Anomalous behavior of nonlinear refractive indexes of CO<sub>2</sub> and Xe in supercritical states

Mareev, E. I.; Aleshkevich, V. A.; Potemkin, F. V.; Баграташвили, В. Н.; Минаев, Н. В.; Gordienko, Vyacheslav M

doi:10.1364/oe.26.013229

Cited by 25 publications

(25 citation statements)

References 26 publications

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“…However, both these approaches have fundamental disadvantages, since anharmonicity effects are taken into account phenomenologically. Recently, excitation spectra in fluids have become of great interest in context of studies concerning the thermodynamics and collective dynamics of fluids 8,26,32–62 , and, in particular, crossover from fluid- to gas-like collective dynamics (Frenkel line) 8,32,34–41,47,48,50–53,56,58 and in the framework of inelastic X-ray or neutron scattering experiments 63–76 . Note that these experiments have already revealed a problem of mixing of longitudinal and transverse modes, which plays an important role in a proper analysis of excitation spectra as will be shown below.…”

Section: Introductionmentioning

confidence: 99%

Excitation spectra in fluids: How to analyze them properly

Kryuchkov

Mistryukova

Бражкин

et al. 2019

Sci Rep

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Although the understanding of excitation spectra in fluids is of great importance, it is still unclear how different methods of spectral analysis agree with each other and which of them is suitable in a wide range of parameters. Here, we show that the problem can be solved using a two-oscillator model to analyze total velocity current spectra, while other considered methods, including analysis of the spectral maxima and single mode analysis, yield rough results and become unsuitable at high temperatures and wavenumbers. To prove this, we perform molecular dynamics (MD) simulations and calculate excitation spectra in Lennard-Jones and inverse-power-law fluids at different temperatures, both in 3D and 2D cases. Then, we analyze relations between thermodynamic and dynamic features of fluids at (Frenkel) crossover from a liquid- to gas-like state and find that they agree with each other in the 3D case and strongly disagree in 2D systems due to enhanced anharmonicity effects. The results provide a significant advance in methods for detail analysis of collective fluid dynamics spanning fields from soft condensed matter to strongly coupled plasmas.

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Section: Introductionmentioning

confidence: 99%

Excitation spectra in fluids: How to analyze them properly

Kryuchkov

Mistryukova

Бражкин

et al. 2019

Sci Rep

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“…Thereby, to identify the current state of matter, it is especially important to have a robust and simple diagnostic tool. The clustering of molecules that occurs in supercritical fluid leads to the modification in the molecular refraction, an increase of the nonlinear refractive index [18] and the offset of the rotational-vibrational transitions [19]. Specific modifications in the SCF microstructure can be detected by optical techniques such as Raman spectroscopy.…”

Section: Introductionmentioning

confidence: 99%

“…The optical diagnostics is not limited by spectroscopic methods [24]. We recently demonstrated that by determining the linear and nonlinear refractive indices, the relative concentration of clusters and the average number of molecules in a cluster can be estimated [18]. Moreover, the nonlinear refractive index measurements are one of the most sensitive approaches due to nonlinear (for instance, n 2 has a quadratic dependence on the cluster concentration) processes [25].…”

Section: Introductionmentioning

confidence: 99%

Optical Diagnostics of Supercritical CO2 and CO2-Ethanol Mixture in the Widom Delta

et al. 2020

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The supercritical CO2 (scCO2) is widely used as solvent and transport media in different technologies. The technological aspects of scCO2 fluid applications strongly depend on spatial–temporal fluctuations of its thermodynamic parameters. The region of these parameters’ maximal fluctuations on the p-T (pressure-temperature) diagram is called Widom delta. It has significant practical and fundamental interest. We offer an approach that combines optical measurements and molecular dynamics simulation in a wide range of pressures and temperatures. We studied the microstructure of supercritical CO2 fluid and its binary mixture with ethanol in a wide range of temperatures and pressures using molecular dynamics (MD) simulation. MD is used to retrieve a set of optical characteristics such as Raman spectra, refractive indexes and molecular refraction and was verified by appropriate experimental measurements. We demonstrated that in the Widom delta the monotonic dependence of the optical properties on the CO2 density is violated. It is caused by the rapid increase of density fluctuations and medium-sized (20–30 molecules) cluster formation. We identified the correlation between cluster parameters and optical properties of the media; in particular, it is established that the clusters in the Widom delta acts as a seed for clustering in molecular jets. MD demonstrates that the cluster formation is stronger in the supercritical CO2-ethanol mixture, where the extended binary clusters are formed; that is, the nonlinear refractive index significantly increased. The influence of the supercritical state in the cell on the formation of supersonic cluster jets is studied using the Mie scattering technique.

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“…The entrance and exit windows of the cuvette were made of 10 mm thick sapphire and had an aperture diameter of 8 mm. The cuvettes, which were fabricated specially for our experiments, are described in details in our earlier publication [22]. Carbon dioxide with a purity of 99.9% and atmospheric air with humidity less than 10% were used as media for THz generation.…”

Section: Methodsmentioning

confidence: 99%

THz generation from laser-induced breakdown in pressurized molecular gases: on the way to terahertz remote sensing of the atmospheres of Mars and Venus

et al. 2020

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The present paper studies the generation of terahertz (THz) radiation in CO 2 in comparison with atmospheric air at a wide range of pressures. We established experimentally and explained theoretically that for these gases there are optimal pressures at about 1bar for air and 0.5bar for CO 2 under which the efficiency of conversion from near-infrared to THz frequencies is the highest. We consider the possibility of applying femtosecond laser-induced THz generation for the study of the atmosphere of Mars and found that the overall THz yield near the surface of Mars is just a factor of 6 lower than on Earth. Comparable THz energy on the two planets is associated with underdense plasma on Earth (∼10% of neutrals) and full double ionization of carbon dioxide on Mars (∼200% of neutrals), the latter opening great perspective for THz remote sensing of trace gases in the Martian atmosphere.

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Anomalous behavior of nonlinear refractive indexes of CO₂ and Xe in supercritical states

Cited by 25 publications

References 26 publications

Excitation spectra in fluids: How to analyze them properly

Excitation spectra in fluids: How to analyze them properly

Optical Diagnostics of Supercritical CO2 and CO2-Ethanol Mixture in the Widom Delta

THz generation from laser-induced breakdown in pressurized molecular gases: on the way to terahertz remote sensing of the atmospheres of Mars and Venus

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Anomalous behavior of nonlinear refractive indexes of CO2 and Xe in supercritical states

Cited by 25 publications

References 26 publications

Excitation spectra in fluids: How to analyze them properly

Excitation spectra in fluids: How to analyze them properly

Optical Diagnostics of Supercritical CO2 and CO2-Ethanol Mixture in the Widom Delta

THz generation from laser-induced breakdown in pressurized molecular gases: on the way to terahertz remote sensing of the atmospheres of Mars and Venus

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Anomalous behavior of nonlinear refractive indexes of CO₂ and Xe in supercritical states