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At least 685 records · Page 38

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NASA NTRS1983

Feasibility of airborne detection of laser-induced fluorescence emissions from green terrestrial plants

The present investigation provides a demonstration of the feasibility of the airborne detection of the laser-induced fluorescence spectral emissions from living terrestrial grasses, shrubs, and trees using existing levels of lidar technology. Airborne studies were performed to ascertain system requirements necessary to detect laser-induced fluorescence from living terrestrial plants, to assess the practical acquisition of useful single-shot laser-induced fluorescence (LIF) waveforms over vegetative canopies, and to determine the comparative suitability of laser system, airborne platform, and terrestrial environmental parameters. The field experiment was conducted on May 3, 1982, over the northern portion of Wallops Island, VA. Attention is given to airborne lidar results and the description of laboratory investigations.

Hoge, F. E.↗

NASA NTRS1982

Fluorescent discharge lamp

A rapidly illuminating fluorescent lamp 1,200 mm long and 32.5 mm in diameter with an interior conducting strip which is compatible with conventional fixtures and ballasts is described. The fluorescent lamp is composed of a linear glass tube, electrodes sealed at both ends, mercury and raregas sealed in the glass tube, a fluorescent substance clad on the inner walls of the glass tube, and a clad conducting strip extending the entire length of the glass tube in the axial direction on the inner surface of the tube.

Mukai, E.↗

NASA NTRS1983

Comments on laser-excited fluorescence of the hydroxyl radical: Relaxation coefficients at atmospheric pressure, appendix 5

The lifetime of the excited state of a atom or molecule is often determined from the rate of fluorescence decay originating as a function of buffer gas pressure, an accurate determination is made of the rates of collision induced transitions away from the excited state. Deconvolution can in principle be employed to resolve fluorescence times shorter than the response times of the system. However, attainable reproducibility and accuracy in actual experiments usually set a limit beyond which no meaningful results are expected. Prudence thus dictates that the results of deconvolution be viewed with extreme caution whenever fluorescence time much shorter than the response of times of the system are indicated.

Wang, C. C.↗

NASA NTRS1984

Fluorescence from photofragments as an aid in identifying new molecular states - The N2 case

Synchrotron radiation has been used to measure the excitation function for producing fluorescence in the 1050-1800 A region through photodissociative excitation of N2 in the 400-620 A region and the photoabsorption cross section of N2 in the 600-670 A region. From the fluorescence excitation function there is found (1) a molecular state(s) in the 21-22.5 eV region which has not been observed in absorption and photoionization spectra, (2) structures which correlate with known two-electron excited Rydberg states, and (3) a decrease in fluorescence cross section at photon energies greater than 23.8 eV, which indicates the existence of new competing processes. The observed molecular state(s) as well as the weak absorption structure in the 19-21 eV region are tentatively assigned as a Rydberg series leading to the D2Pi(g) of N2(+). The newly observed Rydberg states in the 21-22.5 eV region may account for the unusual behavior recently reported in photoelectron spectroscopy results.

Wu, C. Y. R.↗

NASA NTRS1984

Spectrum of squeezing in resonance fluorescence

A spectral analysis of the squeezing in resonance fluorescence is accomplished via a study of a phase sensitive correlation function. In the case of resonance fluorescence from a two-level system, the initial state is a coherent state for the laser mode, a vacuum for the other states of the radiation field, and the atoms in the lower level. Maximum squeezing for resonance excitation is obtained for near resonant frequencies of the fluorescent light. Possible experimental configurations are discussed in order to detect spectral squeezing.

Collett, M. J.↗

NASA NTRS1985

The role of reabsorption in the spectral distribution of phytoplankton fluorescence emission

A theoretical model has been developed to describe an experimentally observed spectral shift in the fluorescence emission from phytoplankton as a result of the internal reabsorption of that emission. This model accounts for both the absorption of the primary excitation and the modification of the fluorescence through the reabsorption of the emitted light by the chloroplast and by the surrounding medium. Comparisons are made between the results of the theoretical model and data derived from experiments using a number of different phytoplankton species, each adapted to varying light conditions. The details of the model are discussed, and the consequences of its interpretation on the spectral distribution of the fluorescence emission from phytoplankton are examined.

Collins, D. J.↗

NASA NTRS1987

Containerless Atomic-Fluorescence Property Measurements

Report describes studies conducted to establish and verify use of laser-induced fluorescence in monitoring and controlling high-temperature containerless processes. Specimens levitated by gas jets or electromagnetic fields and heated by laser beams or electromagnetic induction while being irradiated and detected by fluorescence technique. Makes quantitative and qualitative comparisons among three new methods of temperature measurement; all rely on laser-induced fluorescence. One method gas-density thermometry with seed gas. Other two methods involve measurements of velocities of evaporating atoms or of population ratios of different electronic states.

Nordine, P.↗

NASA NTRS1987

Measuring Gases With Laser-Induced Fluorescence

Temperature, density, and pressure at selected point in low-temperature, turbulent gaseous flow measured simultaneously by pulsed laser-induced fluorescence (LIF). Measurements made with spatial and temporal resolution. LIF measurements nonintrusive and constitute first alternative means of measuring turbulent fluctuations in temperature and density that compares with conventional hot-wire-anemometer data. Laser beams focused to small spots in wind tunnel and reference cell induce fluorescence in nitric oxide, small amount s mixed with main gas flow. Fluorescence radiation depends on main-gas temperature, pressure, and density and measured to deduce these quantities.

Mckenzie, Robert L.↗

NASA NTRS1986

Laser and sunlight-induced fluorescence from chlorophyll pigments

Fluorescence properties of chlorophyll pigment bearing plant foliage utilizing a 337 nm nitrogen laser and integrating sphere were studied. Measured yields, in terms of number of photons emitted per 100 photons absorbed, range from 1.5 to 0.1 for the 685 nm peak, and from 4.2 to 0.2 for the 730 nm peak. Decreasing order of magnitude puts herbaceous leaves ahead of all others followed by broad leaves of hardwoods and coniferous needles. Meaningful quantization for the fluorescence peaks at 430 and 530 nm could not be attained. Passive monitoring of these fluorescence peaks is successful only for the 685 nm from the ocean surface. Field data show the reflectance changes at 685 nm due to the algae presence amounts to 1% at most.

Kim, H. H.↗

NASA NTRS1986

Laser induced fluorescence and phosphorescence of matrix isolated glyoxal - Evidence for exciplex formation in the A 1Au and a 3Au states

Laser-induced fluorescence and phosphorescence as well as infrared and visible absorption spectra of glyoxal in Ar, N2, and CO matrices are presented and analyzed. Glyoxal in its first excited electronic state is shown to form an exciplex with its nearest neighbors in all three matrices, and transitions normally forbidden dominate the emission spectra. The spectral characteristics of these complexes are similar to those of the Ar-glyoxal complex found in supersonic beam experiments. Due to the matrix cage effect, no vibrational predissociation is observed. The phosphorescence lifetime is determined and an upper limit is given for the fluorescence lifetime. This, in combination with the relative intensities of fluorescence and phosphorescence, can be used to place limits on the quantum yields of the various relaxation processes.

Van Ijzendoorn, L. J.↗

NASA NTRS1987

The excitation function of OH(A2Sigma/+/ - X2Pi) fluorescence produced through photodissociation of H2O

The absolute cross section for production of OH(A2Sigma/+/ X2Pi) fluorescence through photodissociative excitation of H2O was obtained using the University of Wisconsin synchrotron radiation source. The absolute fluorescence cross section function shows three broad features, roughly centered at 710, 400, and 240 A; the maximum value fluorescence cross section was found to be 3.4 x 10 to the -20th sq cm at 703 A. The broad features in the 700-A and 400-A regions are considered to be due to the second and the third excited states of H2O, respectively; it is argued that the broad feature at 240 A may have an origin similar to that of the 400-A broad feature. From present and previous data on quantum yields, it is estimated that part of the yield may be attributable to the production of neutral ground state OH(X) and H(1 2S) fragments. Such a process could provide a significant source of high kinetic energy OH and H observed in cometary atmospheres.

Wu, C. Y. Robert↗

NASA NTRS1988

Fluorescence processes and line identifications in the UV spectra of cool stars

Fluorescence processes active in the outer atmospheres of noncoronal cool stars and the UV lines they produce are summarized. Eight pumping processes and 21 fluorescent line products are discussed. The processes, which produce 12 lines, involves energy levels not previously known to be radiatively populated. Four of these are examples of self-fluorescence, whereby one or more lines of Fe II photo-excite through coincident lines the upper levels of other Fe II lines lines seen in emission, while two others explain the selective excitation of solitary Ni II and Si I lines. Nine of the line products are decays from levels in Fe I and Fe II already known to be radiatively populated.

Carpenter, Kenneth G.↗

NASA NTRS1988

CH3O(A-X) fluorescence from photodissociation of dimethyl ether

The paper reports the quantitative photoabsorption and fluorescence cross sections of dimethyl ether (DME) measured with synchrotron radiation. Determinations were also made of the absolute fluorescence cross sections, the fluorescence quantum yield, and the radiative lifetime.

Suto, Masako↗

NASA NTRS1990

Saturated fluorescence measurements of the hydroxyl radical in laminar high-pressure flames

The efficacy of laser saturated fluorescence (LSF) for OH concentration measurements in high pressure flames was studied theoretically and experimentally. Using a numerical model describing the interaction of hydroxyl with nonuniform laser excitation, the effect of pressure on the validity of the balanced cross-rate model was studied along with the sensitivity of the depopulation of the laser-coupled levels to the ratio of rate coefficients describing: (1) electronic quenching to (sup 2) Sigma (+) (v double prime greater than 0), and (2) vibrational relaxation from v double prime greater than 0 to v double prime = 0. At sufficiently high pressures and near-saturated conditions, the total population of the laser-coupled levels reaches an asymptotic value, which is insensitive to the degree of saturation. When the ratio of electronic quenching to vibrational relaxation is small and the rate of coefficients for rotational transfer in the ground and excited electronic states are nearly the same, the balanced cross-rate model remains a good approximation for all pressures. When the above ratio is large, depopulation of the laser-coupled levels becomes significant at high pressures, and thus the balanced cross-rate model no longer holds. Under these conditions, however, knowledge of the depletion of the laser-coupled levels can be used to correct the model. A combustion facility for operation up to 20 atm was developed to allow LSF measurements of OH in high pressure flames. Using this facility, partial saturation in laminar high pressure (less than or equal to 12.3 atm) C2H6/O2/N2 flames was achieved. To evaluate the limits of the balanced cross-rate model, absorption and calibrated LSF measurements at 3.1 and 6.1 atm were compared. The fluorescence voltages were calibrated with absorption measurements in an atmospheric flame and corrected for their finite sensitivity to quenching with: (1) estimated quenching rate coefficients, and (2) an in situ measurement from a technique employing two fluorescence detection geometries.

Carter, Campbell D.↗

NASA NTRS1990

Discovery of molecular hydrogen fluorescence in the diffuse interstellar medium

The first detection of molecular hydrogen fluorescence in the diffuse interstellar medium is reported. Using the Berkeley UVX Shuttle Spectrometer, H2 Lyman band fluorescence has been observed in four directions, each with high significance. Molecular hydrogen fluorescence is detected in all directions that have previously been found to contain significant CO emission. A simple equilibrium model has been developed that includes attenuation of the incident UV radiation field by H2 line and dust continuum absorption. Evidence is found that the gas in the CO emission portions of the clouds may be clumpy, with a filling factor less than 0.2 and an average density greater than 30/cu cm in most cases.

Martin, Christopher↗

NASA NTRS1989

Laser-fluorescence measurements of nitric oxide in low-pressure H2/O2/NO flames

The concentration profiles of NO in low-pressure (76 Torr) H2/O2/Ar flames to which nitric oxide is added are measured by pulsed laser-induced fluorescence. Temporally resolved fluorescence measurements are used to determine the collisional deexcitation rates needed to convert time-integrated fluorescence signal into oxide concentration. Five flames are studied with H2/O2 equivalence ratios of 0.88, 0.98, 1.22, 1.37, and 1.50. In these flames the collisional deexcitation rate decreases rapidly above the burner surface as the density decreases with increasing temperature. A 20 percent decrease is observed for the lean flames, and a 30 percent decrease for the rich flames. Within the precision of the measurement technique (+ or - 10 percent), no significant removal of nitric oxide is observed in these flames.

Cattolica, R. J.↗

NASA NTRS1991

Imaging of molecular hydrogen and oxygen by single and two-photon fluorescence using laser and flashlamp sources

Two flow visualization techniques, i.e., simultaneous two-dimensional fluorescence imaging of H2 and O2 in a diffusion flame, and quasi-linear fluorescence imaging of O2, are presented. The first uses an injection-locked argon-fluoride excimer laser and a partial overlap of a two-photon ground state absorption in H2 with a single photon absorption from a vibrational level in O2. The second uses a simple, high-intensity ultraviolet flashlamp which provides a flux of photons in the 180-195 nm range, sufficient to produce a quasi-one-dimensional fluorescence image of hot/room temperature oxygen. Both techniques do not require that a seed material be introduced into the flow, they can image major flow constituents, and provide an instantaneous snapshot of the flow.

Diskin, Glenn S.↗

NASA NTRS1991

The ultraviolet continuum and the fluorescent H2 lines in low-excitation Herbig-Haro objects

Long-exposure IUE observations are used here to determine the spatial distribution of the fluorescent H2 line emission in the low-excitation objects HH 43 and HH 47. In HH 43, the spatial distribution of the fluorescent H2 line emission has a width equal to or smaller than the width of the point-spread function of IUE, while both the short-wavelength UV continuum and the optical forbidden S II lines show distributions which are wider by more than a factor of two. In HH 47, the results are qualitatively analogous, but the differences between the fluorescent HH 23 emission distribution on the one hand, and the UV continuum and optical forbidden S II distribution on the other, are smaller. These distributions indicate that the appropriate conditions for the formation of fluorscent H2 lines are fulfilled only in rather narrow regions for these low-excitation objects. A continuous energy distribution in HH 43 and HH 47 is found which differs only slightly from that in the high-excitation objects HH 1 and HH 2.

Boehm, K. H.↗