Anisotropic Intermolecular Force Effects in Spectra of H2– and D2–Rare-Gas Complexes

Spectra of H₂–Ar, H₂–Kr, and H₂–Xe Van der Waals complexes, accompanying the $Q_1{\text{(0), S}}_1{\text{(0), Q}}_1\text{(1)}$ , and $S_1\text{(1)}$ transitions of the pressure‐induced fundamental absorption band of hydrogen, have been studied in a path length of 165 m at temperatures in the range 85–158°K. At the low total gas densities used, 1–2 amagat, the lifetimes of the complexes were sufficiently long to show a great deal of new detail in the spectra. The $R$ and $P$ branches ($\text{δl\hspace{0.17em}=\hspace{0.17em}±\hspace{0.17em}1}$ , where $l$ is the angular momentum of the complex) accompanying the overlap‐induced $Q_1\text{(0)}$ transitions, could be analyzed on a nonrigid rotation model to give Lennard‐Jones $\epsilon$ and $\Sigma$ parameters for the molecular pairs. The spectra accompanying the quadrupole‐induced $S_1{\text{(0), Q}}_1\text{(1)}$ , and $S_1\text{(1)}$ transitions show $T$ and $\text{N (δl\hspace{0.17em}=\hspace{0.17em}±\hspace{0.17em}3)}$ as well as $P$ and $R$ branches, and many of the lines are split by the anisotropy of the intermolecular forces. The splitting is qualitatively in agreement with a model with loose coupling between J, the angular momentum of the H₂ molecule, and 1. Well‐resolved spectra of D₂–Ar, D₂–Kr, and D₂–Xe complexes accompanying the $S_1\text{(0)}$ transition of deuterium were also obtained; these show more bound states than the H₂–rare‐gas spectra and anisotropic interaction effects are evident. The H₂–N₂ complex shows a spectrum with a diffuse structure not previously observed.