Knowledge Bank

Matrix-isolated hydrogen chloride has been extensively studied by infrared absorption techniques. Attention has been focused on the spectrum of both monomeric and polymeric species. We have obtained the Raman spectra of HC1 and DC1 in Ar, Kr, and Xe matrices down to $7\circ K$ I. Monomers: The interest lies in the problem of the rotation of the guest molecule. The infrared absorption spectrum of HCl in Ar at $7\circ K$ exhibits three principal lines at 2887, 2867, and $2852cm-1$ which, from temperature dependence intensity measurements, were assigned to the $R0$, Q, and $P1$ treansitions, respectively. In the Raman spectrum we observe an intense the absence of the $R0$ peak the latter cannot be assigned to $P1$. Thus we can conclude that the so-called $P1$ infrared line is the result of two transitions, one vibrational-rotational and the other pure vibrational which may be due to guest molecules trapped at a different site or to non first nearest neighbor dimers. This conclusion applies for Kr matrices. The temperature dependence of the Q branch width has been studied and will be discussed. II. Polymers: With respect to polymeric species, we observe the characteristic features of dimers, trimers, and higher order aggregates. The dimer frequency has the same value in both the Raman and the infrared spectrum, and this observation reules out the conclusion of previous workers who, from far infrared experiments, claimed that the dimer should be cyclic. Our observations favor an open-chain structure. III. External modes: Two external modes are observed. One of them is a combination between a localized translational mode and a stretching mode. The other a host frequency induced by the impurity.