COLLISION-INDUCED SPECTRA: AN AVENUE TO INVESTIGATE MICROSCOPIC-SCALE DIFFUSION IN FLUIDS

New data on the IR spectra induced by intermolecular interactions in liquid cryogenic mixtures at T=89 K (O2 in LAr and LN2 and binary O2-Ar solutions in LN2) are reported. The induced fundamental bands appear as diffuse pedestals (with FWHH≈100 cm−1) on which weak, paradoxically sharp lines (FWHH≈2 cm−1) develop at the 2326 and 1552 cm−1 frequencies of the free-molecule vibrational transitions in N2 and O2, respectively. In LAr and LN2 these lines were carefully separated and studied at varied O2 concentrations up to c =0.23 mole fractions (mf). While the 1552 cm−1 line scales as c[O2] and thus is induced by the O2-O2 interactions in a bulk of cryosolvent (Ar, N2), the 2326 cm−1 feature varies linearly with c[O2] and hence is caused by interaction of a guest (O2) with a vibrating host (N2). The impurity induction mechanism was further supported by our data on the binary O2-Ar solutions in LN2 recorded at the fixed c[O2] (0.03 and 0.06 mf) and the varied c[Ar]≤ 0.2 mf. Both series revealed the same (linear) enhancement of the sharp N2 line by argon, in an accord with our previous studies of the Ar-LN2 systema. The results suggest that the resonance 2326 cm−1 feature is primarily due to the local distortion of the first coordination sphere around a vibrating N2 by a guest molecule. We also notice that the resonance lines should be due to the dispersionand overlap-induced dipole moments independent on the rotational degrees of freedomb. As our previous studies of the H2-LNe system showedc, the unusual line sharpness is a conspicuous manifestation of the relative solvent-solute and solute-solute translations dramatically retarded in a liquid by a fast velocity relaxation, an effect directly related to the microscopic-scale diffusion. The collision-induced spectra thus open up new vistas for studies of microscopic liquid dynamics.