Intermolecular Potentials, Internal Motions and the Spectra of Van Der Waals Molecules

Abstract

The infrared spectra of N2Ar, 02-Ar, N2-N2 and O2-O2 seem to indicate that these Van der Waals molecules possess orientationally localized (librational) states, as well as (nearly) free internal rotor states. Using the full anisotropic potential, we have calculated the rovibrational bound states of N2-N2 and N2-Ar, and also the rotational predissociation states of the latter complex, and we have evaluated the contributions of these states to the infrared spectra. Thus the infrared spectrum of N2-Ar, and in principle also the spectra of the other dimers, can be understood. The onset of the regular free internal rotor structure cannot be explained from the bound states; it is due to rather narrow (0.2 to 3 cm−1) rotational resonances lying in the collision continuum. The structure in the lower frequency part of the spectra is caused by transitions between localized librational states; especially this part will be sensitive to the detailed intermolecular potential. In order to exploit this sensitivity one has to measure the infrared spectra at very low temperature.