Knowledge Bank

The internal rotation of a diatomic molecule, $A2$, in an $A2$- X Van der Waals ‘ molecule can be strongly perturbed by anisotropic intermolecular interactions. Such perturbations are clearly revealed in the vibraional-rotational spectra of $O2-Ar$ and $N2-Ar$ reported elsewhere in this symposium (paper H4). A variational calculation of the internal rotatory energies will be presented in which the angular dependent intermolecular ineraction is approximated by a sinusoidal potential function. As the perturbation increases the free rotor levels are shifted and split somewhat like a Stark effect. An energy diagram corrlating the free rotor levels with bound librator levels is obtained by systematically varying the rotational barrier height. A comparison of calculated librator and hindered rotor transition energies with observed fine strucure yields he preferred geometry and an estimate of the internal rotational barrier of the $A2$ -X Van der Waals ‘ molecule.