Infrared Spectra and Intensity Enhancements in Solutions of Hydrogen Halides in Liquid Xenon

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Abstract
The intensities and shapes of the vibrational fundamental bands of HCl, HBr, and HI dissolved in liquid xenon at 165°K are examined at the resolution of a Beckman IR‐7. While no rotational fine structure was observed, rotational branch maxima were exhibited in all three cases. The spectra of the dissolved molecules are different from those of the gaseous molecule in that they exhibit a strong Q branch and probably also O and S branches. This observation should be compared with spectra of hydrogen halides in dense gases where a Q branch is prominent, and with spectra of such molecules in solid rare gases where ordinarily a strong Q branch is not produced. The extended selection rules in the liquid are probably caused by anisotropic fields which are capable of mixing J states. In this sense, the rotational motions of the hydrogen halides in liquid xenon are not ``free.'' The large intensity enhancements, 2.8 for HCl, 3.1 for HBr, and 44 for HI, are inadequately explained on the basis of dielectric polarization effects. Nuclear distortion effects on the intensity enhancements are shown to be negligible. It is concluded that electronic distortion effects must account for the enhancement. A semiquantitative description based on the lowering of ionic states of the hydrogen halides in the presence of the dielectric medium indicates that for each of the halides studied about one‐sixteenth of an electron moves from the proton to the halogen when the molecule is dissolved. This is sufficient electronic distortion to account for the intensity enhancement in all three molecules. This increased ionicity should manifest itself not only in infrared intensity enhancements but also in permanent dipole moment corrections for dissolved polar molecules. The permanent moment corrections may exceed those given by the Onsager dielectric theory.

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