First vibrational overtone bandshape of HCl in fluid SF6: An experimental and theoretical study

The first overtone band profiles are recorded in the IR spectra of the HCl molecular probes diluted in SF6, both below and above the critical point of the solvent. The spectral density distribution is considered as an additiv e superposition of the contributions due to the quasifree rotating and the rotationally-hindered probes. A recently proposed spectral theory is applie d for description of the quasifree contribution. Good agreement is obtained between the experimental and calculated spectral profiles. The vibration-r otation coupling constant for the HCl molecules is found to decrease in hig h-density fluids. The central Q-branch components associated with the hinde red solute states are separated from the measured spectra and their transfo rmation is studied in a broad range of well-controlled thermodynamic condit ions. We discuss the possible influence of the interaction-induced dipole t ransitions on the absorption in the vicinity of the band center.