Relative Raman Scattering Cross Sections Research Articles

Quantitative Mineralogy of Planetary Silicate Ternary Mixtures Using Raman Spectroscopy

AbstractA 532‐nm‐excited lunar Raman spectrometer (LRS) has been selected as a scientific payload of the Chang'e‐7 mission, exploring mineralogy assemblages in the lunar south polar region. However, the quantification of dark‐colored silicate minerals via Raman spectroscopy is an urgent requirement for upcoming Raman applications in future lunar and planetary explorations. Therefore, we conducted detailed laboratory studies on the Raman quantification of lunar silicate minerals using ternary mixtures of feldspar, olivine, and augite. Quantitative models were established employing the observed linear relationship between Raman integrated intensities and mineral proportions. The significant correlation coefficients (>0.94) and small RMSE (≤4.20 wt.%) confirmed the performance of these models. A series of methods (multipoint sampling, multispectral averaging, peak area extraction, and spectral parameter ratios) were jointly used to ensure that the models were not significantly affected by crystal orientation, chemical inhomogeneity, and instruments. Factors (σ2/σ1) describing the relative Raman scattering cross sections were introduced to calibrate the Raman counts. Our results indicated that the relative Raman scattering efficiency of feldspar, olivine, and augite is 1.4:2.4:1, which can be used to improve the quantitative accuracy of the point‐counting method if polymineralic mixing spectra are dominant. The models were validated across different samples using laboratory mixtures and lunar soil (CE5C0600). The lithology of the Chang'e‐5 soils is basaltic/gabbroic according to the quantitative mineralogy returned from our models that is consistent with the results from traditional methods. This research will be of particular significance for accurately determining the mineral abundances for Chang'e‐7 and other planetary missions. As such, crucial information can be inferred to understand the geological evolution of the exploration regions.

Calibration data for simultaneous determination of P-V-X properties of binary and ternary CO2 - CH4 - N2 gas mixtures by Raman spectroscopy over 5–600 bar: Application to natural fluid inclusions

The P-V-X properties of two-component fluid inclusions (FIs) are generally determined from microthermometry data using appropriate thermodynamic models (i.e., VX diagrams) and/or equations of state (EoS). However, some limitations can hamper the applicability of this technique such as the small size, low density or complex composition of the analyzed FI. Raman spectroscopy is known as the best-suited alternative method to microthermometry for the investigation of natural FIs because it can provide simultaneously non-destructive qualitative and possible quantitative analyses after specific calibrations. The present work aims to provide calibration data to directly determine the P-V-X properties of binary or ternary mixtures of CH4, CO2, and N2. The variation of spectral features as a function of composition and pressure (or density) was investigated by using Raman spectroscopy coupled with an improved High-Pressure Optical Cell (HPOC) system and a customized heating-cooling stage. From our experimental data, the relative Raman scattering cross-section (RRSCS) of CH4 (νCH4∗) was demonstrated to be constant at 7.73 ± 0.16 over the investigated range of pressure (5–600 bars) and for any composition. This parameter can thus be used for the determination of composition with an uncertainty of ~0.5 mol%. Several calibration equations were calculated for different PX domains, linking the Fermi diad splitting of CO2 (Δ) or the relative variation of the CH4 peak position (νCH4∗) to the pressure (or density) and composition of CO2-CH4, CH4-N2, and CO2-N2-CH4 mixtures at 22 and 32 °C. The pressure and density of the fluids can henceforth be directly measured from Raman spectra with an uncertainty of ~20 bars and ~0.01 g·cm−3, respectively. Our calibration equations were then validated on natural FIs by comparing the results obtained from Raman and microthermometry. We also interpreted the variation of the peak position of CH4 based on the change of intermolecular interaction. Finally, we discussed the applicability of the obtained calibration data into another laboratory by comparing it with the data of pure CO2 and CH4 published in literature. A small shift between calibration curves implies a systematic error which is perhaps due to the difference in the configuration or the day-to-day deviation of the instruments. Therefore, standards of well-known P-V-X properties should be regularly measured to prevent and to correct any variation or shifting of the instrumental responses.

Quantitative Measurements of Composition, Pressure, and Density of Microvolumes of CO2-N2 Gas Mixtures by Raman Spectroscopy.

Quantitative analysis of gases by Raman spectroscopy is based on relative Raman scattering cross sections (RRSCS) and the evolution of different spectral parameters (peak position, peak area, peak intensity, etc.). However, most of the calibration data were established at low pressure (low density) and without evaluating the effect of the composition. Using these data may lead to considerable errors, especially when applied to gas mixtures at high pressure as found in natural fluid inclusions. The aim of this study is to reevaluate the RRSCS of CO2 and to establish new calibration data based on the variation of CO2 Fermi diad splitting as a function of pressure (density) and composition over a pressure range of 5-600 bar at 22 and 32 °C. A high-pressure optical cell system (HPOC) and a heating-cooling stage were used for Raman in situ analyses at controlled PTX conditions. Our experimental results show that the RRSCS of CO2 varies slightly with pressure but can be considered constant over the studied pressure range. It can be used to measure the proportion of CO2 in gas mixtures with an uncertainty of about ±0.5 mol%. Different polynomial equations were provided to calculate pressure and density of CO2-N2 gas mixtures with an uncertainty of ±20 bar or 0.01 g·cm-3. A comparison of PVTX properties of natural CO2-N2 fluid inclusions hosted in quartz from the Central Alps (Switzerland) obtained by Raman measurement and as derived from phase transition temperatures by microthermometry experiments shows comparable values.

The Relative Raman Scattering Cross Sections of H2O and D2O, with Implications for In Situ Studies of Isotope Fractionation

The relative Raman scattering cross sections of liquid H2O and D2O were measured using Na2SO4 as an internal standard. Measurements were carried out both at ambient conditions and at elevated tempe...

Identification of Multiple Water-Iodide Species in Concentrated NaI Solutions Based on the Raman Bending Vibration of Water.

The influence of aqueous electrolytes on the water bending vibration was studied with Raman spectroscopy. For all salts investigated (NaI, NaBr, NaCl, and NaSCN), we observed a nonlinear intensity increase of the water bending vibration with increasing concentration. Different lasers and a tunable frequency-doubled optical parametric oscillator system were used to achieve excitation wavelengths between 785 and 374 nm. Focusing on NaI solutions, the relative enhancement of the water bending vibration was found to increase strongly with excitation photon energy, in line with a preresonance effect from the iodide-water charge-transfer transition. We used multivariate curve resolution (MCR) to decompose the measured Raman spectra of NaI solutions into three interconverting spectral components assigned to bulk water and water molecules interacting with one (X···H-O-H···O) and two (X···H-O-H···X) iodide ions (X = I(-)). The Raman spectrum of solid sodium iodide dihydrate supports the assignment of the latter. Using the MCR results, relative Raman scattering cross sections of 4.0 ± 0.6 and 14.0 ± 0.1 were calculated for the mono- and di-iodide species, respectively (compared to that of bulk water set to unity). In addition, it was found that at relatively low concentrations each iodide ion affects the Raman spectrum of roughly 22 surrounding water molecules, indicating that the influence of iodide extends beyond the first solvation shell. Our results demonstrate that the Raman bending vibration of water is a sensitive probe, providing new insights into anion solvation in aqueous environments.

Quantitative Evaluation of the Sensitivity of Library-Based Raman Spectral Correlation Methods

Library-based Raman spectral correlation methods are widely used in surveillance applications in multiple areas including the pharmaceutical industry, where Raman spectroscopy is commonly used in verification screening of incoming raw materials. While these spectral correlation methods are rapid and require little or no sample preparation, their sensitivity to the presence of contaminants has not been adequately evaluated. This is particularly important when dealing with pharmaceutical excipients, which are susceptible to economically motivated adulteration by substances having similar physical/chemical/spectroscopic properties. We report a novel approach to evaluating the sensitivity of library-based Raman spectral correlation methods to contaminants in binary systems using a hit-quality index model. We examine three excipient/contaminant systems, glycerin/diethylene glycol, propylene glycol/diethylene glycol, and lactose/melamine and find that the sensitivity to contaminant for each system is 18%, 32%, and 4%, respectively. These levels are well-correlated to the minimum contaminant composition that can be detected by both verification and identification methods. Our studies indicate that the most important factor that determines the sensitivity of a spectral correlation measurement to the presence of contaminant is the relative Raman scattering cross section of the contaminant.

Remote Continuous Wave and Pulsed Laser Raman Detection of Chemical Warfare Agents Simulants and Toxic Industrial Compounds

This study describes the design, assembly, testing and comparison of two Remote Raman Spectroscopy (RRS) systems intended for standoff detection of hazardous chemical liquids. Raman spectra of Chemical Warfare Agents Simulants (CWAS) and Toxic Industrial Compounds (TIC) were measured in the laboratory at a 6.6 m source-target distance using continuous wave (CW) laser detection. Standoff distances for pulsed measurements were 35 m for dimethyl methylphosphonate (DMMP) detection and 60, 90 and 140 m for cyclohexane detection. The prototype systems consisted of a Raman spectrometer equipped with a CCD detector (for CW measurements) and an I-CCD camera with time-gated electronics (for pulsed laser measurements), a reflecting telescope, a fiber optic assembly, a single-line CW laser source (514.5, 488.0, 351.1 and 363.8 nm) and a frequency-doubled single frequency Nd:YAG 532 nm laser (5 ns pulses at 10 Hz). The telescope was coupled to the spectrograph using an optical fiber, and filters were used to reject laser radiation and Rayleigh scattering. Two quartz convex lenses were used to collimate the light from the telescope from which the telescope-focusing eyepiece was removed, and direct it to the fiber optic assembly. To test the standoff sensing system, the Raman Telescope was used in the detection of liquid TIC: benzene, chlorobenzene, toluene, carbon tetrachloride, cyclohexane and carbon disulfide. Other compounds studied were CWAS: dimethylmethyl phosphonate, 2-chloroethyl ethyl sulfide and 2-(butylamino)-ethanethiol. Relative Raman scattering cross sections of liquid CWAS were measured using single-line sources at 532.0, 488.0, 363.8 and 351.1 nm. Samples were placed in glass and quartz vials at the standoff distances from the telescope for the Remote Raman measurements. The mass of DMMP present in water solutions was also quantified as part of the system performance tests.

On the chemical bonding effects in the Raman response: Benzenethiol adsorbed on silver clusters

We study the effects of chemical bonding on Raman scattering from benzenethiol chemisorbed on silver clusters using time-dependent density functional theory (TDDFT). Raman scattering cross sections are computed using a formalism that employs analytical derivatives of frequency-dependent electronic polarizabilities, which treats both off-resonant and resonant enhancement within the same scheme. In the off-resonant regime, Raman scattering into molecular vibrational modes is enhanced by one order of magnitude and shows pronounced dependence on the orientation and the local symmetry of the molecule. Additional strong enhancement of the order of 10(2) arises from resonant transitions to mixed metal-molecular electronic states. The Raman enhancement is analyzed using Raman excitation profiles (REPs) for the range of excitation energies 1.6-3.0 eV, in which isolated benzenethiol does not have electronic transitions. The computed vibrational frequency shifts and relative Raman scattering cross sections of the metal-molecular complexes are in good agreement with experimental data on surface enhanced Raman scattering (SERS) from benzenethiol adsorbed on silver surfaces. Characterization and understanding of these effects, associated with chemical enhancement mechanism, may be used to improve the detection sensitivity in molecular Raman scattering.

Validation of a novel ultraviolet lidar system with relative Raman-scattering cross sections determined from atmospheric measurements.

We have developed an ultraviolet lidar system in which the upwelled laser beam and the telescope field of view can be made to overlap at any specified location in space. We refer to this system as the Selected Overlap Lidar Experiment. We discuss validation of our system by calculating relative Raman-scattering cross sections (with respect to the nitrogen scattering cross section) for oxygen and water vapor using data collected during field operations of our lidar. Our relative cross sections are consistent with those obtained by other researchers making similar measurements in laboratory environments.

Temperature Dependence for the Relative Raman Cross Section of the Ammonia/Water Complex

The Raman spectra of aqueous ammonia solutions have been obtained between −40 and 25°C. The Raman spectrum of neat water was also obtained at 25°C and is characterized by two broad peaks observed at 3200 and 3400 cm−1. The spectrum due to water is subtracted to determine the NH3 spectrum at all temperatures. In ammonia–water solutions, the spectrum shows three features at measured displacements of 3250, 3316, and 3400 cm−1. The feature at 3316 cm−1 is assigned to the Q branch of the symmetric stretch. The broad, weak features at 3250 and 3400 cm−1, previously assigned to rotational bands, are assigned to combination bands. The NH3 combination bands are assigned by comparing with sum frequency generation (SFG) experiments, monitoring changes with temperature, and analyzing the polarization data. The rotational structure of the Q band is also discussed. As the temperature is lowered from 25 to −40°C, an increase in the Raman intensity is observed for all bands. The relative Raman scattering cross section is determined from the numerically integrated area of the NH3Q branch at each temperature.

Ultraviolet Raman scattering from simple alcohols and a related diol as a study of low-lying Rydberg-valence state characteristics

Relative Raman scattering cross sections of totally symmetric vibrational modes of liquid methanol, ethanol, and ethylene glycol are measured as a function of excitation wavelength in the preresonant spectral region 37 593–47 393 cm−1. Raman spectra are also obtained with 50 043 and 54 198 cm−1 excitation, the latter energy being directly resonant with the first excited state of the alcohols. The data indicate that the first three excited electronic states (with transition energies of approximately 54 600, 62 500, and 66 000 cm−1, respectively) make no significant contribution to the Raman scattered intensities and are nearly entirely Rydberg in character. The low-lying electronic structure of ethylene glycol appears to be primarily Rydberg in character as well. The dominant C–H and O–H stretching modes gain intensity largely from valence state(s) with transition energies near 70 000 cm−1, while the C–O stretching and CH2,3 bending modes gain intensity from even higher energy states (∼85 000 cm−1).

MICRO-RAMAN CHARACTERIZATION OF THIN FILMS AND STUDY OF THE TRANSFORMATIONS INDUCED BY A PULSED ELECTRIC-FIELD

Micro-Raman spectra of thin films of various carbonaceous compounds are presented. The relative Raman scattering cross sections of films of graphite and diamond are revised (a factor of two is found instead of 50 as in literature). Second order and third order Raman spectra of graphite and diamond like films are presented and discussed in terms of resonance Raman effect. Study of carbonaceous thin films under electric field is in progress.

Relative Raman scattering cross sections for chemisorbed and physisorbed pyridine molecules adsorbed on surface-enhanced Raman-scattering-active Ag films in ultrahigh vacuum: Local-field effects in the low-coverage regime.

We have studied the Raman scattering from pyridine adsorbed on silver films evaporated on cleaved mica (100 K) in ultrahigh vacuum. These silver films have only a small number of chemisorption sites (\ensuremath{\sim}2%). The chemisorption sites are filled preferentially during initial stages of adsorption, followed by filling of the physisorption sites. Both physisorbed and chemisorbed pyridine exhibit strongly enhanced Raman scattering. At one monolayer coverage the average scattering enhancement is a factor-of-55 larger for the chemisorbed molecules. When pyridine-${d}_{5}$ molecules are used to saturate the chemisorption sites, subsequent adsorption of pyridine-${h}_{5}$ results in partial displacement of the pyridine-${d}_{5}$ to physisorbed sites. This displacement phenomenon allows us to measure the ratio of the Raman cross sections for chemisorbed and physisorbed pyridine in the neighborhood of chemisorption sites. This ratio is less than 4. Combining this measurement with the factor-of-55-larger overall average enhancement for chemisorbed molecules compared to phys- isorbed molecules, we conclude that molecules (including those just physisorbed) in the neighborhood of the chemisorption site exhibit an extra enhancement. We have also analyzed the coverage dependence of the Raman intensity and found it to be well described by a simple depolarization model. The average optical polarizability in this model is \ensuremath{\sim}10 A${\r{}}^{3}$. This value is essentially the same as the value reported for pyridine adsorbed on annealed surface-enhanced Raman-scattering-inactive silver films.

Raman intensity calculations of formamide with the CNDO method

Within the polarizability theory, absolute and relative Raman scattering cross sections are calculated using a variational method and the finite field perturbation theory. The calculations are performed within the CNDO approximation and the basis set in the variational scheme is extended to include polarization functions. Satisfactory numerical results are obtained for the formamide molecule with the variational procedure. The FFP theory which was used up to now only for calculating relative Raman scattering cross sections is adequate in reproducing the relative values for formamide. The absolute scattering cross sections are too small. Because we are interested in calculations of biological relevant molecules, it is necessary to consider also the FFP method in the standard CNDO version, which is much more economical with respect to computer time and capacity than the variational method.

Relative Raman cross section of O3 for four Ar+ laser frequencies

We have measured the relative Raman scattering cross sections of O3 (referred to N2) and the depolarization ratios at four argon-ion laser pump frequencies to be, respectively, 2.3 and 0.05 at 20 981 cm−1, 2.5 and 0.03 at 20 487 cm−1, 2.8 and 0.04 at 20 135 cm−1, and 2.2 and 0.10 at 19 430 cm−1. The estimated uncertainty in cross section is ±12% and in depolarization, ±0.01, although the cross sections at different frequencies are thought to intercompare to ±5% or less. An improved sample preparation technique was used. We found no dependence of cross section on pressure and only weak evidence for any resonance dependence on pump frequency.