photon energy sum. The method is applied to the near-IR spectrum of NO-Ar using hot band detection via the electronic A state of the complex. In the frequency range from 3718 to 3765 cm(-1), we recorded the previously known vibrational bands with improved frequency resolution. The increased sensitivity of the present experiment allowed us to measure for the first time their overtone, combination, and hot bands. Through the comparison with results of a close-coupling (CC) calculation, we were able to assign most of the rovibrational structures of the spectrum. Except for the first intermolecular stretch level, the band positions and rotational structures of the observed bands are in good agreement with the predictions of the CC calculations.