Effect of density on the Raman scattering of molecular fluids. I. A detailed study of the scattering polarization, intensity, frequency shift, and spectral shape in gaseous N2

Pressure broadening in Raman spectra of CH4-N2, CH4-CO2, and CH4-C2H6 gas mixtures

Different molecular environments change the spectrum of a given gas sample involved in a mixture compared to the spectrum of a pure gas. It is necessary to account for this effect to improve the accuracy of the analysis of the natural gas composition by Raman spectroscopy. First, the change in the main components of natural gas (methane, nitrogen, carbon dioxide, and ethane) must be considered. This work is devoted to the mutual influence of CH₄-N₂, CH₄-CO₂, and CH₄-C₂H₆ on their characteristic Raman bands in the range of 300-2500 cm^-1. The half-width and asymmetry of the Q branches of N₂, CO₂, and C₂H₆ as a function of methane concentration were obtained in the range of 1-50 bar. The averaged broadening coefficients of the rotational-vibrational lines of the ν₂ band of CH₄ perturbed by N₂, CO₂, and C₂H₆ are measured. A high-sensitivity spectrometer with a resolution of 0.5 cm^-1 based on spontaneous Raman scattering was used to obtain reliable results. The algorithm and all the necessary parameters for simulating the effect of various molecular environments on the Raman bands of the main components of natural gas are presented.

Interlaboratory Application of Raman CO2 Densimeter Equations: Experimental Procedure and Statistical Analysis Using Bootstrapped Confidence Intervals

Raman spectroscopy has been used extensively to calculate CO₂ fluid density in many geological environments, based on the measurement of the Fermi diad split (Δ; cm^-1) in the CO₂ spectrum. While recent research has allowed the calibration of several Raman CO₂ densimeters, there is a limit to the interlaboratory application of published equations. These calculate two classes of density values for the same measured Δ, with a deviation of 0.09 ± 0.02 g/cm³ on average. To elucidate the influence of experimental parameters on the calibration of Raman CO₂ densimeters, we propose a bottom-up approach beginning with the calibration of a new equation, to evaluate a possible instrument-dependent variability induced by experimental conditions. Then, we develop bootstrapped confidence intervals for density estimate of existing equations to move the statistical analysis from a sample-specific to a population level. We find that Raman densimeter equations calibrated based on spectra acquired with similar spectral resolution calculate CO₂ density values lying within standard errors of equations and are suitable for the interlaboratory application. The statistical analysis confirms that equations calibrated at similar spectral resolution calculate CO₂ densities equivalent at 95% confidence, and each Raman densimeter does have a limit of applicability, statistically defined by a minimum Δ value, below which the error in calculated CO₂ densities is too high.

Interpretation of the pressure-induced Raman frequency shift of the ν1 stretching bands of CH4 and N2 within CH4-CO2, N2-CO2 and CH4-N2 binary mixtures

The relationships between the frequency shift of the ν₁ stretching bands of CH₄ and N₂ with pressure (or density) and composition have been previously provided in the literature as reliable parameters for accurate empirical barometers and densimeters for the direct determination of the pressure or density of gas mixtures. However, the latter results still remain a pure description of the experimental data without any interpretation of the physical mechanisms hidden behind the variation trend of the observed peak position. The present paper is devoted to interpreting the origin of the pressure-induced vibrational frequency shifts of the ν₁ stretching bands of CH₄ and N₂ within CH₄-CO₂, N₂-CO₂ and CH₄-N₂ binary mixtures at the molecular level. Two different theoretical models (i.e., the Lennard-Jones 6-12 potential approximation - LJ, and the generalized perturbed hard-sphere fluid - PHF) are used to intuitively and qualitatively assess the variation trend as well as the magnitude of the frequency shift of the CH₄ and N₂ν₁ bands for an in-depth understanding. Thereby, the contribution of the attractive and repulsive solvation-mean forces to the variation of the Raman frequency shift as a function of pressure and composition is assessed. A predictive model of the variation trend of the frequency shift of the CH₄ν₁ band as a function of pressure (up to 3000 bars), density and composition within CH₄-N₂ and CH₄-CO₂ binary mixtures is then provided.

Transition from Gas-like to Liquid-like Behavior in Supercritical N2

We have studied in detail the transition from gas-like to rigid liquid-like behavior in supercritical N₂ at 300 K (2.4 T_C). Our study combines neutron diffraction and Raman spectroscopy with ab initio molecular dynamics simulations. We observe a narrow transition from gas-like to rigid liquid-like behavior at ca. 150 MPa, which we associate with the Frenkel line. Our findings allow us to reliably characterize the Frenkel line using both diffraction and spectroscopy methods, backed up by simulation, for the same substance. We clearly lay out what parameters change, and what parameters do not change, when the Frenkel line is crossed.

Quantitative estimation of NH3 partial pressure in H2 desorbed from the Li–N–H system by Raman spectroscopy

The partial pressure of NH3 gas estimated by Raman spectroscopy indicates that approximately 0.1% NH3 inevitably contaminates the H2 desorbed from a hydrogen storage material composed of LiH and LiNH2 at any temperature up to 400 degrees C in a closed system.

Quantitative Raman imaging investigations of mixing phenomena in high-pressure cryogenic jets

A two-dimensional Raman technique was used to investigate mixing phenomena of cryogenic jets under both supercritical and transcritical conditions. The aim of this study was to enlarge the experimental data basis for modeling purposes and to provide quantitative information to help to improve the design of injectors for high-pressure rocket engine combustion chambers. Cryogenic nitrogen, which served as substitute for liquid O(2), was injected into N(2) at room temperature at pressures up to 6.0 MPa. The liquid N(2) jet could be atomized by a coaxial H(2) flow. Raman scattering was generated with a XeF excimer laser. The resulting signal images were discriminated against background by spectral filtering and preferential detection of light with a polarization corresponding to the polarization of the laser, thus making use of the conserved polarization of the Raman-scattered light. The Raman images were converted into density distributions of N(2) and H(2), respectively, as well as into temperature distributions for a variety of experimental conditions.