Interpretation of Raman spectra of van der Waals dimers in argon

The rotational and vibrational eigenenergies of all bound and predissociating states of the ${\Sigma }_g^{+}{\mathrm{Ar}}_2$ dimer are computed, together with certain averages of the nuclear wavefunctions needed for the computation of the Raman intensities for each transition. The MSV III potential of Parson, Siska, and Lee is used. With this information the complete rotational and vibration‐rotation Raman spectra of the argon van‐der‐Waals molecule is obtained and found to consist of truncated and superimposed line sequences. They are thus characteristically different from such spectra of ordinary molecules. A theoretical ``low‐resolution'' spectrum is obtained by superposition of approximated instrumental profiles for each line in the spectrum. At the lowest temperature (103°K) of the experiment nearly all of the observed intensity distribution is due to the dimer Ar₂. At high temperatures (300°K) the experimental spectrum is due largely to the scattering of light by pairs of atoms in fly‐by collisions, not to molecules. The Raman intensity maxima at ±8.3 cm⁻¹ shift, therefore, must be explained in terms of the former. The apparent insignificance of the rotation‐vibration Raman bands in comparison to the pure rotational bands can be explained by adoption of a simple model for the anisotropy of the polarizability tensor.