Pressure dependence of the measured line intensity and super-Lorentzian effects in the absorption spectra of pure HCl

Super-Lorentzian effects in the troughs between lines and the pressure dependence of the line intensities retrieved from fits of absorption spectra of pure HCl have been investigated both experimentally and theoretically. For that, spectra of pure HCl gas in the 2-0 band were recorded with a Fourier Transform spectrometer at room temperature and for pressures ranging from 1 to 10 atm. The line intensities, retrieved from fits of the measurements with the Voigt profile using a single spectrum fitting technique, reveal large decreases with increasing pressure - up to 3% per atm - with a relatively weak rotational dependence. We also show that the absorptions in-between successive P and R transitions are significantly larger than those predicted using Voigt profiles. Requantized classical molecular dynamics simulations were made in order to predict absorption spectra of pure HCl matching the experimental conditions. The pressure dependence of the intensities retrieved from the calculated spectra as well as the predicted super-Lorentzian behavior between lines are in good agreement with the measurements. Our analysis shows that these effects are essentially due to incomplete collisions, which govern the dipole auto-correlation function at very short times.