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All sourcesNASA NTRSDOE OSTI
NASA NTRS1983

Performance and materials aspects of Ge:Be photoconductors

Ge:Be photoconductors have been developed for low photon background applications in the 30 - 50 micron wavelength region. These detectors provide higher responsivity and lower noise equivalent power (NEP) than the Ge:Ga detectors currently operating in this wavelength range. Beryllium-doped single crystals were grown by the Czochralski method from a carbon susceptor under a vacuum of approximately 10 to the -6th torr. An optimum detective quantum efficiency of 46 percent at a background flux of 1.5 x 10 to the 8th photons/second (7 x 10 to the -13th W is reported. Ge:Be detector performance is strongly influenced by the absolute concentrations and the concentration ratio of residual shallow donors and shallow acceptors.

Haegel, N. M.↗
NASA NTRS1979

The challenge of detecting gravitational radiation is creating a new chapter in quantum electronics: Quantum nondemolition measurements

Future gravitational wave antennas will be approximately 100 kilogram cylinders, whose end-to-end vibrations must be measured so accurately (10 to the -19th power centimeters) that they behave quantum mechanically. Moreover, the vibration amplitude must be measured over and over again without perturbing it (quantum nondemolition measurement). This contrasts with quantum chemistry, quantum optics, or atomic, nuclear, and elementary particle physics where measurements are usually made on an ensemble of identical objects, and care is not given to whether any single object is perturbed or destroyed by the measurement. Electronic techniques required for quantum nondemolition measurements are described as well as the theory underlying them.

Braginsky, V. B.↗
NASA NTRS1989

Low-cost DH and quantum well laser array development

The intial results of a program aimed at developing low-cost diode laser arrays for use as solid-state laser pumps are reported. MOCVD is used to demonstrate excellent run-to-run reproducibility in emission wavelength, threshold current density, and quantum efficiency. For this first experimental series, J(th) values of approximately 1310 Amps/sq cm were obtained for broad-area unthinned devices from the growth runs. Differential quantum efficiencies of between 41 percent and 47 percent were measured on the non-facet-coated devices from all four runs. Single quantum well, separate confinement heterostructure lasers fabricated from wafers grown in the same MOCVD reactor exhibited near single-mode emission, with J(th) values of approximately 300 Amps/sq cm. Photoluminescence data confirm quantum well widths of 80 A and 150 A for two different MOCVD growth runs.

Linden, Kurt J.↗
NASA NTRS1980

A laboratory study on the dissociative recombination of vibrationally excited O2/+/ions

The dissociative recombination of vibrationally excited O2(+) ions is studied in light of the possible importance of this reaction in upper atmospheric chemistry. A plasma spectroscopy experiment was performed in a microwave cavity filled by an argon-oxygen mixture, with O(1S) production monitored by measurements of the 5577-A afterglow, the O2(+) density and the electron concentration. Plasma and optical data reveal the predominant afterglow ions to be Ar2(+) and O2(+), with an effective O(1S) dissociative recombination coefficient of 2.1 x 10 to the -8th cu cm/sec, corresponding to a quantum yield of 10%. Experiments with an argon-krypton-oxygen mixture reveal that vibrationally excited O2(+) ions are the chief source of the O(1S) atoms, with a specific recombination coefficient for the dissociation of O2(+)(2 pi g) into O(1S) and O(1D) of 4.2 x 10 to the -9th cu cm/sec. A comparison of the laboratory results with Atmospheric Explorer data on the 5577-A airglow implies that O2(+) ions in the sunlit ionosphere are vibrationally excited to the same degree as in the laboratory, with the vibrational relaxation of these ions much slower than dissociative recombination. Results also predict a dawn-twilight asymmetry in the effective O(1S) yield due to the normal variation of electron content.

Zipf, E. C.↗
NASA NTRS1991

Dissociative recombination of the ground state of N2(+)

Large-scale calculations of the dissociative recombination cross sections and rates for the v = 0 level of the N2(+) ground state are reported, and the important role played by vibrationally excited Rydberg states lying both below and above the v = 0 level of the ion is demonstrated. The large-scale electronic wave function calculations were done using triple zeta plus polarization nuclear-centered-valence Gaussian basis sets. The electronic widths were obtained using smaller wave functions, and the cross sections were calculated on the basis of the multichannel quantum defect theory. The DR rate is calculated at 1.6 x 10 to the -7th x (Te/300) to the -0.37 cu cm/sec for Te in the range of 100 to 1000 K, and is found to be in excellent agreement with prior microwave afterglow experiments but in disagreement with recent merged beam results. It is inferred that the dominant mechanism for DR imparts sufficient energy to the product atoms to allow for escape from the Martian atmosphere.

Guberman, Steven L.↗
NASA NTRS1979

Hydrogen atom initiated chemistry

H Atoms have been created by the photolysis of H2S. These then initiated reactions in mixtures involving acetylene-ammonia-water and ethylene-ammonia-water. In the case of the acetylene system, the products consisted of two amino acids, ethylene and a group of primarily cyclic thio-compounds, but no free sulfur. In the case of the ethylene systems, seven amino acids, including an aromatic one, ethane, free sulfur, and a group of solely linear thio-compounds were produced. Total quantum yields for the production of amino acids were about 3 x 10 to the -5th and about 2 x 10 to the -4th with ethylene and acetylene respectively as carbon substrates. Consideration is given of the mechanism for the formation of some of the products and implications regarding planetary atmosphere chemistry, particularly that of Jupiter, are explored.

Hong, J. H.↗
NASA NTRS1980

Production of OH from photolysis of HOCl at 307-309 nm

Laser-induced fluorescence has been used to measure the amount of OH produced in the laser photolysis of HOCl at 310 nm. The HOCl was generated by the Cl2O + H2O and by the HCl + Cl2O reactions. The laser technique samples OH on a microsecond time scale so that secondary radical reactions cannot contribute significantly to the observed signals. The fluorescence signals were calibrated with O3-H2O mixtures, the reactions O3 + h(nu) yields O(1D) + O2 and O(1D) + H2O yields 2OH yielding known amounts of OH. The value for the absorption cross section of HOCl at 310 nm has been determined to be 6 x 10 to the -20th sq cm, assuming K(eq) = 0.082 for the Cl2O + H2O yields 2HOCl system and assuming unit quantum yield for the production of OH. The present investigation further supports earlier experimental work concluding that HOCl will photodissociate rapidly and will not be an important holding tank for chlorine in the stratosphere. These results are in disagreement with theoretical calculations of the UV absorption spectrum of HOCl in the 300-350-nm wavelength region.

Molina, M. J.↗
NASA NTRS1986

The AXAF CCD imaging spectrometer

The AXAF CCD Imaging Spectrometer (ACIS) is currently being defined as a possible focal plane instrument to be flown on the Advanced X-ray Astrophysics Facility (AXAF). The imaging spectrometer consists of an array of charge coupled devices (CCD's) to be placed at the focus of the AXAF mirrors. The array will provide high angular resolution (0.5 arc seconds), moderate spectral resolution (150 eV) over the energy range 0.1 to 10 keV, temporal resolution down to 60 microseconds, and single photon quantum detection efficiencies of up to 90 percent. X-ray sensitivity for a point source exceeds 10 to the -15th ergs/sq cm sec for a 10 to the 5th sec exposure. When used in conjunction with the objective gratings, the array will yield wavelength resolution of up to 200.

Garmire, G. P.↗
NASA NTRS1987

The production of NH(A 3Pi i) through photodissociative excitation of NH3

The NH(A-X) emission produced by photodissociation of NH3 is examined using a line emission source in the 600-1340 A region. It is observed that the production of the NH(A 3Pi i) photofragment involves a spin-forbidden transition. Fluorescence spectra of NH3 are derived and analyzed. The relationship between fluorescence cross sections and fluorescence count rates and gas pressure is studied. The fluorescence cross sections of NH(A-X) are calculated as 2.1 x 10 to the -22nd, 3.1 x 10 to the -22nd, 3.0 x 10 to the -21st, 1.5 x 10 to the -20th, and 3.4 x 10 to the -20th sq cm for the primary photon wavelength of 1239, 1184, 1065, 1037, and 976 A, respectively; and the corresponding fluorescence quantum yields are 0.00002, 0.00003, 0.0012, 0.0085, and 0.015.

Wu, C. Y. Robert↗
NASA NTRS1991

Photodissociation of the hydroxyl radical (OH) at 157 nm

The photodissociation of the OH radical was studied at 157 nm via the detection of the product H atoms with the resonance fluorescence technique. OH radicals were produced in a fast-flow cell from the reaction between H and NO2 and subsequently photodissociated by an excimer laser operating on the F2 emission. The quantum yield for photodissociation of OH was measured to be 1.10 + or - 0.28. The photodissociation cross section was calculated to be 6.6 x 10 to the -18th sq cm (+ or - 25 percent).

Cody, R. 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 NTRS1977

Stimulated-emission cross section at 1064 and 1342 nm in Nd:YVO4

The stimulated-emission cross sections at 1064 and 1342 nm in Nd:YVO4 were determined by measurement of laser thresholds as a function of output mirror reflectivity. Small samples of Nd:YVO4 and Nd:YAG were end pumped at room temperature with a CW 514.5-nm argon ion laser in an almost identical geometrical arrangement which permitted comparative values to be obtained. Assuming that the pumping quantum efficiency of Nd:YVO is similar to that of Nd:YAG, the stimulated-emission cross sections obtained for the 1064- and 1342-nm transitions were respectively 12 x 10 to the -19th and 6.0 x 10 to the -19th/sq cm.

Tucker, A. W.↗
NASA NTRS1996

Finding New Math Identities by Computer

Recently a number of interesting new mathematical identities have been discovered by means of numerical searches on high performance computers, using some newly discovered algorithms. These include the following: pi = ((sup oo)(sub k=0))(Sigma) (1 / 16) (sup k) ((4 / 8k+1) - (2 / 8k+4) - (1 / 8k+5) - (1 / 8k+6)) and ((17 pi(exp 4)) / 360) = ((sup oo)(sub k=1))(Sigma) (1 + (1/2) + (1/3) + ... + (1/k))(exp 2) k(exp -2), zeta(3, 1, 3, 1, ..., 3, 1) = (2 pi(exp 4m) / (4m+2)! where m = number of (3,1) pairs. and where zeta(n1,n2,...,nr) = (sub k1 (is greater than) k2 (is greater than) ... (is greater than) kr)(Sigma) (1 / (k1 (sup n1) k2 (sup n2) ... kr (sup nr). The first identity is remarkable in that it permits one to compute the n-th binary or hexadecimal digit of pu directly, without computing any of the previous digits, and without using multiple precision arithmetic. Recently the ten billionth hexadecimal digit of pi was computed using this formula. The third identity has connections to quantum field theory. (The first and second of these been formally established; the third is affirmed by numerical evidence only.) The background and results of this work will be described, including an overview of the algorithms and computer techniques used in these studies.

Bailey, David H.↗
NASA NTRS1984

Molecular processes in a high temperature shock layer

Models of the shock layer encountered by an Aeroassisted Orbital Transfer Vehicle require as input accurate cross sections and rate constants for the atomic and molecular processes that characterize the shock radiation. From the estimated atomic and molecular densities in the shock layer and the expected residence time of 1 m/s, it can be expected that electron-ion collision processes will be important in the shock model. Electron capture by molecular ions followed by dissociation, e.g., O2(+) + e(-) yields 0 + 0, can be expected to be of major importance since these processes are known to have high rates (e.g., 10 to the -7th power cu/cm/sec) at room temperature. However, there have been no experimental measurements of dissociative recombination (DR) at temperatures ( 12000K) that are expected to characterize the shock layer. Indeed, even at room temperature, it is often difficult to perform experiments that determine the dependence of the translational energy and quantum yields of the product atoms on the electronic and vibrational state of the reactant molecular ions. Presented are ab initio quantum chemical studies of DR for molecular ions that are likely to be important in the atmospheric shock layer.

Guberman, S. L.↗
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 NTRS1991

Calculations concerning the reaction C + H3(+) - CH(+) + H2

The ion-molecule reaction of C + H3(+) - CH(+) + H2 has not been studied in the laboratory but is thought to be important in the gas phase synthesis of organic molecules in dense interstellar clouds. This reaction was studied theoretically by performing quantum chemical ab initio calculations on the potential surface. It is found that there is no activation barrier to the reaction and that it proceeds smoothly to the first excited electronic state of CH(+). The rate coefficient as a function of temperature can then be estimated using the proper long-range potentials. The rate coefficient at 10 K is calculated to be 2.9 x 10 to the -9th cu cm/sec.

Talbi, D.↗
NASA NTRS1986

SQUID readout and ultra-low magnetic fields for Gravity Probe-B (GP-B)

The superconducting readout system to be used for resolving 0.001 arcsec changes in the gyroscope spin direction in the Relativity Gyroscope (GP-B) experiment is described. This system couples the London magnetic moment flux of the spinning gyro to a low noise superconducting quantum interference device (SQUID) detector. Resolution limits and noise performance of the detection system are discussed, and improvements obtained and expected with advanced SQUIDs are presented. Also described is the novel use of superconducting magnetic shielding techniques to obtain a 250 dB attenuation of the earth's magnetic field at the location of the gyroscopes. In this approach, expanded superconducting foil shields are coupled with fixed cylindrical superconducting shields and special geometric considerations to obtain the extremely high attenuation factor required. With these shielding techniques, it appears that the 0.5-Gauss earth field (which appears to the gyroscopes as an ac field at the satellite roll rate) can be reduced to the 10 to the -13th G level required by the experiment. Recent results concerning improvements in the performance of the superconducting foil techniques obtained with the use of a new computer-controlled cooling system are presented.

Lockhart, James M.↗
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.↗