Fourier-transform infrared spectroscopy of hydrogen fluoride dimers in solid parahydrogen

We investigate the Fourier-transform infrared spectra of hydrogen fluoride dimers in solid parahydrogen, the detailed analysis of which has remained unexplored. We propose a plausible analysis based on concentration dependence, light polarization, annealing, and time evolution. The absorption lines exhibited multiple peaks, with intensity ratios significantly altered by annealing and by time evolution at a constant temperature. The spectral patterns and isotopic effects suggest that the dimers do not rotate freely in solid parahydrogen, while multiple peaks arise from different stable structures, including single and double substitution sites. Unlike in the gas phase and helium droplets, no tunneling splitting was observed. The broad ν1 band suggests that some dimer structures may exhibit axial rotation. Spectral changes due to annealing likely result from site conversion, while observed IR-induced changes indicate preferential dissociation of dimers in double substitution sites. These findings still remain tentative, necessitating further experimental and theoretical studies.