Vibration—Rotation Spectra of Monomeric HCl, DCl, HBr, DBr, and HI in the Rare-Gas Lattices and N2-Doping Experiments in the Rare-Gas Lattices

Abstract

Extensive experiments are performed on trapping of the HCl, DCl, HBr, DBr, and HI molecules in rare-gas lattices. Concentration, deposition rate, and temperature-dependent studies yield the monomer spectra of the molecules. The HCl and DCl monomers in Ar, Kr, and Xe show hindered rotational spectra that is interpreted on the basis of 0h site symmetry. HCl in Ne also appears to rotate. HBr and DBr in Ar shows additional splitting which is attributed to either site splitting due to loss of one of the near-neighbor atoms or site distortion toward D3h symmetry. HBr and DBr in the larger Kr lattice give spectra similar to the HCl spectra. The spectra indicate the predominant local symmetry is 0h and the barriers to rotation in the lattices are given from an interpretation of the spectra. The barriers to rotation are interpreted by extension of an electrostatic model. Calculations of rotational energy levels have been performed for the linear molecule in a lattice site of D3h symmetry. The perturbed rigid-rotor levels are given as a function of lattice—molecule coupling constants. No experimental evidence was found in these studies for D3h site symmetry. N2-doping experiments are performed with HCl in the argon lattice. N2 doping (1%—4%) is found to give predominately two sites for the HCl molecule; one site similar to the pure HCl in Ar case and another site in which the rotation is frozen out giving a single transition for HCl. The frozen site is attributed to the nearest-neighbor presence of N2. Detailed discussions are given in interpreting the spectra in terms of a hindered rotor or a librator. Most of the spectra is interpreted on the basis of a hindered rotor but some spectra such as the N2-doping experiments require the librator analogy similar to CO in Ar and CN− in KCl. The vibrational shifts of the monomers in the rare-gas lattices are given for all cases studied where the predominant shift is to lower energy. These shifts are also qualitatively explained by localized nonbonded dispersion and overlap repulsion effects.