Knowledge Bank

The first spectroscopic observations of $O2-Ar$ and $N2-Ar$ Van der Waals ‘ molecules were achieved with a long path length, cryogenic cell. Both spectra exhibit fine structure corresponding to hindered internal rotation of $O2$ or $N2$, within the weakly bonded complex. The internal rotors of lowest rotational energy are found to be trapped by the angular part of the intermolecular potential and exhibit librational bands in combination with an infrared active fundamental. A quantum mechanical treatment of internal rotation of Van der Waals ‘ molecules (discussed elsewhere in the symposium, Paper W4) together with these experimental data provides a barrier height as well as an equilibrium configuration for both $O2-Ar$ and $N2-Ar$. The equilibrium intermolecular distances are estimated from the rotational envelopes of fundamental bands. The analysis reveals the promiment effects of rotational dissociation and anharmonicity on the absorption band shapes.