Rayleigh–Brillouin scattering of gases with internal relaxation

Bulk viscosity of CO2 from Rayleigh-Brillouin light scattering spectroscopy at 532 nm

Rayleigh-Brillouin scattering spectra of CO₂ were measured at pressures ranging from 0.5 to 4 bars and temperatures from 257 to 355 K using green laser light (wavelength 532 nm, scattering angle of 55.7°). These spectra were compared to two line shape models, which take the bulk viscosity as a parameter. One model applies to the kinetic regime, i.e., low pressures, while the second model uses the continuum, hydrodynamic approach and takes the rotational relaxation time as a parameter, which translates into the bulk viscosity. We do not find a significant dependence of the bulk viscosity with pressure or temperature. At pressures where both models apply, we find a consistent value of the ratio of bulk viscosity over shear viscosity η_b/η_s = 0.41 ± 0.10. This value is four orders of magnitude smaller than the common value that is based on the damping of ultrasound and signifies that in light scattering only relaxation of rotational modes matters, while vibrational modes remain "frozen."

Temperature Dependence of the Rayleigh Brillouin Spectrum Linewidth in Air and Nitrogen

The relation between spontaneous Rayleigh Brillouin (SRB) spectrum linewidth, gas temperature, and pressure are analyzed at the temperature range from 220 to 340 K and the pressure range from 0.1 to 1 bar, covering the stratosphere and troposphere relevant for the Earth's atmosphere and for atmospheric Lidar missions. Based on the analysis, a model retrieving gas temperature from directly measured linewidth is established and the accuracy limitations are estimated. Furthermore, some experimental data of air and nitrogen are used to verify the accuracy of the model. As the results show, the retrieved temperature shows good agreement with the reference temperature, and the absolute difference is less than 3 K, which indicates that this method provides a fruitful tool in satellite retrieval to extract the gaseous properties of atmospheres on-line by directly measuring the SRB spectrum linewidth.

Rayleigh-Brillouin scattering of carbon dioxide

The spectral line shape of spontaneous Rayleigh-Brillouin scattering in CO2 is studied in a range of pressures. The spectrum is influenced by the bulk viscosity η(b), which is a relaxation phenomenon involving the internal degrees of freedom of the molecule. The associated relaxation rates can be compared to the frequency shift of the scattered light, which demands precise measurements of the spectral line shape. We find η(b)=(5.7±0.6)×10(-6)  kg m(-1) s(-1) for the range of pressures p=2-4 bar and for room temperature conditions.

Single-shot coherent Rayleigh-Brillouin scattering using a chirped optical lattice

We present a method for obtaining coherent Rayleigh-Brillouin scattering (CRBS) spectra on timescales of hundreds of nanoseconds using rapidly chirped, pulsed, optical lattices. This enables us to transfer the spectral profile to a temporal profile which can be easily recorded on a single shot of an oscilloscope. These spectra are demonstrated to have sufficient signal-to-noise ratio to study CRBS models over a wide range of gas densities.

A Rayleigh-Brillouin scattering spectrometer for ultraviolet wavelengths

A spectrometer for the measurement of spontaneous Rayleigh-Brillouin (RB) scattering line profiles at ultraviolet wavelengths from gas phase molecules has been developed, employing a high-power frequency-stabilized UV-laser with narrow bandwidth (2 MHz). The UV-light from a frequency-doubled titanium:sapphire laser is further amplified in an enhancement cavity, delivering a 5 W UV-beam propagating through the interaction region inside a scattering cell. The design of the RB-scattering cell allows for measurements at gas pressures in the range 0-4 bars and at stably controlled temperatures from -30 °C to 70 °C. A scannable Fabry-Perot analyzer with instrument resolution of 232 MHz probes the RB profiles. Measurements on N(2) and SF(6) gases demonstrate that the high signal-to-noise ratio is achievable with the instrument at the 1% level at the peak amplitude of the scattering profile.

Coherent Brillouin scattering

We measure the spectrum of coherent Brillouin scattering (CBS) in a gas as a function of time and observe for the first time additional spectral sidebands and line shape narrowing of the Brillouin peak. We find that both effects result from the interference of the density modulation induced by the moving dipole force of the pump beams with the acoustic waves induced by their fast thermalization and are predicted by a hydrodynamic-light scattering model. These line shapes differ from both spontaneous and stimulated Brillouin scattering spectra and also from previous coherent Rayleigh-Brillouin measurements.

Spontaneous Rayleigh-Brillouin scattering of ultraviolet light in nitrogen, dry air, and moist air

Atmospheric lidar techniques for the measurement of wind, temperature, and optical properties of aerosols rely on the exact knowledge of the spectral line shape of the scattered laser light on molecules. We report on spontaneous Rayleigh-Brillouin scattering measurements in the ultraviolet at a scattering angle of 90 degrees on N(2) and on dry and moist air. The measured line shapes are compared to the Tenti S6 model, which is shown to describe the scattering line shapes in air at atmospheric pressures with small but significant deviations. We demonstrate that the line profiles of N(2) and air under equal pressure and temperature conditions differ significantly, and that this difference can be described by the S6 model. Moreover, we show that even a high water vapor content in air up to a volume fraction of 3.6vol.% has no influence on the line shape of the scattered light. The results are of relevance for the future spaceborne lidars on ADM-Aeolus (Atmospheric Dynamics Mission) and EarthCARE (Earth Clouds, Aerosols, and Radiation Explorer).

Melting curve and fluid equation of state of carbon dioxide at high pressure and high temperature

The melting curve and fluid equation of state of carbon dioxide have been determined under high pressure in a resistively heated diamond anvil cell. The melting line was determined from room temperature up to 11.1+/-0.1 GPa and 800+/-5 K by visual observation of the solid-fluid equilibrium and in situ measurements of pressure and temperature. Raman spectroscopy was used to identify the solid phase in equilibrium with the melt, showing that solid I is the stable phase along the melting curve in the probed range. Interferometric and Brillouin scattering experiments were conducted to determine the refractive index and sound velocity of the fluid phase. A dispersion of the sound velocity between ultrasonic and Brillouin frequencies is evidenced and could be reproduced by postulating the presence of a thermal relaxation process. The Brillouin sound velocities were then transformed to thermodynamic values in order to calculate the equation of state of fluid CO2. An analytic formulation of the density with respect to pressure and temperature is proposed, suitable in the P-T range of 0.1-8 GPa and 300-700 K and accurate within 2%. Our results show that the fluid above 500 K is less compressible than predicted from various phenomenological models.

Temperature imaging in nonpremixed flames by joint filtered Rayleigh and Raman scattering

Joint fuel Raman and filtered Rayleigh-scattering (FRS) imaging is demonstrated in a laminar methane-air diffusion flame. These experiments are, to our knowledge, the first reported extension of the FRS technique to nonpremixed combustion. This joint imaging approach allows for correction of the FRS images for the large variations in Rayleigh cross section that occur in diffusion flames and for a secondary measurement of fuel mole fraction. The temperature-dependent filtered Rayleigh cross sections are computed with a six-moment kinetic model for calculation of major-species Rayleigh-Brillouin line shapes and a flamelet-based model for physically judicious estimates of gas-phase chemical composition. Shot-averaged temperatures, fuel mole fractions, and fuel number densities from steady and vortex-strained diffusion flames stabilized on a Wolfhard-Parker slot burner are presented, and a detailed uncertainty analysis reveals that the FRS-measured temperatures are accurate to within +/- 4.5 to 6% of the local absolute temperature.

Molecular backscatter heterodyne lidar: a computational evaluation

The application of heterodyne lidar to observe molecular scattering is considered. Despite the reduced Rayleigh cross section, infrared systems are predicted to require mean power levels comparable with those of current and proposed direct detection lidars that operate with the thermally broadened spectra in the visible or ultraviolet. Rayleigh-Brillouin scattering in the kinetic and hydrodynamic (collisional) regimes encountered in the infrared is of particular interest because the observed spectrum approaches a triplet of relatively narrow lines that are more suitable for wind, temperature, and pressure measurements.

Rayleigh-Brillouin scattering to determine one-dimensional temperature and number density profiles of a gas flow field

Rayleigh-Brillouin spectra for heated nitrogen gas were measured by imaging the output of a Fabry-Perot interferometer onto a CCD array. The spectra were compared with the theoretical 6-moment model of Rayleigh-Brillouin scattering convolved with the Fabry-Perot instrument function. Estimates of the temperature and a dimensionless parameter proportional to the number density of the gas as functions of position in the laser beam were calculated by least-squares deviation fits between theory and experiment.

Airborne and spaceborne lidar measurements of water vapor profiles: a sensitivity analysis

This paper presents an evaluation of the random and systematic error sources associated with differential absorption lidar (DIAL) measurements of tropospheric water vapor (H(2)O) profiles from airborne and spaceborne platforms. The results of this analysis are used in the development and performance evaluation of the Lidar Atmospheric Sensing Experiment (LASE) H(2)O DIAL system presently under development at the NASA Langley Research Center for operation on a high altitude ER-2 (advanced U-2) aircraft. The analysis shows that a <10% H(2)O profile measurement accuracy is possible for the LASE system with a vertical and horizontal resolution of 200 m and 10 km, respectively, at night and 300 m and 20 km during the day. Global measurements of H(2)O profiles from spaceborne DIAL systems can be made to a similar accuracy with a vertical resolution of 500 m and a horizontal resolution of 100 km.