Knowledge Bank

Spectra due to the Van der Waals Complex $(H2)2$ have been obtained with much higher resolution than $previously1$ possible, and analogous spectra due to $(D2)2$ and $H2-D2$ have been observed for the first time. The experiments were performed using a specially designed 3.5 m multiple-traversal cell which permits the study of absorption spectra of gases at temperatures as low as 17K. In these experiments, the absorption path was 110m. The spectra are manifested as fine structure accompanying the various $H2$ and $D2$ transitions in the collision-induced fundamental bands of hydrogen (2.4 $\mu$) and deuterium (3.3 $\mu$). The observed structure for $(H2)2$ and $H2-D2$ may be unambiguously assigned to rotational transitions governed by the selection rule $\Delta \ell =\pm 1,\pm 3$, where $\ell$ is the rotational quantum number of the complex itself. The spectrum of $(D2)2$ is more complicated and can be only partly assigned; the reasons for this will be discussed. Present address of A.R.W. McKellar: Division of Physics, National Research Council of Canada, Ottawa, Ontario, Canada