Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “Photoexcitations”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 631 records · Page 35
All sourcesNASA NTRSDOE OSTI
NASA NTRS1978

Emission lines of H2 in the extreme-ultraviolet solar spectrum

A sunspot EUV spectrum covering the wavelength range from 1175 to 1714 A with high spatial and spectral resolution is examined which contains about 200 lines not previously reported in solar spectra. Many of the lines are identified as transitions in the Lyman bands of H2. It is shown that the H2 lines are photoexcited not only by H L-alpha, as reported previously, but also by the strong transition-region lines of C II, Si IV, and O IV. The line intensities are analyzed as far as is possible at present, and differences between the quiet chromosphere and the sunspot chromosphere are deduced. The polarization of the observed lines is briefly discussed, the importance of the H L-alpha fluorescence mechanism for the excitation of the H2 Lyman bands is demonstrated, and it is concluded that the H2 lines could also be observable in stars of spectral type later than the sun.

Jordan, C.↗
NASA NTRS1980

Modeling of a solar-pumped iodine laser

The direct conversion in space of solar radiation into laser radiation for power transmission to earth, satellites, or deep space probes shows promise as a reasonably simple technology and may have cost advantage in deployment and greater reliability compared to other methods of space power generation and transmission. The main candidates for solar pumping are the gas dynamic, photochemical, and direct photoexcited lasers. Here consideration is given to the photochemical reaction of alkyliodides which predominantly excite the I(2P1/2) state which then lases at 1.315 microns. The iodine ground state is eventually lost to reconstituting the gas or in the formation of molecular iodine. The rates at which the gas is required to be recycled through the laser system are modest. The side exposure at 100-fold solar concentration of a 100-m long tube with a 1 sq m cross section is estimated to provide 20 kW of continuous laser output. Scaling laws and optimum operating conditions of this system are discussed.

Wilson, J. W.↗
NASA NTRS1981

GaAs-oxide interface states - Gigantic photoionization via Auger-like process

Spectral and transient responses of photostimulated current in MOS structures were employed for the study of GaAs-anodic oxide interface states. Discrete deep traps at 0.7 and 0.85 eV below the conduction band were found with concentrations of 5 x 10 to the 12th/sq cm and 7 x 10 to the 11th/sq cm, respectively. These traps coincide with interface states induced on clean GaAs surfaces by oxygen and/or metal adatoms (submonolayer coverage). In contrast to surfaces with low oxygen coverage, the GaAs-thick oxide interfaces exhibited a high density (about 10 to the 14th/sq cm) of shallow donors and acceptors. Photoexcitation of these donor-acceptor pairs led to a gigantic photoionization of deep interface states with rates 1000 times greater than direct transitions into the conduction band. The gigantic photoionization is explained on the basis of energy transfer from excited donor-acceptor pairs to deep states.

Lagowski, J.↗
NASA NTRS1983

Optical characteristics of young quasars as sources of the cosmic X-ray background

The sources which dominate the thermal cosmic X-ray background cannot have X-ray spectra similar to the power laws measured for bright active galactic nuclei. The optical consequences of this disparity are pursued by considering a standard model for the photoexcitation and heating of the line emitting gas surrounding a central source (e.g., such as a quasar). The optical line emission to be associated with compact young quasar sources having the same X-ray spectrum as the X-ray background is found to be substantially different from that characteristic of typical quasars. Implications on quasar source counts and the identification of such new objects are discussed.

Boldt, E.↗
NASA NTRS1984

Optical characteristics of young quasars as sources of the cosmic X-ray background

The sources which dominate the thermal cosmic X-ray background cannot have X-ray spectra similar to the power laws measured for bright active galactic nuclei. The optical consequences of this disparity are pursued by considering a standard model for the photoexcitation and heating of the line-emitting gas surrounding a central source (e.g., such as a quasar). The optical line emission to be associated with compact young quasar sources having the same X-ray spectrum as the X-ray background is found to be substantially different from that characteristic of typical quasars. Implications on quasar source counts and the identification of such new objects are discussed.

Boldt, E.↗
NASA NTRS1984

Study of the sulphur-containing molecules in the EUV region. IV - The dissociation processes of SO2 in the 760 A region

The fluoresence throughout the VUV and violet regions has been observed from photoexcitation of SO2, and the absolute fluorescence cross sections of the VUV emissions have been measured at selected atomic emission lines and in the synchrotron radiation continuum from 550 to 760 A. Emissions from the O I 1304 A and several S I lines account for the fluorescence observed in the VUV region, while the fluorescence observed in the UV and violet regions is due to excited SO and, also, possibly to excited SO2(+) and SO(+). Structure in the VUV fluorescence excitation function is attributed to predissociation of Rydberg states. The VUV fluorescence resulting from an O2 elimination process, SO2 + h times nu yielding metastable S + O2, has been observed in the present work although the threshold for this process has not been determined. The threshold corresponding to the total dissociation process, SO2 + h times nu yielding metastable S + 2O, has been measured and found to agree reasonably with the expected value calculated from thermochemical data. The processes for the formation of S(+) are also discussed.

Wu, C. Y. R.↗
NASA NTRS1984

Variable ultraviolet emission in SY Muscae

The results of continued IUE monitoring of the symbiotic variable SY Muscae, following an earlier report of a radical enhancement in UV emission from the star, are reported. Over the course of one year, the prominent emission lines of N V, O V, C IV, and He II appear to be gradually decreasing in absolute intensity. This appears to coincide with a steady decline in electron density in the emission line forming region. The data are consistent with a sudden ejection event in which material expelled from the surface of a hot subdwarf has exposed the underlying UV continuum of the star. A number of strong emission lines that are photoexcited by the intense radiation field of the secondary also exhibit broad pedestal emission that suggests turbulent velocities of about 150-300 km/s in an expanding shell or possibly in an accretion disk.

Michalitsianos, A. G.↗
NASA NTRS1985

Population mechanisms for the He(+) n = 3 levels determined from measurements of the solar 1640 A emission

The 1640 A line profile is analyzed using solar data obtained by the NRL slit spectrograph on Skylab. Data from coronal hole regions, quiet sun regions, active regions, prominence regions, and flare regions are presented. The relative densities of 3s, 3p, and 3d levels are determined from the data, and the dominant population mechanisms for these levels are inferred using a rate equation model for the He(+) ion. The relative importance of collisional excitation, radiative recombination, and photoexcitation is determined for each of the solar regions.

Seely, J. F.↗
NASA NTRS1985

Comprehensive solar cell modeling and correlation studies

Modeling and correlation studies of solar cells was discussed. Recursive relationships were used to generate solutions at a number of mesh points within the emitter region. Photoexcited hole concentration and built-in electric field were calculated as a function of position. Simulated and experimentally determined I-V curves were shown to have good fit.

Lamorte, M. L.↗
NASA NTRS1985

Native hole trap in bulk GaAs and its association with the double-charge state of the arsenic antisite defect

A dominant hole trap has been identified in p-type bulk GaAs employing deep level transient and photocapacitance spectroscopies. The trap is present at a concentration up to about 4 x 10 to the 16th per cu cm, and it has two charge states with energies 0.54 + or - 0.02 and 0.77 + or - 0.02 eV above the top of the valence band (at 77 K). From the upper level the trap can be photoexcited to a persistent metastable state just as the dominant midgap level, EL2. Impurity analysis and the photoionization characteristics rule out association of the trap with impurities Fe, Cu, or Mn. Taking into consideration theoretical results, it appears most likely that the two charge states of the trap are the single and double donor levels of the arsenic antisite As(Ga) defect.

Lagowski, J.↗
NASA NTRS1986

Measurement of HO2 chemical kinetics with a new detection method

Reaction rate constants of HO2+O3 were measured at various temperatures using a newly developed HO2 detection method. HO2 was detected by the OH(A-X) emission produced from photodissociative excitation of HO2 at 147 nm. In order to examine the possible interference of other emitting species with the HO2 detection, the photoexcitation processes of all the chemical species existing in the discharge flow tube were also investigated. The results are summarized.

Lee, Long C.↗
NASA NTRS1986

CF2 and CFCl fluorescence from VUV excitation of C2F3Cl

The photoexcitation process of C2F3Cl molecule was investigated in the 106 to 230 nm region using synchrotron radiation as a light source. Photoabsorption and fluorescence cross sections were measured and used to determine the fluorescence quantum yield. Fluorescence yield starts to appear at 170 nm and increases to about 2% at 155 nm. The fluorescence spectra were dispersed to identify the emitting species. At the excitation wavelength of 155 nm, the emission system is CFCl (Hermitian conjugate of A - Hermitian conjugate of X), and at 123.9 nm, both the CF2 (Hermitian conjugate of A - Hermitian conjugate of X) and CFCl (Hermitian conjugate of A - Hermitian conjugate of X) systems are observed. The dissociation processes that produced these excited species are discussed.

Nee, J. B.↗
NASA NTRS1986

Neutral oxygen and sulfur densities in the Io torus

Ultraviolet spectra (1150-1750 A) of the Io torus, obtained using a rocket-borne telescope and spectrometer in May 1981, show emissions of neutral sulfur and neutral oxygen, as well as ionic emissions of S II, S III, and S IV. The emissions are observed almost 180 deg from the location of the satellite Io and imply the existence of an extended neutral atomic cloud associated with the torus. In the present work, neutral densities and ion mixing ratios are derived by modeling the observations using torus electron temperature and density profiles proportional to those at the time of the Voyager 1 encounter. The model includes the effects of photoexcitation, which can be considerable if the neutral cloud is large enough. At the time of the observation, substantial densities of neutral oxygen and neutral sulfur are found in regions of the torus far from Io. These lower limits on the neutral densities are compared to theoretical models of the neutral source, and the ion mixing ratios are compared to IUE observations of the torus obtained over a period of 5 yr. Arguments that tend to rule out Europa as the source of the neutral emission are also presented.

Skinner, T. E.↗
NASA NTRS1986

Quantitative photoabsorption and fluorescence study of H2O and D2O at 50-190 nm

The photoabsorption cross sections and the fluorescence quantum yields of H2O and D2O were measured in the 50-190 nm region using synchrotron radiation as a light source. The oscillator strengths for the Rydberg states of H2O and D2O were determined from the absorption cross sections measured. The processes for the production of fluorescence from the excited species H(asterisk) (n greater than 2), D(asterisk) (n greater than 2), OH(asterisk) (A) and OD(asterisk) (A) are discussed. The upper limit for the dissociation energy of D(D-OD) was determined, from the threshold of the OD(A-X) fluorescence, to be 5.14 + or - 0.01 eV. The upper limit for the cross section of visible fluorescence from the excited H2O(+) ions was determined to be 2 x 10 to the -19th sq cm. A comparison between the photoexcitation spectra of H2O and D2O is made.

Lee, L. C.↗
NASA NTRS1988

Solar generation and storage of O2 (a 1 delta g)

An investigation was performed of the technical steps required to design a solar powered oxygen-iodine laser. Singlet delta oxygen is formed upon transfer of energy from selected photoexcited dye molecules to ground state molecular oxygen and then is concentrated and stored as an endoperoxide by reaction with an aromatic hydrocarbon. The endoperoxide, when heated, releases singlet oxygen in high yield thus providing a regenerable source of laser fuel. Energy transfer from dye molecules to molecular oxygen was investigated. When dye molecules were adsorbed to polymer substrates it was observed that the dye became embedded in the polymer matrix. Porphin dyes were incorporated into films of 1,4-dimethyl-2-poly(vinylnaphthalene), 2PVN. An endoperoxide was formed when porphin-doped 2PVN was exposed to visible radiation. This demonstrates the possibility of generating singlet oxygen using solar energy and concentrating and storing it in one simple step. Transport of energy by exciton migration in polycrystalline dye films was also investigated.

Twarowski, Allen J.↗
NASA NTRS1987

Nonlinear effects in coplanar GaAs/InGaAs strained-layer superlattice directional couplers

The performance characteristics of InGaAs/GaAs strained-layer superlattice coplanar ridge-type directional couplers realized by molecular beam epitaxy are reported. The measured power transfer characteristics with 1.15 micron incident photoexcitation demonstrate nonlinear coupling due to absorption associated with the tails of the excitonic resonances in the quantum wells. From a theoretical fit of the measured data, the nonlinear refractive index coefficient, n2, of the multiquantum well is found to be 2.25 x 10 to the -7th sq cm/W. This agrees very well with a value of n2 = 1.9 x 10 to the-7th sq cm/W obtained independently on the same material from interferometric measurents.

Das, Utpal↗
NASA NTRS1988

Discrete fluorescent saturation regimes in multilevel systems

Using models of multilevel atoms, the fluorescent process was examined for the ratio of the photooxidation rate, Pij, to the collisional oxidation rate, Cij, in the pumped resonance transition i-j. It is shown that, in the full range of the parameter Pij/Cij, there exist three distinct regimes (I, II, and III) which may be usefully exploited. These regimes are defined, respectively, by the following conditions: Pij/Cij smaller than about 1; Pij/Cij much greater than 1 and Pij much lower than Cki; and Pij/Cij much greater than 1 and Pij much higher than Cki, where Cki is the collisional rate populating the source level i. The only regime which is characterized by the sensitivity of fluorescent-fluorescent line intensity ratios to Pij is regime I. If regime III is reached, even fluorescent-nonfluorescent line ratios become independent of Pij. The analysis is applied to the resonant photoexcitation of a carbonlike ion.

Kastner, S. O.↗
NASA NTRS1987

CF2 and CFCl fluorescence from VUV excitation of C2F3Cl

The photoexcitation process of the C2F3Cl molecule was investigated in the 106-230-nm region using synchrotron radiation as a light source. Photoabsorption and fluorescence cross sections were measured and used to determine the fluorescence quantum yield. Fluorescence yield starts to appear at 170 nm and increases to about 2 percent at 155 nm. The fluorescence spectra were dispersed to identify the emitting species. The (A-X) systems of CFCl (at excitation wavelengths 155 and 123.9 nm) and CF2 (at 123.9 nm) are observed. The dissociation processes that produced these excited species are discussed.

Nee, J. B.↗