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At least 145 records · Page 8

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NASA NTRS1986

Experimental apparatus for photon/ion coincidence measurements of dielectronic recombination

An inclined beams apparatus for the measurement of absolute cross sections for dielectronic recombination between free electrons and singly or multiply charged ions is described. The collision products, a photon and a lower-charge-state ion, are detected in delayed coincidence. Measurements of dielectronic recombination in C(3+) are described to illustrate the use of the apparatus and techniques. Verification of the calibrations and operation of the apparatus is demonstrated through measurements of charge transfer and electron impact excitation.

Gardner, L. D.↗

NASA NTRS1987

Determination of lifetime and surface recombination velocity of p-n junction solar cells and diodes by observing transients

The unified view of transient methods for the determination of recombination lifetime tau and back surface recombination velocity S presented here for silicon solar cells and diodes attempts to define limitations of existing methods and to evolve improvements. The presence of sizable junction capacitance for silicon devices under forward voltage invalidates the use of conventional open-circuit voltage decay (OCVD) and reverse recovery. This led Green (1983) to his method of compensated open-circuit voltage decay, in which the addition of an external resistor shunting the solar cell partially corrects for the presence of the junction capacitance. Setting this resistance to zero produces an electrical short-circuit current-decay method, which has the advantage of enabling determination of both tau and S. In an alternate approach, one may insert the functional dependence of the junction capacitance on forward voltage. This new method, denoted by the acronym OCVDCAP, enables the determination of tau with apparently greater accuracy than that obtained by previous methods utilizing voltage transients. But OCVDCAP has in common with the previous methods that it determines tau only and has practical utility only for determining tau of long-base devices. This means that it is useful only for thick base regions. In principle, however, it has an advantage over short-circuit current decay: it requires only pressure contacts, not ohmic contacts, and therefore may be used to determine tau after key processing steps in manufacturing.

Lindholm, Fredrik A.↗

NASA NTRS1987

Variations in the electrical short-circuit current decay for recombination lifetime and velocity measurements

An improved measurement system for electrical short-circuit current decay is presented that extends applicability of the method to silicon solar cells having an effective lifetime as low as 1 microsec. The system uses metal/oxide/semiconductor transistors as voltage-controlled switches. Advances in theory developed here increase precision and sensitivity in the determination of the minority-carrier recombination lifetime and recombination velocity. A variation of the method, which exploits measurements made on related back-surface field and back-ohmic contact devices, further improves precision and sensitivity. The improvements are illustrated by application to 15 different silicon solar cells.

Jung, Tae-Won↗

NASA NTRS1987

Radio recombination lines from fast shocks in molecular clouds, with application to bipolar flows

Detailed shock calculations are presented which demonstrate that fast dissociative shocks in molecular clouds produce detectable radio recombination lines and continuum lines, in addition to optical and infrared recombination lines. The theoretical results are applied to outflows from young stellar objects; bipolar flows are treated as massive, radiative winds. The calculations support Hasegawa and Akabane's (1984) interpretation of the H51-alpha line from Orion-KL as being due to a shock, and are consistent with the two-shock model of Chernoff, Hollenbach, and McKee (1982).

Mckee, Christopher F.↗

NASA NTRS1988

Recombination line intensities for hydrogenic ions. II - Case B calculations for C VI, N VII and O VIII

The intensities of recombination lines formed in extended, optically thim, photoionized plasmas such as those found in PN, H II regions, and winds of certain hot stars are an important source of information on chemical abundances and can sometimes provide estimates of electron temperature. In this paper, the intensities of the recombination lines of C VI, N VII, and O VIII are calculated, accounting for both electron and heavy particle collisions and assuming case B of Baker and Menzel. The computational procedure is explained. The intensities of lines formed by transitions n(u) - n(l) are tabulated for n(u) of 50 or less, n(l) of 29 or less, at log N(e) = 4(1)13 and 10 values of electron temperature in the interval 10,000 K to 500,000 K.

Storey, P. J.↗

NASA NTRS1989

Atom-atom inelastic collisions and three-body atomic recombination in weakly ionized argon plasmas

A stationary collisional-radiative model including both inelastic electron-atom and atom-atom collisions is used to examine nonequilibrium weakly ionized argon plasmas with atomic densities 10 to the 16th to 10 to the 20th/cu cm, temperatures below 6000 K, and with different degrees of radiation trapping. It is shown that three-body atomic recombination becomes important at high particle densities. Comparison is made between the present approach and Thomson's theory for atomic recombination.

Braun, C. G.↗

NASA NTRS1989

Intersubband Auger recombination and population inversion in quantum-well subbands

The intersubband-Auger-recombination time of electrons under population-inversion conditions in a single quantum well is calculated by taking into account momentum- and energy-conservation rules, and by employing Fermi-Dirac statistics. The screened matrix element of the electron-electron interaction and the overlap integral are calculated for an infinitely deep quantum well. The results are in a good agreement with published experimental data. As a major nonradiative process, the Auger recombination is related to threshold current of infrared lasers based on intersubband transitions in quantum-well structures. The realization of these devices and other limitations to achieving population inversion are discussed. In view of the results, development of these lasers for emission wavelengths corresponding to energies below the LO-phonon energy seems feasible.

Borenstain, S.↗

NASA NTRS1989

Mechanistic model for catalytic recombination during aerobraking maneuvers

Several mechanistic models are developed to predict recombination coefficients for use in heat shield design for reusable surface insulation (RSI) on aerobraking vehicles such as space shuttles. The models are applied over a temperature range of 300 to 1800 K and a stagnation pressure range of 0 to 3,000 Pa. A four parameter model in temperature was found to work best; however, several models (including those with atom concentrations at the surface) were also investigated. Mechanistic models developed with atom concentration terms may be applicable when sufficient data becomes available. The requirement is shown for recombination experiments in the 300 to 1000 K and 1500 to 1850 K temperature range, with deliberate concentration variations.

Willey, Ronald J.↗

NASA NTRS1990

Potential technology transfers of research on low-temperature carbon monoxide-oxygen recombination catalysts

Results from research on catalytic recombination of CO-O2 for stable closed-cycle operation of CO2 lasers hold much promise for a variety of technology transfer. Expansion of CO2 laser remote sensing applications toward chemical detection and pollution monitoring would certainly be expected. However, the catalysts themselves may be especially effective in low-temperature oxidation of a number of chemicals in addition to CO. It is therefore of interest to compare the CO-O2 catalysts with chemical systems designed for chemical sensing, air purification and process catalysis. Success in understanding the catalytic mechanisms of the recombination of CO-O2 could help to shed light on how catalyst systems operate. New directions in low-temperature oxidation catalysts, coatings for chemical sensors and sorbents for air purification could well emerge.

Poziomek, Edward J.↗

NASA NTRS1989

Aeronomical determinations of the quantum yields of O (1S) and O (1D) from dissociative recombination of O2(+)

Data from the visible-airglow experiment on the Atmosphere Explorer-E satellite have been used to determine the quantum yields of O (1S) and O (1D) from the dissociative recombination of O2(+) based on a constant total recombination rate from each vibrational level. A range of values between 0.05 and 0.18 has been obtained for the quantum yield of O (1S) and shows a positive correlation with the extent of the vibrational excitation of O2(+). The quantum yield of O (1D) has been measured to be 0.9 + or - 0.2, with no apparent dependence on the vibrational distribution of O2(+).

Yee, Jeng-Hwa↗

NASA NTRS1991

Oxygen recombination in the sealed nickel-cadmium cell

The purpose of this study was to determine what parameters are most critical for controlling overcharge pressure in a sealed nickel cadmium cell. Topics covered in viewgraph format are: parameters examined; oxygen evolution and recombination; experimental test description; and effect of parameters on recombination rate.

Zimmerman, Al H.↗

NASA NTRS1990

Solvent-tuned intramolecular charge-recombination rates in a conjugated donor-acceptor molecule

The nonradiative charge-recombination rates from the charge-transfer state of a new conjugated donor-acceptor molecule (p-cyano-p-prime-methylthiodiphenylacetylene) can be tuned over almost an order of magnitude by varying the polarity of the solvent. These measurements of intramolecular recombination show a turnover of rates as a function of emission energy, consistent with the 'normal' and 'inverted' behavior of Marcus theory. Steady-state spectra and time-resolved measurements make it possible to quantitatively compare thermal and optical electron-transfer rates as a function of driving force and demonstrate their correspondence.

Khundkar, Lutfur R.↗

NASA NTRS1992

Hydrogen recombination at high optical depth and the spectrum of SN 1987A

A general theory is presented for hydrogen recombination line formation in an expanding medium in which some of the lines are optically thick. This theory is used to calculate the time evolution of the hydrogen lines of SN 1987A at t equal to or greater than 150 days, assuming that the supernova envelope is a homologously expanding uniform sphere. The theoretical luminosities and ratios of the recombination lines agree remarkably well with the observations. For the first 2 yr, the supernova envelope is optically thick to Balmer continuum. For t equal to or less than 400 days, hydrogen is ionized primarily from the n = 2 level by Balmer continuum photons, which are provided partly by the two-photon decay of the 2s state and partly by emission lines of heavy elements.

Xu, Yueming↗

NASA NTRS1992

Recombination line intensities for hydrogenic ions. III - Effects of finite optical depth and dust

The effect on the recombination spectrum of hydrogen arising from: (1) finite optical thickness in the Lyman lines; (2) the overlapping of Lyman lines near the series limit; (3) the absorption of Lyman lines by dust or photoionization, and (4) the long-wave radiation emitted by dust is examined. Full account is taken of electron and heavy particle collisions in redistributing energy and angular momentum. It is seen that each of these deviations from the classical Case B leads to observable effects, and that dust influences the recombination spectrum in characteristic ways that may make possible new observational constraints on dust properties in nebulosities. On the basis of these calculations it is believed that the uncertainty in the determination of the helium-to-hydrogen abundance ratio in the universe may be larger than currently claimed.

Hummer, D. G.↗

NASA NTRS1992

Model for oxygen recombination on silicon-dioxide surfaces. II - Implications toward reentry heating

This paper briefly reviews the model for recombination of oxygen on a silicon-dioxide surface presented in detail in a previous paper. New data supporting the model is also presented. The ramifications of the model toward the production of excited molecular oxygen is examined as it pertains to surface heating. A reentry simulation is given and compared to STS-2 reentry data, and conclusions are drawn as to the implications of the recombination model toward reentry heating. A possible buffering of the heating above a critical temperature associated with the physics of the model is also discussed.

Jumper, E. J.↗

NASA NTRS1992

Dielectronic recombination as a function of electric field strength

Dielectronic recombination (DR) is the dominant recombination mechanism at coronal temperatures and densities. We present a procedure for calculating DR rate coefficients as a function of electric field strength and apply this method to carbon ions. We focus on the competing effects of enhancement by plasma microfields and rate decrease through collisional excitation and ionization. We find that, in the case of C(3+), a significant rate enhancement results, leading to a reinterpretation of C IV emission-line intensities in the sun and late-type stars. We further consider how macroscopic electric fields, in particular motional electric fields, can affect DR rate coefficients, demonstrating dramatic rate increases for a number of the carbon ions.

Reisenfeld, Daniel B.↗

NASA NTRS1992

Radio continuum and radio recombination line observations of Sagittarius B1 and G0.6-0.0

Continuum emission and H110-alpha recombination line emission from Sgr B1 and G0.6-0.0 have been observed using the VLA. It is shown that Sgr B1 is a region of great complexity, both spatially and kinematically. The continuum observations show that this region is dominated by many extended features rather than compact sources. On the other hand, Sgr B2 is dominated by several ultracompact H II regions. The two regions may be in different stages of evolution, with Sgr B1 being older, perhaps by as much as 0 exp 6 yrs. The recombination line study shows that Sgr B1 is composed of two distinct kinematic regions, a simple western one and a more complex eastern one. G0.6-0.0 is a region composed of at least four ultracompact H II regions that is situated between Sgr B1 and Sgr B2. There is an arc of ionized gas that lies to the east and to the south of these compact regions. The velocity of G0.6-0.0 is intermediate between those of Sgr B1 and Sgr B2. These facts strengthen the argument that these regions are physically associated.

Mehringer, David M.↗

NASA NTRS1992

The role of engineered materials in superconducting tunnel junction X-ray detectors - Suppression of quasiparticle recombination losses via a phononic band gap

An engineered structure is proposed that can alleviate quasi-particle recombination losses via the existence of a phononic band gap that overlaps the 2-Delta energy of phonons produced during recombination of quasi-particles. Attention is given to a 1D Kronig-Penny model for phonons normally incident to the layers of a multilayered superconducting tunnel junction as an idealized example. A device with a high density of Bragg resonances is identified as desirable; both Nb/Si and NbN/SiN superlattices have been produced, with the latter having generally superior performance.

Rippert, Edward D.↗