New Line Narrowing Effects in the Infrared Collision-Induced Spectra of Molecular Hydrogens in Liquid Neon

The first spectroscopic observation of the relative (solute-solute) diffusion in a fluid environment is reported. New, unusually sharp $Q_1^q(J)$ lines developing against the diffuse background in the collision-induced fundamental IR bands of hydrogen isotopomers (${\mathrm{H}}_2$, ${\mathrm{D}}_2$, and HD) dissolved in liquid Ne ($T\approx 25\mathrm{K}$) are studied as functions of the solute concentration $x$. In all cases, the $Q_1^q$ intensity parabolically scales with $x$, accompanied by a striking narrowing of the line shapes. The narrowing, as revealed by the $p\mathrm{\text{−}}{\mathrm{H}}_2$ solution studies, is due to a faster growth with $x$ of the sharper solute-solute induced component of the single $Q_1^q(0)$ line. The latter as well as other observed solute-solute lines are strongly narrowed by fast velocity decorrelations and are signatures of microscopic-scale diffusion. Also, a first observation of the solute-solvent induced $J\to J+4$ transitions is reported.