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 Q₁^q(J) lines developing against the diffuse background in the collision-induced fundamental IR bands of hydrogen isotopomers (H₂, D₂, and HD) dissolved in liquid Ne (T≈25  K) are studied as functions of the solute concentration x. In all cases, the Q₁^q intensity parabolically scales with x, accompanied by a striking narrowing of the line shapes. The narrowing, as revealed by the p-H₂ solution studies, is due to a faster growth with x of the sharper solute-solute induced component of the single Q₁^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→J+4 transitions is reported.