Knowledge Bank

The infrared absorption spectra induced by the liquid state environment, have been studied of solutions of $H2$ and $D2$ in neon $(27.2\circ K)$. Absorption features were found which differed by less than $7cm-1$ from frequencies of vibration-rotation transitions expected for the freely rotating $H2$ or $D2$ molecule. Rotatory energy levels of $H2$ and $D2$ in neon solution are, therefore, essentially the same as for gas phase molecules. Evidence for quantized translational energy levels for the solute was found. Broad absorption features which shift in frequency with isotope change in solute are assigned to vibration-translation combination bands in which the $H2$ (or $D2$) molecule, in addition to increasing its vibrational level, increases its translational level in its solvent cage. The spectroscopic determination of quantized translational energy levels permits the estimation of a solute-solvent interaction potential. A harmonic oscillator potential to describe the intermolecular interaction is consistent with the observation that the translational energy levels for $D2$ in neon are $\surd 2$ smaller than those of $H2$ in neon. This study has provided a spectroscopic estimate of the liquid state interaction potential for the $H2$ and $D2$ neon system and an approximate evaluation of all the types of energy of the solute:vibration, rotation and translation.