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At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
NASA NTRS1972

Physical interpretation of glory undulations in scattering cross sections.

The relation between glory undulations and the detailed nature of the potential energy of interaction of two molecules having a minimum containing one or more bound states is clarified, i.e., a physical interpretation is given to the undulations. Although the number, amplitude, and spacing of the oscillations all give information about the potential, the spacing is most often used because it can be most accurately measured. Results are given which show fairly explicitly just what information on the interaction potential is contained in the spacing of the glory oscillations. Glory oscillations put strong known constraints on any potential model.

Greene, E. F.↗
NASA NTRS1973

Calculation of total electron excitation cross-sections and partial electron ionization cross-sections for the elements

Computer programs were used to calculate the total electron excitation cross-section for atoms and the partial ionization cross-section. The approximations to the scattering amplitude used are as follows: (1) Born, Bethe, and Modified Bethe for non-exchange excitation; (2) Ochkur for exchange excitation; and (3) Coulomb-Born of non-exchange ionization. The amplitudes are related to the differential cross-sections which are integrated to give the total excitation (or partial ionization) cross-section for the collision. The atomic wave functions used are Hartree-Fock-Slater functions for bound states and the coulomb wave function for the continuum. The programs are presented and the results are examined.

Green, T. J.↗
NASA NTRS1974

CO2 photodissociation and vibrational excitation in the planetary atmospheres

The principal subjects of investigation were the determination of the CO2 photodissociation quantum yields at the wavelengths from 1200 A to 1500 A, and the efficiency of electronic-to-vibrational energy transfer in the systems 0(1D) + CO, N2, CO2 yields 0(3P) + CO N2, CO2 vibrational energies. Measurements on the photodissociation quantum yield of CO2 in the 1200-1500 A region show that it is wavelength dependent, and for the six atomic line sources used, the quantum yield varied from 0.2 to 0.8. The data appear to fit the interpretation of stable CO2 bound states mixed with repulsive or predissociating states, since the low quantum yields coincide with the maximum structure in the CO2 absorption spectrum. The first reliable measurements were made on the efficiency of electronic-to-vibrational energy transfer in the systems 0(1D)-CO and 0(1D)-N2, using a uv resonance fluorescence technique. The 0(1D)-CO2 interaction was investigated by infrared techniques.

Slanger, T. G.↗
NASA NTRS1975

Asymptotic form of the charge exchange cross section in the three body rearrangement collisions

A three body general rearrangement collision is considered where the initial and final bound states are described by the hydrogen-like wave functions. Mathematical models are developed to establish the relationships of quantum number, the reduced mass, and the nuclear charge of the final state. It is shown that for the low lying levels, the reciprocal of n cubed scaling law at all incident energies is only approximately satisfied. The case of the symmetric collisions is considered and it is shown that for high n and high incident energy, E, the cross section behaves as the reciprocal of E cubed. Zeros and minima in the differential cross sections in the limit of high n for protons on atomic hydrogen and positrons on atomic hydrogen are given.

Omidvar, K.↗
NASA NTRS1975

Positron annihilation in solar flares

The gamma ray line at 0.51 MeV originates from the annihilation of positrons. When a fraction of the positrons annihilate from bound states of positronium, the 0.51-MeV line is accompanied by a continuum of 3-gamma annihilation radiation at energies up to 0.51 MeV. Accurate calculations of the rates of free annihilation and positronium formation in a solar flare plasma are presented and positronium formation by charge exchange is discussed. The observability of the 3-gamma annihilation is increased by the inherent delay in the production and slowing down time of the positrons. It was concluded that such radiation could be detected at times late in solar gamma ray events when the continuum and prompt line emissions have essentially disappeared.

Crannell, C. J.↗
NASA NTRS1975

Useful extremum principle for the variational calculation of matrix elements. II

Recent work (Gerjuoy et al., 1974) on variational principles for diagonal bound state matrix elements of arbitrary Hermitian operators is extended. In particular, it is shown that the previously derived minimum principle for the trial auxiliary function appearing in such variational principles can be constructed using a modified Hamiltonian possessing not heretofore recognized positive definite properties. Thus there is at least one alternative to the particular modified Hamiltonian on which the results of Gerjuoy et al. (1974) originally were based.

Gerjuoy, E.↗
NASA NTRS1975

Asymptotic form of the charge-exchange cross section in three-body rearrangement collisions

A three-body general-type rearrangement collision is considered in which the initial and final bound states are described by hydrogen-like wave functions. It is shown that the charge-exchange amplitude in the first Born approximation can be expanded at all incident energies in terms of the inverse powers of the principal quantum number (n). By expanding the exchange amplitude in this way, it is demonstrated conclusively that the cross section for capture into the s, p, and d states as well as for the sum over all the angular-momentum states is proportional to 1/n-cubed plus terms proportional to higher inverse odd powers of n. It is found that the low-lying levels cannot be scaled to the 1/n-cubed law irrespective of the value of the incident energy except in the case of capture into the s states in accordance with the Oppenheimer-Brinkman-Kramers approximation. Zeros and minima in the differential cross sections are given in the limit of high principal quantum number for electron capture by protons from atomic hydrogen and for positronium formation by proton-atomic hydrogen collisions.

Omidvar, K.↗
NASA NTRS1976

Formation of the 0.511.-MeV line in solar flares

The gamma-ray line produced at 0.51-MeV was studied and is shown to be the result of either of free annihilation of positrons with electrons or of the decay of positronium by 2-photon emission. Positron annihilation from the bound state of positronium may also proceed by 3-photon emission, resulting in a continuum with energies up to 0.51-MeV. Accurate calculations of the rates of free annihilation and positronium formation in a solar-flare plasma are presented. Estimates of the positronium-formulation rates by charge exchange and the rates of dissociation and quenching are also considered. The temperature and density dependence of the ratio of 3-photon to 2-photon emission was obtained. It is shown that when the ratio of free electrons to neutral atoms in the plasma is approximately unity or greater, the Doppler width of the 0.51-MeV line is a function of the temperature of the annihilation region. For the small ion densities characteristics of the photosphere, the width is predominantly a function of the density.

Crannell, C. J.↗
NASA NTRS1975

An archetype hydrogen atmosphere problem

Populations for the first three bound states and the continuum of hydrogen are determined for an isothermal hydrostatic atmosphere at 20,000 K. The atmosphere is treated as optically thin in the Balmer and Paschen continua and illuminated by continuum radiation at these wavelengths with prescribed radiation temperatures. The atmosphere is optically thick in the 2-1, 3-1, 3-2 and c-1 transitions. Three stages of approximation are treated: (1) radiative detailed balance in the 2-1, 3-1 and 3-2 transitions, (2) radiative detailed balance in the 3-1 and 3-2 transitions, and (3) all transitions out of detailed balance. The solution of this problem is nontrivial and presents sufficient difficulty to have caused the failure of at least one rather standard technique. The problem is thus a good archetype against which new methods or new implementations of old methods may be tested.

Athay, R. G.↗
NASA NTRS1975

Positron annihilation in solar flares

The gamma-ray line at 0.51 MeV originates from the annihilation of positrons. When a fraction of the positrons annihilate from bound states of positronium, the 0.51 MeV line is accompanied by a continuum of 3-gamma annihilation radiation at energies up to 0.51 MeV. We present accurate calculations of the rates of free annihilation and positronium formation in a solar flare plasma, and we also discuss positronium formation by charge exchange. The observability of the 3-gamma annihilation is increased by the inherent delay in the production and slowing down time of the positrons. We conclude that such radiation could be detected at times late in solar gamma-ray events when the continuum and prompt line emissions have essentially disappeared.

Crannell, C. J.↗
NASA NTRS1976

Magnetic monopoles and cosmology

The number density of magnetic monopoles is calculated in the framework of big-bang cosmology on the basis of a solution to the Boltzmann transport equation in evolutionary cosmological models. The product of the mass and the transition probability of the annihilation process for magnetic monopoles is obtained relative to those for protons and is shown to be greater than about 10 to the 20th power. Based on this result, two likely conclusions are made: (1) monopoles exist, but possess a pole-antipole bound state with binding energy of the order of the monopole rest mass; or (2) magnetic monopoles do not exist.

Adams, P. J.↗
NASA NTRS1979

Stellar atmosphere in statistical equilibrium

A static atmosphere with only Lyman continuum radiation in radiative equilibrium is studied for the effects of radiative and collisional ionization on deviations from local thermodynamic equilibrium (LTE). Large increases and decreases of the kinetic temperature (range in T of about factor 2) and, correspondingly, very large over- and underpopulation of the bound state (range in b of about factor 1,000,000) are found, depending on the frequency dependence of the photoionization cross section. Despite these large deviations from LTE, which strongly modify the emergent spectrum, there is almost no effect on the particle densities, the degree of ionization, and the basic structure of the atmosphere.

Kalkofen, W.↗
NASA NTRS1980

Ab initio projected-unrestricted Hartree-Fock calculation of some potential energy curves for carbonyl fluoride

Some potential energy curves for CF2O were calculated using projected-unrestricted Hartree-Fock (PUHF) theory. The calculations employed a contracted (4s 3p) Gaussian-type atomic orbital basis set. Bound states were found for the X-tilde 1A1 and 1,3A2 states while the 1,3B1 and 1,3B2 states were repulsive in the valence representation. The merits of the PUHF treatment for excited states are discussed. The results are discussed in terms of available experimental information and previous calculations with particular emphasis on the question of the photolysis channels open in the solar spectral region.

Brewer, D. A.↗
NASA NTRS1983

Dissociative recombination of O2(+), NO(+) and N2(+)

A new L(2) approach for the calculation of the threshold molecular capture width needed for the determination of DR cross sections was developed. The widths are calculated with Fermi's golden rule by substituting Rydberg orbitals for the free electron continuum coulomb orbital. It is shown that the calculated width converges exponentially as the effective principal quantum number of the Rydberg orbital increases. The threshold capture width is then easily obtained. Since atmospheric recombination involves very low energy electrons, the threshold capture widths are essential to the calculation of DR cross sections for the atmospheric species studied here. The approach described makes use of bound state computer codes already in use. A program that collects width matrix elements over CI wavefunctions for the initial and final states is described.

Guberman, S. L.↗
NASA NTRS1984

Variational calculations of muonic-molecule energy levels

The best variational energies obtained to date of the bound states of muonic molecules containing nucleons of unit charge or isotopes of hydrogen are reported. Hylleraas wave functions are used to describe the three-particle systems, and the convergence of the energies is carefully studied; as many as 440 terms have been included in some cases. These results are compared with the best previous values.

Bhatia, A. K.↗
NASA NTRS1985

Photodetachment of the positronium negative ion

The photodetachment cross section of the negative positronium ion is calculated. The description of the initial bound state is simplified by representing it by an asymptotic form whose normalization comes from the most accurate Hylleraas wave function of the ion. The final state is assumed to be a plane wave.

Bhatia, A. K.↗
NASA NTRS1988

Accuracy of analytic energy level formulas applied to hadronic spectroscopy of heavy mesons

Linear and harmonic potential models are used in the nonrelativistic Schroedinger equation to obtain article mass spectra for mesons as bound states of quarks. The main emphasis is on the linear potential where exact solutions of the S-state eigenvalues and eigenfunctions and the asymptotic solution for the higher order partial wave are obtained. A study of the accuracy of two analytical energy level formulas as applied to heavy mesons is also included. Cornwall's formula is found to be particularly accurate and useful as a predictor of heavy quarkonium states. Exact solution for all partial waves of eigenvalues and eigenfunctions for a harmonic potential is also obtained and compared with the calculated discrete spectra of the linear potential. Detailed derivations of the eigenvalues and eigenfunctions of the linear and harmonic potentials are presented in appendixes.

Badavi, Forooz F.↗
NASA NTRS1988

Eikonal solutions to optical model coupled-channel equations

Methods of solution are presented for the Eikonal form of the nucleus-nucleus coupled-channel scattering amplitudes. Analytic solutions are obtained for the second-order optical potential for elastic scattering. A numerical comparison is made between the first and second order optical model solutions for elastic and inelastic scattering of H-1 and He-4 on C-12. The effects of bound-state excitations on total and reaction cross sections are also estimated.

Cucinotta, Francis A.↗