What is measured by hyper-Rayleigh scattering from a liquid?

Abstract

Polarization and angle dependence of hyper-Rayleigh scattering (HRS) measured for liquid acetonitrile and dimethyl sulfoxide (DMSO) is analyzed in terms of contributions from randomly oriented molecules and additional contributions produced during intermolecular collisions and induced by the electric field of dissolved ions. All three contributions show the effect of long-range correlation, and the correlation functions are determined using the HRS observations combined with the results of molecular dynamics simulations. HRS from acetonitrile is polarized transverse to the scattering vector. This is due to long-range molecular orientation correlation produced by the dipole-dipole interaction, and correlation at distances r > 100 nm must be included to account for the HRS observations. Analysis of the HRS measurements for acetonitrile determines the length scale a = 0.185 nm for the long-range longitudinal and transverse orientation correlation functions B=-2B=a/r. Transverse polarized collision-induced HRS is also observed for acetonitrile, indicating long-range correlation of intermolecular modes. Strong longitudinal HRS is induced by the radial electric field of dissolved ions in acetonitrile. For DMSO, the angle between the molecular dipole and the vector part of the first hyperpolarizability tensor is about 100°. As a result, HRS from the randomly oriented molecules in DMSO is nearly unaffected by dipole correlation, and ion-induced HRS is weak. The strong longitudinal polarized HRS observed for DMSO is due to the collision-induced contribution, indicating long-range correlation of intermolecular modes. The HRS observations require correlation that has r long-range asymptotic form, for molecular orientation and for intermolecular vibration and libration, for both acetonitrile and DMSO.

Long-Range Orientational Organization of Dipolar and Steric Liquids

“…Polarized resolved shs intensity measurements have demonstrated the ability to catch the long-range molecular organization in neat solvents. In the case where molecules have a strong dipolar symmetry character, the long-range organization is driven by dipole–dipole interaction in agreement with the dipolar hard sphere model . However, when the geometry of the molecule is far from that of a sphere or when other local intermolecular forces play an important role, the long-range organization is dramatically different, even if the molecular permanent dipole is strong.…”

“…In the case where molecules have a strong dipolar symmetry character, the long-range organization is driven by dipole−dipole interaction in agreement with the dipolar hard sphere model. 30 However, when the geometry of the molecule is far from that of a sphere or when other local intermolecular forces play an important role, the long-range organization is dramatically different, even if the molecular permanent dipole is strong. Whereas the dipolar hard sphere model has been well studied in the past, the hard ellipsoid models as well as other models have not been as thoroughly investigated, especially in view of their consequences on the long-range molecular organization.…”

“…This result is summarized in Figure d, where the dipolar hard sphere model leads to i 4 values weighted by their depolarization ratio for the three solvents: trichloromethane, water, and acetonitrile. However, numerical calculations have provided molecular hyperpolarizability tensor values for these solvent molecules and have evidenced a strong octupolar response for trichloromethane, a strong dipolar response for acetonitrile, , and an intermediate case for water, , even though these numerical calculations showed a rather strong dispersion. Assuming values for h 112 given by eq , the account of the depolarization ratios for each solvent provides a good agreement with the experimental data, confirming the presence of the long-range organization with local reporters of different molecular symmetry; see Figure e.…”

What Does Second-Harmonic Scattering Measure in Diluted Electrolytes?

“…Double-quantum light-scattering techniques, such as hyper Raman scattering (HRS), already have a long history. They were shown to probe the third-rank hyperpolarizability tensor β and to be able to measure orientational correlations in both crystalline and liquid phases. , Recently, new HRS as well as femtosecond elastic second-harmonic scattering (fs-ESHS) experimental setups have been developed to probe long-range orientational order in bulk liquids − and in diluted electrolyte solutions. − In particular, the recent fs-ESHS experiments of Chen et al on aqueous solutions have shown that the dissolution of ions induces an enhancement of the measured signal for concentrations as low as a few micromolars, and thus the presence of ions is able, even at those concentrations, to alter the water–water orientational correlations . Those findings, and their interpretation, suggesting a specific ability of water to support ion-induced long-range correlations, have received much attention in the liquid-state and “water” community.…”

“…9,10 In this Letter, we go back to the fundamentals, and, before anything can be said on the observed long-range orientational correlations, we interrogate ourselves on what is really measured by the fs-ESHS experiments of Chen et al The same question was addressed by Shelton in his numerous HRS studies of polar liquids and raised recently as such. 8 We thus start with a derivation of the expression of the SHS signal corresponding to the 90°experimental setup of Chen et al The derived expression is next applied to the SHS measurements from water and diluted aqueous electrolytes.…”