Spectroscopy of HF and HF-containing clusters in solid parahydrogen

We report measurements of FT-IR absorption spectroscopy of HF, DF, and their clusters in solid parahydrogen (pH ₂). The observed spectra contain many absorption lines which were assigned to HF monomers, HF polymers, and clusters with other species, such as N ₂, O ₂, orthohydrogen (oH ₂), etc. The rotational constants of HF and DF monomers were determined from the cooperative transitions of the vibration of solid pH ₂ and the rotation of HF and DF. Small reduction of the rotational constants indicates that HF and DF are nearly free rotors in solid pH ₂. Time dependence of the spectra suggests that HF and DF monomers migrate in solid pH ₂ and form larger polymers, probably via tunneling reactions through high energy barriers on inserting another monomer to the polymers. The line width of HF monomers in solid pH ₂ was found to be 4 cm ^-1, which is larger than that of other hydrogen halides in solid pH ₂. This broad line width is explained by rapid rotational relaxation due to the accidental coincidence between the rotational energy of HF and the phonon energy with maximum density of states of solid pH ₂ and the rotational - translational coupling in a trapping site. (Figure presented)