Long Range Molecular Vibration Correlation in Liquids

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In this project funded by the Chemical Structure, Dynamics, and Mechanisms A (CSDM-A) program of the Chemistry Division, Professor David Shelton of the University of Nevada Las Vegas is using inelastic laser light scattering techniques to probe molecular vibrations in liquids. The blue color of the sky at mid-day and red color at dawn or dusk are the result of elastic scattering of light; the photons of light maintain the same color (i.e., energy) as they scatter off molecules or dust particles in the atmosphere. Sometimes, the photons of light lose some of their energy to the molecule or object from which they scatter. This is called inelastic scattering, and analysis of the scattered photon energy can reveal interesting information about the scattering system. Prof. Shelton is exploring the vibrations of molecules in liquids. The generally accepted view is that molecular vibrations in liquids are localized, with the influence of one vibrating molecule on the motion of another molecule extending at most a few molecular diameters. This project is exploring the possibility (based on some preliminary observations) that vibrational interactions between molecules can occur at inter-molecular distances of hundreds of molecular diameters. If these preliminary observations are confirmed, our understanding of liquids would be changed significantly: despite being composed of apparently randomly moving individual molecules, some liquids would appear to possess some properties normally associated with highly ordered solid crystals. The principal tool used in this project (hyper-Raman scattering spectroscopy) is an advanced type of inelastic scattering instrument that detects events when two scattering photons combine to form one high-energy photon (this is an example of what is called a “nonlinear” optical process). The students engaged in this project are undergraduate researchers who are gaining an unusually valuable experience in advanced nonlinear laser light scattering techniques and spectroscopic analysis, both of which are important in other areas of science and increasingly important in sensing and communications technologies.The project focuses on long range molecular vibration correlation measured by hyper-Raman scattering (HRS) in several representative non-polar, polar and hydrogen-bonding molecular liquids. HRS is observed with a linear polarized laser beam incident on the sample and linear polarized scattered light collected at 90 degree scattering angle. HRS for each vibration mode appears at a scattered light frequency that is shifted from the second harmonic frequency of the laser by the vibration frequency of the mode. Spectral intensity is measured with incident (I) and scattered (S) light polarized either parallel (H) or perpendicular (V) to the horizontal scattering plane, for the configurations with IS = VV, HV, VH and HH. The HV and VH HRS intensities are equal by symmetry for uncorrelated molecular vibrations in an isotropic fluid, but when the molecular vibrations are spatially correlated, then coherent addition of the scattered fields from individual molecules produces a dependence on the scattering wavevector that breaks the symmetry between the HV and VH polarization configurations. The HRS signal is decomposed into contributions from transverse and longitudinal polar collective modes based on their distinctive polarization (and angle) dependence. This experiment suggests that molecular vibrations in liquids are correlated over macroscopic distances, which is a fundamental change in the usual description of liquids. This work contributes to a better understanding of the liquid state of matter, which is important for technology, physics, chemistry and biology. The hyper-Raman scattering experiments have been designed to be accessible to undergraduates researchers without extensive prior experience; the new experience is preparing them well for graduate school and other advanced pursuits.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.