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At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
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 NTRS2001

Total Electron-Impact Ionization Cross-Sections of CFx and NFx (x = 1 - 3)

The discrepancy between experimental and theoretical total electron-impact ionization cross sections for a group of fluorides, CFx, and NFx, (x = 1 - 3), is attributed to the inadequacies in previous theoretical models. Cross-sections calculated using a recently developed siBED (simulation Binary-Encounter-Dipole) model that takes into account the shielding of the long-range dipole potential between the scattering electron and target are in agreement with experimentation. The present study also carefully reanalyzed the previously reported experimental data to account for the possibility of incomplete collection of fragment ions and the presence of ion-pair formation channels. For NF3, our experimental and theoretical cross-sections compare well with the total ionization cross-sections recently reported by Haaland et al. in the region below dication formation.

Huo, Winifred M.↗
NASA NTRS2004

Molecular Data for a Biochemical Model of DNA Radiation Damage: Electron Impact Ionization and Dissociative Ionization of DNA Bases and Sugar-Phosphate Backbone

As part of the database for building up a biochemical model of DNA radiation damage, electron impact ionization cross sections of sugar-phosphate backbone and DNA bases have been calculated using the improved binary-encounter dipole (iBED) model. It is found that the total ionization cross sections of C3'- and C5'-deoxyribose-phospate, two conformers of the sugar-phosphate backbone, are close to each other. Furthermore, the sum of the ionization cross sections of the separate deoxyribose and phosphate fragments is in close agreement with the C3'- and C5'-deoxyribose-phospate cross sections, differing by less than 10%. Of the four DNA bases, the ionization cross section of guanine is the largest, then in decreasing order, adenine, thymine, and cytosine. The order is in accordance with the known propensity of oxidation of the bases by ionizing radiation. Dissociative ionization (DI), a process that both ionizes and dissociates a molecule, is investigated for cytosine. The DI cross section for the formation of H and (cytosine-Hl)(+), with the cytosine ion losing H at the 1 position, is also reported. The threshold of this process is calculated to be 17.1 eV. Detailed analysis of ionization products such as in DI is important to trace the sequential steps in the biochemical process of DNA damage.

Dateo, Christopher E.↗
NASA NTRS2000

Ionization Properties of Molecules Commonly Used for Plasma Processing of Semi-Conductors

Two types of processes are involved in plasma processing of semi-conductors. They are: plasma etching or cleaning and plasma deposition of the semi-conducting materials. For plasma etching of semi-conductors mostly halogen containing gases are used as additives to gases such as O2 and N2. For plasma deposition gases such as C2H2, SiH4, Si2H6 have been tested in the past. For an optimal performance of a reactor it is important to model the plasma. In this modeling effort electron impact excitation and ionization cross sections play a central role. For ionization balance calculations values of ionization cross sections are needed. Ion molecule reactions determine the ultimate composition of the plasma. Recently it has been discovered that the by products of many of these plasmas are per fluro hydrocarbons (PFCs) which are highly infrared absorbing species and have long life times in the atmosphere. They cause global warming. A lot of research is being pursued at the present time to find alternative molecules which do not produce global warming gases as the and product of the plasma processing reactor. There is also interest in the ionization and dissociative ionization properties of these molecules from the point view of the plasma abatement of the pollutant gases at the exhaust of the semi-conductor processing reactors. At the conference ionization and dissociative ionization properties of some of these molecules will be presented.

Srivastava, S. K.↗
NASA NTRS1995

Simplified Numerical Description of SPT Operations

A simplified numerical model of the plasma discharge within the SPT-100 stationary plasma thruster was developed to aid in understanding thruster operation. A one dimensional description was used. Non-axial velocities were neglected except for the azimuthal electron velocity. A nominal operating condition of 4.5 mg/s of xenon anode flow was considered with 4.5 Amperes of discharge current, and a peak radial magnetic field strength of 130 Gauss. For these conditions, the calculated results indicated ionization fractions of 0.99 near the thruster exit with a potential drop across the discharge of approximately 250 Volts. Peak calculated electron temperatures were found to be sensitive to the choice of total ionization cross section for ionization of atomic xenon by electron bombardment and ranged from 51 eV to 60 eV. The calculated ionization fraction, potential drop, and electron number density agree favorably with previous experiments. Calculated electron temperatures are higher than previously measured.

Manzella, David H.↗
NASA NTRS1992

Cross-sections for electron impact ionization of O2

Using a crossed electron beam-molecular beam collision geometry and the relative flow technique measurements have been made to generate a uniform set of electron impact ionization cross-section data for O2 for electron impact energies from threshold to 1000 eV. Present values of total, partial and dissociative ionization cross-sections are compared with previously published data. Cross-sections for the formation of O2(+) from O2 have been measured for the first time.

Krishnakumar, E.↗