Spectroscopy and thermodynamics of ArHCl

A general model for analysis of the infrared spectrum of atom–diatom van der Waals molecules is developed. Using energy levels and spectroscopic parameters based on the Hutson and Howard M5 potential, this model quantitatively reproduces the previously observed ArHCl spectrum in the null gap of the monomer HCl spectrum. The thermodynamics of atom–atom and atom–diatomic systems is discussed. An approximate quantum mechanical method of evaluating bound and metastable partition functions is derived. Simple expressions for the partition functions are given which are shown to accurately reproduce the well established results for atom–atom complexes. The generalization of the approach to atom–diatomics is discussed. These partition functions are used to calculate thermodynamic properties such as heat capacities and equilibrium constants.