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At least 559 records · Page 31
All sourcesNASA NTRSDOE OSTI
NASA NTRS1983

Laser-induced fluorescence spectroscopy for combustion diagnostics

The types of spectroscopic and collisional measurements that are needed to develop laser-induced fluorescence as a diagnostic technique are discussed, with emphasis placed on combustion measurements. The spectroscopic measurements under collision-free conditions include production of radicals, excitation scan studies, lifetime measurements, and fluorescence scans. The collisional studies discussed here are quenching, energy transfer, and polarization phenomena. The results of recent laboratory experiments are presented.

Crosley, D. R.↗
NASA NTRS1983

Photodiagnostics of turbulent flows using laser-induced fluorescence

An optical probe technique that will allow remote measurements of temperature (and density), along with their time dependent fluctuations, to be made in a supersonic turbulent wind tunnel flow was developed. Laser-induced fluorescence from nitric oxide which was seeded into the flowing gas medium (nitrogen) at low concentrations was used. The fluorescence emission intensity following laser excitation of the nitric oxide (NO) ground state rotational levels is then related to thermodynamic quantities of the bulk fluid.

Bershader, D.↗
NASA NTRS1984

Saturated laser fluorescence in turbulent sooting flames at high pressure

The primary objective was to develop a quantitative, single pulse, laser-saturated fluorescence (LSF) technique for measurement of radical species concentrations in practical flames. The species of immediate interest was the hydroxyl radical. Measurements were made in both turbulent premixed diffusion flames at pressures between 1 and 20 atm. Interferences from Mie scattering were assessed by doping with particles or by controlling soot loading through variation of equivalence ratio and fuel type. The efficacy of the LSF method at high pressure was addressed by comparing fluorescence and adsorption measurements in a premixed, laminar flat flame at 1-20 atm. Signal-averaging over many laser shots is sufficient to determine the local concentration of radical species in laminar flames. However, for turbulent flames, single pulse measurements are more appropriate since a statistically significant number of laser pulses is needed to determine the probability function (PDF). PDFs can be analyzed to give true average properties and true local kinetics in turbulent, chemically reactive flows.

King, G. B.↗
NASA NTRS1984

Shot noise limited detection of OH using the technique of laser induced fluorescence

Nearly shot-noise limited detection of OH using the technique of laser-induced fluorescence is reported. A LIDAR configuration is used to excite fluorescence in a large volume and a narrow-bandwidth interference filter provides spectral discrimination. This arrangement alleviates the effect of ozone interference and facilitates image processing at relatively close distances. The detection limit is determined mainly by the shot-noise of the solar background. Ground-based measurements in Dearborn indicate a detection limit of better than 1 x 10 to the 6th power OH/cubic cm over a forty-minute acquisition period. Under favorable conditions, a comparable detection limit was also observed for airborne measurements.

Bakalyar, D. M.↗
NASA NTRS1983

Oil film thickness using airborne laser-induced oil fluorescence backscatter

Remote airborne measurement of oil film thickness on ocean surface using laser-induced water Raman backscatter is discussed. It is pointed out that the theoretical model of oil fluorescence by Horvath et al. (1971) contains the necessary constituents to provide for the natural background fluorescence that is also induced by the laser during the course of an oil thickness experiment. How the various parameters of the model are obtained from typical airborne profile data is discussed, and it is shown that the water Raman backscatter may be used to assist further in the application of the data. The regions or water types over which the technique might be most useful or applicable are discussed.

Hoge, F. E.↗
NASA NTRS1985

Analysis of vibronic transitions in titanium doped sapphire using the temperature of the fluorescence spectra

An analysis of the fluorescence spectra of titanium-doped sapphire crystals for temperatures ranging from 13 to 578 K is made. Zero- and multiphoton assisted electronic transitions are observed at temperatures below 125 K. Zero-phonon transition energies agree with previously reported measured and predicted values. Photon-assisted transitions suggest that two principle modes of lattice vibrations with energies of 220 and 260/cm are coupled to the electronic 2T2g state. A single phonon energy of 173/cm is observed to couple to the 2Eg electronic state. Crossover energy is estimated to be about 11,800/cm using a simple harmonic approximation. Lineshape analysis is performed. The fluorescence lifetime as measured to be a constant 3.9 microsec below 150 k decreasing to 2.9 microsec at 300 K.

Byvik, C. E.↗
NASA NTRS1986

Fluorescence from VUV excitation of formaldehyde

The 105-180-nm photoabsorption and fluorescence cross sections of H2CO are determined experimentally using a synchrotron-radiation light source and the apparatus described by Lee (1980). The results are presented in tables and graphs and discussed. VUV emission with threshold wavelength 140.3 nm is detected and attributed to the A 1Pi - X 1Sigma(+) system of CO, and UV emission from the HCO (B-X) system is found below a 147.5-nm threshold, the H-HCO dissociation energy having an upper limit of 3.61 + or - 0.03 eV. At 116 nm, the maximum quantum yields of the VUV and UV fluorescences are about 1.6 and 0.23 percent, respectively.

Suto, Masako↗
NASA NTRS1987

Ocean color spectral variability studies using solar-induced chlorophyll fluorescence

It is suggested that chlorophyll-induced ocean color spectral variability can be studied using only a passive airborne spectroradiometer instrument, with solar-induced chlorophyll fluorescence used as the standard against which all correlations are performed. The intraspectral correlation (ISC) method is demonstrated with results obtained during an airborne mapping mission in the New York Bight. The curvature algorithm is applied to the solar-induced chlorophyll fluorescence at about 690 nm, and good agreement is found with results obtained using active-passive correlation spectroscopy. The ISC method has application to spectral variability and resulting chlorophyll concentration measurement in different environmental conditions and in different water types.

Hoge, Frank E.↗
NASA NTRS1987

Environmental effects on laser-induced fluorescence spectra of natural waters

Laser fluorosensing can be used to monitor dissolved organic carbon (DOC), but analysis of the data can be hindered by several environmental phenomena. These phenomena include attenuation of the laser beam and differential attenuation of the fluorescence by the water column, variability in the molecular weight composition of the DOC, and temperature, pH, and metal ion effects on DOC fluorescence. These factors are discussed in terms of their effect on laboratory and remote field data analysis. Experimental results are provided. Analysis of fluorosensor data of DOC may be improved by compensating for the environmental factors. An improved methodology is discussed, and a suggestion is made for indirect monitoring of pH and metal ion concentration.

Vodacek, Anthony↗
NASA NTRS1987

Measurements of temperature, density, pressure, and their fluctuations in supersonic turbulence using laser-induced fluorescence

A laser-induced fluorescence method has been developed that provides simultaneous measurements of temperature, density, and their fluctuations owing to turbulence in unheated compressible flows. Pressure and its fluctuations are also deduced using the equation of state. Fluorescence is induced in nitric oxide that has been seeded into a nitrogen flow in concentrations of 100 ppm. Measurements are obtained from each laser pulse, with a spatial resolution of 1 mm and a temporal resolution of 125 ns. The method was applied to a supersonic, turbulent, boundary-layer flow with a free-stream Mach number of 2. For stream conditions in the range from 150-300 K and 0.3-1 atm, temperature is measured with an uncertainty of approximately 1 percent rms, while density and pressure uncertainties are approximately 2 percent rms.

Gross, K. P.↗
NASA NTRS1988

Two-photon-excited fluorescence spectroscopy of atomic fluorine at 170 nm

Two-photon-excited fluorescence spectroscopy of atomic fluorine is reported. A doubled dye laser at 286-nm is Raman shifted in H2 to 170 nm (sixth anti-Stokes order) to excite ground-state 2P(0)J fluorine atoms to the 2D(0)J level. The fluorine atoms are detected by one of two methods: observing the fluorescence decay to the 2PJ level or observing F(+) production through the absorption of an additional photon by the excited atoms. Relative two-photon absorption cross sections to and the radiative lifetimes of the 2D(0)J states are measured.

Herring, G. C.↗
NASA NTRS1988

The fluorescence of cometary OH

A new solar spectral atlas, improved oscillator strengths, and additional transitions have been incorporated in calculations of the fluorescent equilibrium of the OH molecule in comets as a function of heliocentric distance and radial velocity. Comparisons of detailed, ultraviolet band profiles have been made with the observed, high-resolution spectra of eight comets. While a purely fluorescent equilibrium calculation is sufficient to reproduce the major variations seen in the band structure from one comet to another, some collisional quenching of the inversion of the X2II3/2 Lambda-doublet ground state is required to match the detailed features. The relative population inversion of the upper and lower levels of the ground state is also calculated as a function of both distance and velocity, with the results being consistent with 18 cm radio observations. Predictions of the infrared OH vibrational bands are made; the 1-0 band should be observable if a sufficiently high spectral resolution is employed.

Schleicher, David G.↗
NASA NTRS1988

CH(A, B-X) fluorescence from vacuum ultraviolet excitation of CH3

The fluorescence due to single-photon and two-photon excitation of CH3 produced by the reaction of CH4 with F or Cl in a flow tube is investigated experimentally. The excitation source is an excimer laser operating with pulse duration 6 nsec and repetition rate 20 Hz, and the fluorescence is observed using a 0.25-m spectrometer and a 1024-channel diode array; the results are presented graphically. Emission is detected at 157 nm (with approximate cross section 8 x 10 to the -19th sq cm) and 193 nm, but not at 248 nm. The applicability of these data to studies of hydrocarbon combustion, the methane oxidation cycle in the atmosphere, interstellar molecule formation, and diamond film fabrication is indicated.

Ye, Chao↗
NASA NTRS1988

Identification of new fluorescence processes in the UV spectra of cool stars from new energy levels of Fe II and Cr II

Two fluorescence processes operating in atmospheres of cool stars, symbiotic stars, and the Sun are presented. Two emission lines, at 1347.03 and 1360.17 A, are identified as fluorescence lines of Cr II and Fe II. The lines are due to transitions from highly excited levels, which are populated radiatively by the hydrogen Lyman alpha line due to accidental wavelength coincidences. Three energy levels, one in Cr II and two in Fe II, are reported.

Johansson, Sveneric↗
NASA NTRS1988

Three color laser fluorometer for studies of phytoplankton fluorescence

A three-color laser fluorometer has been developed for field work operations. Using two tunable dye lasers (excitation wavelengths at 440 nm and 530 nm), broadband wavelength optical filters were selected to obtain maximum fluorescence sensitivity at wavelengths greater than 675 nm (chlorophyll) and 575 + or - 15 nm (phycoerythrin). The laser fluorometer permits the measurement of phytoplankton pigments under static or flowing conditions and more closely resembles the time scales (ns) and energy levels (mW) of other laser-induced fluorescence instruments.

Phinney, David A.↗
NASA NTRS1989

Mystery spot in supernova 1987A - Reflection or fluorescence by an interstellar cloud?

This paper explores fluorescence and reflection models of the companion to SN 1987A obseved by speckle interferometry, recalling a 1901 precedent. The apparent small angular size of the companion is a severe constraint. A fluorescence model cannot reach the observed brightness unless the ultraviolet burst from the supernova contained as many as 2 x 10 to the 58th ionizing photons. This is about 25 times stronger than generous current models. Even then, the expected line ratios and widths do not fit the observations. The absence of narrow H-alpha and H-beta lines in the supernova spectrum, the ratio of fluxes of the companion in H-alpha and forbidden N II line filters, the invisibility of the companion at 4861 (H-beta), and its detection at 5330 fail to agree with theory. A dust-reflection model is more promising, and the color can be reddened by the evaporation of small grains, but the model still falls more than about 1 mag short in brightness. Furthermore, a dust reflection should have increased in relative brightness in May-June 1987, rather than disappearing as the mystery spot did. If all the observations are correct, neither model is likely to work.

Felten, James E.↗
NASA NTRS1989

Spectrophotometry near the atmospheric cutoff of the strongest Bowen resonance fluorescence lines of O III in two planetary nebulae

Spectrophotometric results are presented for the stronger, well-resolved Bowen O III resonance fluorescence emission lines in the planetary nebulae 7027 and NGC 7662 down to and including the intrinsically strong line at 3133 A. These data are combined with results from the IUE atlas of spectra and similar results for the longer wavelength lines by Likkel and Aller (1986) to give the first full coverage of the Bowen lines. Good agreement is found with fluorescence theory for the primary cascade lines, except for the Likkel and Aller results. The efficiency of conversion of the exciting He II Ly-alpha into O III lines is determined, and values comparable to other planetary nebulae are found.

O'Dell, C. R.↗
NASA NTRS1989

NH2 fluorescence efficiencies and the NH3 abundance in Comet Halley

If NH3 is the dominant source of the NH2 observed in comet spectra, then the NH3 abundance can in principle be accurately determined. Fluorescence efficiencies for the (0, v-prime/2/, O) to (0, 0, 0) progression of NH2 bands are calculated for NH2 bands likely to be observed in the 4500-8200-A region of comets. The results differ from previous determinations of the NH2 band fluorescence efficiencies by factors in the range 1.4-5.9, leading t6o significant changes in previously reported NH2 production rates in comets. A recalculation of the NH3/H2O abundance ratio in Comet Halley gives about (0.5 + or - 0.2) percent in better agreement with the Giotto ion-mass-spectrometer results of Allen et al (1987).

Tegler, Stephen↗