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At least 1,801 records · Page 100
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NASA NTRS1974

Effects of anisotropic electron-ion interactions in atomic photoelectron angular distributions

The photoelectron asymmetry parameter beta in LS-coupling is obtained as an expansion into contributions from alternative angular momentum transfers j sub t. The physical significance of this expansion of beta is shown to be that: (1) the electric dipole interaction transfers to the atom a charcteristic single angular momentum j sub t = sub o, where sub o is the photoelectron's initial orbital momentum; and (2) angular momentum transfers indicate the presence of anisotropic interaction of the outgoing photoelectron with the residual ion. For open shell atoms the photoelectron-ion interaction is generally anisotropic; photoelectron phase shifts and electric dipole matrix elements depend on both the multiplet term of the residual ion and the total orbital momentum of the ion-photoelectron final state channel. Consequently beta depends on the term levels of the residual ion and contains contributions from all allowed values of j sub t. Numerical calculations of the asymmetry parameters and partial cross sections for photoionization of atomic sulfur are presented.

Dill, D.↗
NASA NTRS1973

Abundances in the Crab Nebula

The bright visual-wavelength emission lines in spectra of filaments in the Crab Nebula cannot be used to derive unique values for the relative abundances of elements there, even under a simplifying assumption that heating is due entirely to photoionization by the Crab's nonthermal continuum. However, a range of possibilities may be estimated. By mass, helium is more abundant than hydrogen; but the mass fraction of heavier elements may be surprisingly small. Such conclusions are subject to modification if a noticeable fraction of the energy density involves fast particles around 1 MeV.

Davidson, K.↗
NASA NTRS1974

Absolute photon-flux measurements in the vacuum ultraviolet

Absolute photon-flux measurements in the vacuum ultraviolet have extended to short wavelengths by use of rare-gas ionization chambers. The technique involves the measurement of the ion current as a function of the gas pressure in the ion chamber. The true value of the ion current, and hence the absolute photon flux, is obtained by extrapolating the ion current to zero gas pressure. Examples are given at 162 and 266 A. The short-wavelength limit is determined only by the sensitivity of the current-measuring apparatus and by present knowledge of the photoionization processes that occur in the rate gases.

Samson, J. A. R.↗
NASA NTRS1974

Unique properties of SBN and their use in a layered optical memory

Holographic storage experiments using crystal Sr(0.75)Ba(0.25)Nb2O6 (SBN 75/25) as a volume-phase holographic medium show that this material is the most sensitive crystalline storage medium yet discovered. An exposure level of 0.003 J/sq cm at 0.488 micron produces a 1% diffraction efficiency in a 5-mm length of crystal. In addition to high recording sensitivity, SBN 75/25 exhibits interesting electric-field induced effects that include electric-field enhanced recording sensitivity and voltage-switchable latent-to-active holographic reconstruction efficiency. These effects are explained in terms of drift and diffusion of photoionized carriers coupled with the nonlinear electrooptic behavior characteristic of this low Curie temperature ferroelectric crystal. A mechanism is proposed that correlates the observed electrically controlled holographic response with the dielectric behavior of the crystal. These interesting effects are used to implement a novel, layered optical memory having no moving parts and having electrical access for writing or reading selected layers.

Thaxter, J. B.↗
NASA NTRS1974

Twilight airglow. II - N2/+/ emission at 3914 A

One of the experiments aboard a rocket flight carrying instruments to measure the dawn airglow, the ion and electron densities, and the photoelectron spectrum is reported. For a solar zenith angle of 90 deg the emission at 3914 A from N2(+) peaks at about 260 km. The integrated intensity from model calculations suggests that resonance scattering of 3914-A solar photons off N2(+) produces 90% of the emission, whereas simultaneous photoionization excitation of N2(+) produces less than 10% of the emission. Photoelectron impact excitation is found to contribute about 1%.

Sharp, W. E.↗
NASA NTRS1974

QSO envelopes - Optically thin, low density, and normal abundances

Consideration of a family of photoionization models which are optically thin to the ionizing radiation, have low enough density that collisional de-excitation is unimportant, and have normal (cosmic) chemical abundances. The line ratios from all such models with a given form of the ionizing spectrum can be derived by means of a scaling law from a single numerical calculation and expressed as a function of one physical parameter. The value of the parameter is uniquely determined from the observed line ratios. The resulting model is in reasonably good agreement with the typical or composite QSO spectrum and with line intensities for two recently observed individual QSOs. It is therefore suggested that much of the emission may come from optically thin gas, although some of the observational evidence (especially the strength of Mg II 2800 A) indicates that thick gas may also be present in some objects.

Scargle, J. D.↗
NASA NTRS1974

Boltzmann equation for the electron gas of a nondegenerate plasma

The collision terms in the Boltzmann equation associated with various processes are derived. For processes having a Fokker-Planck (F-P) limit, the associated F-P operator is derived by means of physical arguments to determine the form of the operator; its multiplying constant is fixed by calculating the total energy exchange rate and comparing with the rate computed by other means. In this manner, the F-P operator is derived for electron-ion scattering, electron-electron scattering in the high-velocity limit, electron-atom elastic scattering, Compton scattering, and the high-velocity limit for inelastic scattering. Other processes considered are bremsstrahlung, radiative-recombination, photoionization, collisional ionization of atoms, and suprathermal-particle ionization of atoms.

Gould, R. J.↗
NASA NTRS1974

Absorption of laser radiation in a H-He plasma. I - Theoretical calculation of the absorption coefficient

The theory for calculating the absorption of laser radiation by hydrogen is outlined for the temperatures and pressures of common laboratory plasmas. Nonhydrogenic corrections for determining the absorption by helium are also included. The coefficients for the absorption of He-Ne laser radiation at the wavelengths of 0.633, 1.15, and 3.39 microns in a H plasma is presented for temperatures in the range from 10,000 to 40,000 K and electron number densities in the range from 10 to the 15th power to 10 to the 18th power per cu cm. The total absorption of a H-He plasma calculated from this theory is compared with the measured absorption. The theoretical composition of the H-He absorption is analyzed with respect to the significant absorption processes, inverse bremsstrahlung, photoionization, resonance excitation, and photodetachment.

Stallcop, J. R.↗
NASA NTRS1974

Absorption of laser radiation in a H-He plasma. II - Experimental measurement of the absorption coefficient

The absorption coefficients of 0.633-, 1.15-, and 3.39-micron laser radiation for a homogeneous H-He plasma have been measured in the temperature range from 12.2 to 21.7 (x 1000 K) and in the electron number density range 0.45 to 6.5 (x 10 to the 17th power per cu cm). Good agreement is found between the experimentally determined total absorption for each of the wavelengths and that calculated from theory. Furthermore, because the 3.39-micron absorption is dominated by inverse bremsstrahlung, while the 0.633-micron absorption is dominated by photoionization and resonance absorption, the experiment indicates a correct assessment by the theory of these individual absorption mechanisms.

Billman, K. W.↗
NASA NTRS1974

The day side mid-latitude plasma trough

Above 2500 km the electrostatic probe experiment aboard the Isis 1 satellite detects a persistent dayside midlatitude plasma trough. The superposition of measurements taken over two-week intervals at noon is used to illustrate the form and location of the quiet time trough within each season. These measurement results contrast with those observations of a less prominent dayside trough at altitudes below 2500 km. Associated with the trough above 2500 km is an electron temperature maximum of about 6000 K that is sharp at midnight and broad at noon. In spring and summer a second noontime temperature maximum often appears poleward of 70-deg invariant latitude accompanied by an enhancement in the ionization. When it is assumed that the geophysical processes producing a plasmasphere act most directly upon the light ions, one factor contributing to the distinction between the plasma trough and the equatorial plasmapause is the increasing influence of O(+) on total plasma behavior at lower altitudes. Other factors are the local processes, such as dayside F region photoionization by energetic cusp particles that produce enhancements in plasma density and electron temperature.

Miller, N.↗
NASA NTRS1974

Termination of the solar wind in the hot, partially ionized interstellar medium

Theoretical foundations for understanding the problem of the termination of the solar wind are reexamined in the light of most recent findings concerning the states of the solar wind and the local interstellar medium. The investigation suggests that a simple extention of Parker's (1961) analytical model provides a useful approximate description of the combined solar wind, interstellar wind plasma flowfield under conditions presently thought to occur. A linear perturbation solution exhibiting both the effects of photoionization and charge exchange is obtained for the supersonic solar wind. A numerical algorithm is described for computing moments of the non-equilibrium hydrogen distribution function and associated source terms for the MHD equations. Computed using the algorithm in conjunction with the extended Parker solution to approximate the plasma flowfield, profiles of hydrogen number density are given in the solar wind along the upstream and downstream axes of flow with respect to the direction of the interstellar wind. Predictions of solar Lyman-alpha backscatter intensities to be observed at 1 a.u. have been computed, in turn, from a set of such hydrogen number density profiles varied over assumed conditions of the interstellar wind.

Lombard, C. K.↗
NASA NTRS1974

Formation of O3/+/ by the reaction of metastable O2/+/ ions with O2

The high resolution of the photoionization mass spectrophotometer was utilized to resolve some doubts about the participating species in the reaction of metastable oxygen molecular ions with oxygen molecules to yield ozone ions and oxygen radicals. It is found from inspection of the appearance potential of the ozone ion that an a4 Pi-excited state is responsible for the formation of ozone near the appearance potential of these lines.

Ajello, J. M.↗
NASA NTRS1975

Absorption spectrum of the laser-populated 3D metastable levels in barium

This paper deals with the details of the absorption spectrum of the 3D metastable term in barium. The 3D term was selectively populated with a tuneable dye laser. The fundamental triplet series (6s5d 3D-6snf 3F) is identified and extended out to n = 32. In addition, the absolute photoionization cross section was measured at 303 nm. The relative cross section from 303 to 250 nm was also measured with the absolute scale set by the measurement at 303 nm and was found to be nearly constant in the wavelength region measured.

Carlsten, J. L.↗
NASA NTRS1974

Radio occultation exploration of Mars

The present work summarizes the principal results of the radio occultation experiments carried out to date for Mars. The early measurements, made at the time of the Martian dust storm of 1971, showed greatly reduced temperature gradients in the daytime troposphere, indicating the heating effect of the dust. The temperature gradients that were measured later when the atmosphere was apparently free of dust, were still much lower than expected under conditions of radiative-convective balance, indicating that dynamics may play a large part in determining the temperature structure of the Martian troposphere. The surface atmospheric pressure was observed to vary widely with topography ranging from about 1 mb at the summit of the Middle Spot volcano (Pavonis Mons) to over 10 mb in the North circumpolar region. The radius of the planet was measured with accuracies ranging from about 0.25 to about 2.1 km over latitudes ranging from 86 to -80 deg. These measurements have shown that Mars has pronounced equatorial and north-south asymmetries, which make it difficult to represent its shape by a simple triaxial figure. The daytime ionosphere measurements indicated that the main ionization peak was similar in behavior to a terrestrial F1 layer and is probably produced by photoionization of carbon dioxide by solar extreme ultraviolet.

Kliore, A. J.↗
NASA NTRS1975

The electron density in the direction of zeta Oph

It is shown that photoionization of vibrationally excited H2 and photodissociation of the H2(+) ions produced thereby constitute a significant electron production route in high UV flux situations. A significant fraction of the electron density in the direction of zeta Oph (-15 km per sec cloud) deduced from observations may be expected to arise in this way.

Stecher, T. P.↗
NASA NTRS1975

The atmosphere and ionosphere of Io

Models for Jupiter's innermost Galilean satellite's atmosphere, ionosphere, and sodium airglow are developed on the basis of recent observational data. The sodium emission detected by Brown (1973) is seen to require a collisional excitation process in Io's atmosphere, while the extended sodium emission measured by Trafton et al. (1974) may require scattering of the planetary radiation by an extended sodium cloud. The sodium is presumably present in bound form on Io's surface, and may be released by a sputtering mechanism proposed by Matson et al. (1974). The ionosphere detected by a radio occultation experiment on Pioneer 10 could be attributed to photoionization of atmospheric sodium, provided Io's atmosphere could sustain significant upward motions during daytime and downward motions during nighttime. The incomplete hydrogen torus observed by Judge and Carlson (1974) in the vicinity of Io appears to require a large supply of hydrogen from the satellite's atmosphere. Implications of the hydrogen torus for the energy and mass balance of Jupiter's magnetosphere are discussed.

Mcelroy, M. B.↗
NASA NTRS1975

Production of N2O/+/ by reaction of metastable O2/+/ ions with N2

Photoionization mass spectrometry examination of the production of N2O(+) was undertaken to determine whether N2(+) or O2(+) ions are responsible for onset of N2O(+). It appears that the N2(+) ion does not contribute significantly to the production of N2O(+) in this experiment. Therefore, it is clear that excited O2(+) is responsible for the formation of N2O(+) near the appearance potential of these ions.

Ajello, J. M.↗
NASA NTRS1975

Photometry of a bright filament in the Orion nebula

The spectrum of a bright filament in the outer portion of the Orion nebula, 14 min from the Trapezium, was studied. The temperature of the filament, derived from /N II/ lambda 5755/6583 ratio, is 9560 K. The electron density (from /S II/ lambda 6717, 6731) is low: 360 plus or minus 150 per cu cm. The ionic abundances of He(+), N(+), O(++), and S(-) are about the same as Peimbert and Costero (1969) found in regions in Orion closer to the Trapezium. It is concluded that the filament is photoionized, presumably by the Trapezium, rather than a shock front, as had originally been suspected.

Dufour, R. J.↗

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