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At least 181 records · Page 10

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NASA NTRS1992

Verification measurement of a polarization Raman elastic-backscatter lidar

By measuring the depolarization of light Raman scattered from a gas of known number density (nitrogen), it is possible to determine the influence of multiple scattering on lidar signals. In order to realize such measurements, linearly polarized laser light is emitted and two components of the nitrogen Raman signals, with E vectors parallel (parallel P sub lambda R) and perpendicular (perpendicular P sub lambda R) to the plane of polarization of the laser light, are measured. The depolarization ratio, delta sub lambda R = perpendicular P sub lambda R/parallel P sub lambda R, is constant if only the Raman scattering process contributes to the signal. Any variation of the depolarization is caused by additional elastic, and, thus, multiple scattering. If the contribution of multiple scattered light to the lidar signals is known, other parameters determined with the systems such as extinction, backscattering, and the depolarization of elastically scattered light, can be corrected for this influence. The lidar system used for the polarization measurements, especially the receiver setup, is described. The calibration of the apparatus and a clear air measurement are discussed.

Schulze, Christoph↗

NASA NTRS1992

Performance modeling of daytime Raman lidar systems for profiling atmospheric water vapor

We describe results from a comprehensive computer model developed to guide optimization of an ultraviolet Raman lidar system for measuring daytime profiles of atmospheric water vapor. Daytime measurements present added challenges because of the difficulties inherent in detecting Raman signals against solar backgrounds. We are pursuing two concepts for optimizing the daytime performance of a Raman lidar system. The first involves operating the system in the solar blind region of the ultraviolet spectrum. The second concept involves operation of the system in a narrowband, narrow field-of-view mode. Calculations for lidar systems based on both concepts are presented.

Goldsmith, J. E. M.↗

NASA NTRS1992

Laser Raman sensor for measurement of trace-hydrogen gas

A new optical hydrogen sensor based on spontaneous Raman scattering of laser light has been designed and constructed for rugged field use. It provides good sensitivity, rapid response, and the inherent Raman characteristics of linearity and background gas independence of the signal. Efficient light collection and discrimination by using fast optics and a bandpass interference filter compensate for the inefficiency of the Raman-scattering process. A multipass optical cavity with a Herriott-type configuration provides intense illumination from an air-cooled CW gas laser. The observed performance is in good agreement with the theoretical signal and noise level predictions.

Adler-Golden, Steven M.↗

NASA NTRS1993

Compact Raman Spectrometers Would Detect Hydrogen

Compact Raman spectrometers developed to measure concentrations of hydrogen as low as hundreds of parts per million in air, nitrogen, or other carrier gases. Advantages include speed, dynamic range, and ease of calibration. Design concept incorporates Raman-scattering apparatus into compact instrument of hydrogen leaking into stream of gas or into gas enclosed in small space. Should hydrogen-fueled cars and trucks come into widespread use, instruments used to detect leaks from vehicles and supply equipment, to help prevent explosions. Similar spectrometers developed to detect other gases emitting characteristic Raman spectra.

Helms, William R.↗

NASA NTRS1993

Laser Rayleigh and Raman Diagnostics for Small Hydrogen/oxygen Rockets

Localized velocity, temperature, and species concentration measurements in rocket flow fields are needed to evaluate predictive computational fluid dynamics (CFD) codes and identify causes of poor rocket performance. Velocity, temperature, and total number density information have been successfully extracted from spectrally resolved Rayleigh scattering in the plume of small hydrogen/oxygen rockets. Light from a narrow band laser is scattered from the moving molecules with a Doppler shifted frequency. Two components of the velocity can be extracted by observing the scattered light from two directions. Thermal broadening of the scattered light provides a measure of the temperature, while the integrated scattering intensity is proportional to the number density. Spontaneous Raman scattering has been used to measure temperature and species concentration in similar plumes. Light from a dye laser is scattered by molecules in the rocket plume. Raman spectra scattered from major species are resolved by observing the inelastically scattered light with linear array mounted to a spectrometer. Temperature and oxygen concentrations have been extracted by fitting a model function to the measured Raman spectrum. Results of measurements on small rockets mounted inside a high altitude chamber using both diagnostic techniques are reported.

Degroot, Wilhelmus A.↗

NASA NTRS1993

Intense backward Raman lasers in CH4 and H2

The conversion efficiency of the first Stokes radiation generated by backward stimulated Raman scattering is measured in CH4 and H2 that are pumped by a third harmonic of a Nd:YAG laser in the 12-55-mJ energy range and in the 0.86-6.00-MPa pressure range. Our experimental results show that CH4 is a better Raman medium than H2 at moderate pressures (a few megapascals) to efficiently generate backward Raman lines.

Sentrayan, Karuppasamy↗

NASA NTRS1993

Rayleigh rejection filters for 193-nm ArF laser Raman spectroscopy

Selected organic absorbers and their solvents are evaluated as spectral filters for the rejection of 193-nm Rayleigh light associated with the use of an ArF excimer laser for Raman spectroscopy. A simply constructed filter cell filled with 0.5 percent acetone in water and an optical path of 7 mm is shown effectively to eliminate stray Rayleigh light underlying the Raman spectrum from air while transmitting 60 percent of the Raman light scattered by O2.

Mckenzie, Robert L.↗

NASA NTRS1991

UV Raman scattering measurements in a Mach 2 H2-air flame for assessment of CFD models

An UV narrowband tunable excimer laser is used for spontaneous Raman scattering measurements in hydrogen diffusion flames. The UV Raman system is characterized by a repetition rate of about 100 Hz, a temporal resolution of about 20 ns, and a spatial resolution of about 0.4 mm. It is concluded that a single KrF excimer laser based on spontaneous Raman scattering in conjunction with laser-induced predissociative fluorescence is capable of measuring instantaneously and simultaneously major species (H2, O2, N2, H2O), minor species (OH), and temperature.

Cheng, T. S.↗

NASA NTRS1993

Efficient vibrational Raman conversion in O2 and N2 cells by use of superfluorescence seeding

We report first-Stokes vibrational conversion efficiency of 21 percent and 35 percent, respectively, in high-pressure O2- and N2-stimulated Raman cells. Broadband superfluorescence is employed to seed these Raman cells, significantly increasing the conversion efficiences with no measured effect on the Raman spectrum. The addition of helium buffer gas reduces competition from stimulated Brillouin scattering and improves the pulse-to-pulse stability and spatial mode quality by increasing the thermal conductivity. Further improvement of the spatial mode quality is achieved by use of gentle heating on the bottom of the cell to induce convection.

Zhang, Barry↗

NASA NTRS1994

Raman-Augmented Stratospheric-Ozone Lidar

Differential-absorption lidar (DIAL) system measures concentration of ozone in stratosphere augmented with subsystem measuring Raman scattering from nitrogen. One of number of DIAL systems used in long-term monitoring of stratospheric ozone. Raman scattering from nitrogen provides data to correct for effects of aerosols. Channels at wavelengths of 332 and 385 nm added to DIAL receiver to measure Raman backscattering from nitrogen molecules in stratosphere. Data-acquisition electronics sample photon counts at a rate of 250 MHz.

Mcdermid, I. Stuart↗

NASA NTRS1995

Effects of Raman scattering on the water-leaving radiance

The contribution of Raman scattering to the water-leaving radiance is examined using Monte Carlo simulations. Exit angle information is retained, allowing a comparison of different satellite viewing directions. Chlorophyll values of 0.0, 0.01, 0.1, and 1.0 mg Chl/cu m are simulated. Little directional variability is found, with the exception of the direct solar backscatter direction. The wavelength variability is greatest for low chlorophyll concentrations and is negligible for 1.0 mg Chl/cu m. At 550 nm the Raman contribution ranges from approximately 18% of the total water-leaving radiance for pure water to 3% for 1.0 mg Chl/cu m. At 440 nm the range is from 6% to 2%, indicating that Raman scattering will impact radiance ratios for ocean color satellite algorithms.

Waters, Kirk J.↗

NASA NTRS1995

Aggregation-Enhanced Raman Scattering by a Water-Soluble Porphyrin

Much interest in our laboratory has focused on aggregation of organic compounds, particularly cyanine dyes and porphyrins. For this discussion we have applied absorption and Raman scattering spectroscopies to characterize aggregated TSPP (tetrakis-(p-sulfonatophynyl) porphyrin) in aqueous solution. Based on concentration, pH and ionic strength dependence of TSPP absorption, we deduce that aggregation evolves through the formation of TSPP diacid and that the diacid is the repeating unit in the aggregate. The Raman bands of TSPP in strongly acidic solution lead us further to conclude that vibrations of adjacent molecules are perturbed in a fashion that is consistent with the pyrrolic ring in the porphinato macrocycle being ruffled, and that two aggregate arrangements occur: specifically J- and H-type aggregates. Furthermore, aggregation enhancement is advanced as a viable mechanism to explain enhanced Raman Scattering for homogeneous aqueous phase TSPP, where the surface-enhancement mechanism is not applicable.

Akins, Daniel L.↗

NASA NTRS1996

Ring Effect Studies: Rayleigh Scattering, including Molecular Parameters for Rotational Raman Scattering, and the Faunhofer Spectrum

Improved parameters for the description of Rayleigh scattering in air and for the detailed rotational Raman scattering component for scattering by O2 and N2 are presented for the wavelength range 200-1000 nm. These parameters enable more accurate calculations of bulk molecular scattering and of the 'Ring effect' for a variety of atmospheric radiative transfer and constituent retrieval applications. A solar reference spectrum with accurate absolute vacuum wavelength calibration, suitable for convolution with the rotational Raman spectrum for Ring effect calculations, has been produced and is briefly described. The solar-rotational Raman convolved product is available for fitting of atmospheric spectra.

Chance, K. V.↗

NASA NTRS1995

Raman Spectroscopy as a Method for Mineral Identification on Lunar Robotic Exploration Missions

The sharp, nonoverlapping Raman bands for plagioclase, pyroxene, and olivine would be advantageous for on-surface, active mineralogical analysis of lunar materials. A robust, light-weight, low-power, rover-based Raman spectrometer with a laser exciting source, entirely transmission-mode holographic optics, and a charge-coupled device (CCD) detector could fit within a less than 20 cm cube. A sensor head on the end of an optical fiber bundle that carried the laser beam and returned the scattered radiation could be placed against surfaces at any desired angle by a deployment mechanism; otherwise, the instrument would need no moving parts. A modem micro-Raman spectrometer with its beam broadened (to expand the spot to 50-micrometer diameter) and set for low resolution (7/cm in the 100-1400/cm region relative to 514.5-nm excitation), was used to simulate the spectra anticipated from a rover instrument. We present spectra for lunar mineral grains, less than 1 mm soil fines, breccia fragments, and glasses. From frequencies of olivine peaks, we derived sufficiently precise forsterite contents to correlate the analyzed grains to known rock types and we obtained appropriate forsterite contents from weak signals above background in soil fines and breccias. Peak positions of pyroxenes were sufficiently well determined to distinguish among orthorhombic, monoclinic, and triclinic (pyroxenoid) structures; additional information can be obtained from pyroxene spectra, but requires further laboratory calibration. Plagioclase provided sharp peaks in soil fines and most breccias even when the glass content was high.

Wang, Alian↗

NASA NTRS1998

Upper tropospheric water vapor: A field campaign of two Raman lidars, Airborne hygrometers, and Radiosondes

Water vapor in the atmosphere plays an important role in radiative transfer and the process of radiative balance so critical for understanding global change. It is the principal ingredient in cloud formation, one of the most difficult atmospheric processes to model, and the most variable component of the Earth-atmosphere albedo. And as a free molecule, it is the most active infrared absorber and emitter, thus, the most important greenhouse gas. The radiative impact of water vapor is important at all levels of the atmosphere. Even though moisture decreases by several orders-of-magnitude from the Earth's surface to the tropopause, recent research has shown that, from a radiative standpoint, a small percentage change in water vapor at any level is nearly equivalent. Therefore accurate and precise measurements of this important atmospheric constituent are needed at all levels to evaluate the full radiative impact. The need for improved measurements in the upper troposphere is particularly important because of the generally hostile (very dry and cold) conditions encountered. Because of the importance of water vapor to the understanding of radiative transfer, the Department of Energy's Atmospheric Radiation Measurements (ARM) program initiated a series of measurement campaigns at the Cloud And Radiation Testbed (CART) site in Oklahoma, especially focused on atmospheric water vapor. Three water vapor intensive observation period (water vapor IOP) campaigns were planned. Two of the water vapor IOP campaigns have been completed: the first IOP was held during the fall of 1996 with a focus on boundary layer water vapor measurements, and the second was conducted during the fall of 1997 with a focus on both boundary layer moisture e and moisture in the upper troposphere. This paper presents a review of the intercomparisons of water vapor measurements in the upper troposphere aquired during the second water vapor IOP. Data to be presented include water vapor measurements ements from: two Raman Lidars, the NASA Goddard Scanning Raman Lidar (SRL) and the CART Raman Lidar (CARL), a number of Vaisala radiosondes launched during the IOP campaign, and a dew point hygrometer flown on the University of North Dakota Cessna Citation Aircraft.

Melfi, S. Harvey↗

NASA NTRS1998

Stimulated Raman amplification, oscillation, and linewidth in barium nitrate

Measurements of Raman gain in a Ba(NO3)2 crystal are reported at 532 nm using a Raman oscillator/amplifier arrangement for differential absorption lidar measurements of ozone. The experimentally determined gain coefficient will be compared with theoretical results. The effect of single and multi-longitudinal mode pumping upon the amplification process will be discussed. Measurement of the Raman linewidth for 1st 2nd and 3d stokes shifts arc presented.

McCray, Christopher J.↗

NASA NTRS1998

Raman and Infrared Spectroscopy of Yttrium Aluminum Borate Glasses and Glass-ceramics

Raman spectra of glasses and glass-ceramics in the Y2O3-Al2O3-B2O3 system are reported. Glasses with B2O3 contents ranging from 40 to 60 mole percent were prepared by melting 20 g of the appropriate oxide or carbonate powders in alumina crucibles at 1400 C for 45 minutes. Subsequent heat treatments of the glasses at temperatures ranging from 600 to 800 C were performed in order to induce nucleation and crystallization. It was found that Na2CO3 added to the melt served as a nucleating agent and resulted in uniform bulk crystallization. The Raman spectra of the glasses are interpreted primarily in terms of vibrations of boron - oxygen structural groups. Comparison of the Raman spectra of the glass-ceramic samples with spectra of aluminate and borate crystalline materials reveal that these glasses crystallize primarily as yttrium aluminum borate, YAl3(BO3)4.

Bradley, J.↗

NASA NTRS1999

Raman LIDAR Detection of Cloud Base

Advantages introduced by Raman lidar systems for cloud base determination during precipitating periods are explored using two case studies of light rain and virga conditions. A combination of the Raman lidar derived profiles of water vapor mixing ratio and aerosol scattering ratio, together with the Raman scattered signals from liquid drops, can minimize or even eliminate some of the problems associated with cloud boundary detection using elastic backscatter lidars.

Demoz, Belay↗