Evidence for negative cross correlations in vibrational dephasing in liquids: isotropic raman-line shift and width phenomena in isotopic mixtures of N2 and O2

Raman Laser Spectrometer: Application to 12C/13C Isotope Identification in CH4 and CO2 Greenhouse Gases

A compact Raman laser gas spectrometer is developed. It comprises a high-power green laser at 532.123 nm as an excitation source and a specially designed gas cell with an internal volume of less than 0.6 cm3 that can withstand gas pressures up to 100 atm. The resolution of the spectrometer is ~1 cm−1. The Raman spectra of chemically pure isotopically enriched carbon dioxide (12CO2, 13CO2) and methane (12CH4, 13CH4) gases are studied. The expected limit of detection (LOD) is less than 100 ppm for the isotopologues of CO2 and less than 25 ppm for those of CH4 (at a gas pressure of 50 atm.), making the developed spectrometer promising for studying the sources of emissions of greenhouse gases by resolving their isotopologue composition. We also show the suitability of the spectrometer for Raman spectroscopy of human exhalation.

A New Approach to Explain Concentration-Dependent Changes of Isotropic Raman Line Width in the Associated Binary Mixtures

We report on a new empirical relationship to explain the concentration-dependent isotropic Raman line width changes of a vibrational mode in uniform binary mixtures. The factors contributing to the intrinsic line width and several other broadening mechanisms are, in general, concentration-dependent. Concentration fluctuation in a microscopic volume and microviscosity are the two factors that are known to cause a concentration-dependent line width variation. These two factors combined in a specific manner successfully explain the variation of the line width with concentration strongly associated with binary systems. A readily usable empirical relationship for line width is suggested. It has been demonstrated that it can successfully explain the line width variation with concentration in a given class of hydrogen-bonded systems taking some representative binary mixtures.

Concentration-Dependent Frequency Shifts and Raman Spectroscopic Noncoincidence Effect of the CO Stretching Mode in Dipolar Mixtures of Acetone/Dimethyl Sulfoxide. Experimental, Theoretical, and Simulation Results

The nu(C=O) Raman band frequencies of acetone have been analyzed to separate the contributions of the environmental effect and the vibrational coupling to the gas-to-liquid frequency shifts of this band and to elucidate the changes in these two contributions upon dilution in DMSO. We have measured the frequencies of the nu((12)C=O) band in acetone/DMSO binary mixtures, the nu((13)C=O) band of the acetone-(13)C=O present as a natural abundance isotopic impurity in these mixtures, and both the nu((12)C=O) and nu((13)C=O) bands in the acetone-(12)C=O/acetone-(13)C=O isotopic mixtures at infinite dilution. These frequencies are compared with those of the nu((12)C=O) band in the acetone/CCl(4) binary mixtures measured previously. We have found the following three points: (i) The negative environmental contribution for the nu((12)C=O) oscillator of acetone completely surrounded by DMSO is reduced in magnitude by +5.5 cm(-1) and +7.8 cm(-1) upon the complete substitution of DMSO with acetone and CCl(4) molecules, respectively, indicating the progressive reduction of the attractive forces exerted by the environment on the nu((12)C=O) mode of acetone. (ii) In DMSO and other solvents, the contribution of the vibrational coupling to the frequency of the isotropic Raman nu((12)C=O) band of acetone becomes progressively more negative with increasing acetone concentration up to a value of -5.5 cm(-1), while the contribution to the frequency of the anisotropic Raman band remains approximately unchanged. The only difference resides in the curvatures of the concentration dependencies of these contributions which depend on the relative solute/solvent polarity. (iii) The noncoincidence effect (separation between the anisotropic and isotropic Raman band frequencies) of the nu(C=O) mode in the acetone/DMSO mixtures exhibits a downward (concave) curvature, in contrast to that in the acetone/CCl(4) mixtures, which shows an upward (convex) curvature. This result is supported by MD simulations and by theoretical predictions and is interpreted as arising from the reduction and enhancement of the short-range orientational order of acetone in the acetone/DMSO and acetone/CCl(4) mixtures, respectively.