Analytic Approximations for Integrated Electron?Atom Excitations

1981 ◽  
Vol 34 (2) ◽  
pp. 135 ◽  
Author(s):  
IE McCarthy ◽  
BC Saha ◽  
AT Stelbovics
Keyword(s):  
Cross Section ◽  
Cross Sections ◽  
Momentum Space ◽  
Optical Potential ◽  
Analytic Approximations ◽  
Maximum Contribution ◽  
Excitation Cross Sections ◽  
Analytic Expressions ◽  
Reaction Channels ◽  
Atomic Excitations

Accurate calculations of atomic excitations require estimates of the effect of higher excitations on the effective (optical) potential coupling various reaction channels. The total cross section for a particular excitation is proportional to the maximum contribution of that excitation to the imaginary part of the elastic momentum-space optical potential, and is typical of the contribution to the potential in general. Analytic expressions relevant to the calculation of optical potentials are given Their validity is estimated by comparison with more-accurate calculations and with experimental excitation cross sections.

scholarly journals Positron Scattering from Atoms and Molecules

Atoms ◽  
2020 ◽  
Vol 8 (2) ◽  
pp. 29 ◽  
Author(s):  
Sultana N. Nahar ◽  
Bobby Antony
Keyword(s):  
Cross Section ◽  
Electron Scattering ◽  
Cross Sections ◽  
Optical Potential ◽  
Inert Gases ◽  
Positron Impact ◽  
Positron Scattering ◽  
Atoms And Molecules ◽  
Section Data ◽  
Theoretical Approaches

A review on the positron scattering from atoms and molecules is presented in this article. The focus on positron scattering studies is on the rise due to their presence in various fields and application of cross section data in such environments. Positron scattering is usually investigated using theoretical approaches that are similar to those for electron scattering, being its anti-particle. However, most experimental or theoretical studies are limited to the investigation of electron and positron scattering from inert gases, single electron systems and simple or symmetric molecules. Optical potential and polarized orbital approaches are the widely used methods for investigating positron scattering from atoms. Close coupling approach has also been used for scattering from atoms, but for lighter targets with low energy projectiles. The theoretical approaches have been quite successful in predicting cross sections and agree reasonably well with experimental measurements. The comparison is generally good for electrons for both elastic and inelastic scatterings cross sections, while spin polarization has been critical due to its sensitive perturbing interaction. Positron scattering cross sections show relatively less features than that of electron scattering. The features of positron impact elastic scattering have been consistent with experiment, while total cross section requires significant improvement. For scattering from molecules, utilization of both spherical complex optical potential and R-matrix methods have proved to be efficient in predicting cross sections in their respective energy ranges. The results obtained shows reasonable comparison with most of the existing data, wherever available. In the present article we illustrate these findings with a list of comprehensive references to data sources, albeit not exhaustive.

scholarly journals Drift Velocity and Longitudinal Diffusion Coefficient of Electrons in CO2?Ar Mixtures and Electron Collision Cross Sections for CO2 Molecules

1995 ◽  
Vol 48 (3) ◽  
pp. 357 ◽  
Author(s):  
Y Nakamura
Keyword(s):  
Diffusion Coefficient ◽  
Momentum Transfer ◽  
Cross Section ◽  
Drift Velocity ◽  
Cross Sections ◽  
Vibrational Excitation ◽  
Electron Collision ◽  
Longitudinal Diffusion ◽  
Excitation Cross Sections ◽  
Momentum Transfer Cross Section

The drift velocity and longitudinal diffusion coefficient of electrons in 0�2503% and 1� 97% C02-Ar mixtures were measured for 0�03 ~ E/N ~ 20 Td. The measured electron swarm parameters in the mixtures were used to derive a set of consistent vibrational excitation cross sections for the C02 molecule. Analysis of electron swarms in pure C02 using the present vibrational excitation cross sections was also carried out in order to determine a new momentum transfer cross section for the C02 molecule.

Optical potential for light nuclei and momentum-space eikonal phase function

2018 ◽  
Vol 96 (6) ◽  
pp. 642-649
Author(s):  
Charles M. Werneth ◽  
K.M. Maung ◽  
M.D. Vera ◽  
L.W. Townsend
Keyword(s):  
Cross Sections ◽  
Momentum Space ◽  
Phase Function ◽  
Optical Potential ◽  
Solution Method ◽  
Radiation Environment ◽  
Space Representation ◽  
Nuclear Fragmentation ◽  
Position Space ◽  
Space Radiation Environment

The space radiation environment comprises all of the nuclei in the periodic table with energies that extend from a fraction of an MeV/n to TeV/n. The vast range of projectile–target and energy combinations necessitates highly efficient and accurate cross section codes for use in radiation transport codes. As particles in the space radiation environment impinge on shielding materials, nuclear reactions, such as nuclear fragmentation, occur. One way of estimating nuclear fragmentation cross sections is to use an abrasion–ablation model, which describes how nucleons are dislodged from the nuclei as a result of nuclear collisions and the mechanism by which excited pre-fragments decay via particle emission to more stable states. The well-known partial wave solution method cannot be used directly for the computation of abrasion cross sections. Instead, abrasion cross sections may be computed by slightly altering the Eikonal solution method, which is a high energy (small scattering angle) approximation that depends on the nucleus–nucleus optical potential. The aim of the current work is to present two efficient methods for the computation of the Eikonal phase shift function. Analytic formulas of the optical potential are presented in the position-space representation for nuclei that are well-represented by harmonic-well nuclear matter densities (A < 20), which reduces the Eikonal phase factor to an integration over a single dimension. Next, the Eikonal phase function is presented in the momentum-space representation, which is particularly useful when the Fourier transform of the position-space optical potential is known. These new methods increase the computational efficiency by three orders of magnitude and allow for rapid prediction of elastic differential, total, elastic, and reaction cross sections in the Eikonal approximation.

Measurement of absolute electron-impact excitation cross sections in Ar and N2 using H and H2 emissions as secondary standards

1988 ◽  
Vol 66 (4) ◽  
pp. 349-357 ◽  
Author(s):  
J. L. Forand ◽  
S. Wang ◽  
J. M. Woolsey ◽  
J. W. McConkey
Keyword(s):  
Electron Impact ◽  
Cross Section ◽  
Cross Sections ◽  
Emission Cross Section ◽  
Impact Excitation ◽  
Electron Impact Excitation ◽  
Dissociative Excitation ◽  
Excitation Cross Sections ◽  
Line Following ◽  
Good Agreement

A detailed description is given of a technique in which emissions from H and H2 are used to calibrate an apparatus used for electron-impact emission cross-section measurements in the wavelength range 90–130 nm. Absolute emission cross sections have been measured at 200 eV electron-impact energy for the 120 nm N I line following dissociative excitation of N2 and for the Ar and Ar+ lines at 104.8, 106.7, 92.0, and 93.8 nm respectively. Good agreement with earlier works is obtained in the case of the N I line when earlier data are renormalized to take into account the recent revision of the cross section for production of Lyman α from H2. Measurements of the 104.8 and 106.7 nm lines suggest a 40% cascade component for the latter line at energies of 200 eV and above.

scholarly journals Positron Impact Excitation of the nS States of Atomic Hydrogen

Atoms ◽  
2020 ◽  
Vol 8 (1) ◽  
pp. 9 ◽  
Author(s):  
Anand K. Bhatia
Keyword(s):  
Cross Section ◽  
Cross Sections ◽  
Born Approximation ◽  
Atomic Hydrogen ◽  
Impact Excitation ◽  
Orbital Method ◽  
Positron Impact ◽  
Excitation Cross Sections ◽  
Theoretical Results ◽  
Incident Positron

The excitation cross sections of the nS states, n = 2 to 6, of atomic hydrogen at various incident positron energies (10.23 to 300 eV) were calculated using the variational polarized-orbital method. Nine partial waves were used to obtain converged cross sections. The present results should be useful for comparison with results obtained from other theories and approximations. The positron-impact cross section was found to be higher than the electron-impact cross sections. Experimental and other theoretical results are discussed. The threshold law of excitation is discussed and the cross sections in this region were seen to obey the threshold law proportional to ( ln k f ) − 2 . Cross sections were calculated in the Born approximation also and compared to those obtained using the variational polarized orbital method.

Measurement of Excitation Cross-Sections for Electron Impact at low Densities of Target Atoms

1973 ◽  
Vol 28 (11) ◽  
pp. 1871-1872 ◽  
Author(s):  
F. Karstensen ◽  
J. Pohl
Keyword(s):  
Electron Impact ◽  
Cross Section ◽  
Cross Sections ◽  
Atomic Beam ◽  
Excitation Cross Section ◽  
Particle Density ◽  
Measured Cross Section ◽  
Beam Experiment ◽  
Excitation Cross Sections

In a crossed-beam experiment Na atoms are excited by electron impact with an energy of 11 eV. Determination of the excitation cross-section for the D-lines shows an increase of the measured cross-section with decreasing particle density in the atomic beam below 1012 cm-3.

scholarly journals Analytic approximations to photoabsorption cross sections of once-ionized helium in magnetar atmospheres

2021 ◽  
Vol 2103 (1) ◽  
pp. 012030
Author(s):  
I V Demidov ◽  
A Y Potekhin
Keyword(s):  
Magnetic Field ◽  
Cross Sections ◽  
Binding Energies ◽  
Oscillator Strengths ◽  
Theoretical Studies ◽  
Centre Of Mass ◽  
Substantial Fraction ◽  
Analytic Approximations ◽  
Internal Motions ◽  
Analytic Expressions

Abstract Magnetar atmospheres can contain a substantial fraction of once-ionized helium. At the magnetic fields about 1014 −1015 G, typical of magnetars, Landau quantization is important not only for the electrons, but also for the centre-of-mass (CM) motion of the He+ ion. The CM and internal motions are mutually dependent, which complicates theoretical studies of the He+ characteristics. We present asymptotic analytic expressions for the binding energies, oscillator strengths, and photoionization cross sections of the moving hydrogenlike ions in an ultra-strong magnetic field, which can be used to construct approximate models of magnetar atmospheres.

scholarly journals A METHOD FOR EXTRACTING THE RESONANCE PARAMETERS FROM EXPERIMENTAL CROSS-SECTIONS

2013 ◽  
Vol 22 (05) ◽  
pp. 1350032 ◽  
Author(s):  
S. A. RAKITYANSKY ◽  
N. ELANDER
Keyword(s):  
Experimental Data ◽  
Cross Section ◽  
Cross Sections ◽  
Single Channel ◽  
Analytic Structure ◽  
Resonance Parameters ◽  
S Matrix ◽  
Analytic Expressions ◽  
Data Points ◽  
Pseudo Data

Within the proposed method, a set of experimental data points are fitted using a multi-channel S-matrix. Then the resonance parameters are located as its poles on an appropriate sheet of the Riemann surface of the energy. The main advantage of the method is that the S-matrix is constructed in such a way that it has proper analytic structure, i.e. for any number of two-body channels, the branching at all the channel thresholds is represented via exact analytic expressions in terms of the channel momenta. The way the S-matrix is constructed makes it possible not only to locate multi-channel resonances but also to extract their partial widths as well as to obtain the scattering cross-section in the channels for which no data are available. The efficiency of the method is demonstrated by two model examples of a single-channel and a two-channel problems, where known resonance parameters are rather accurately reproduced by fitting the pseudo-data artificially generated using the corresponding potentials.

Electron-impact excitation of W40+–W43+ ions: Cross-section and polarization

2020 ◽  
Vol 34 (26) ◽  
pp. 2050241
Author(s):  
Neelam Shukla ◽  
Rajesh Srivastava
Keyword(s):  
Electron Impact ◽  
Cross Section ◽  
Cross Sections ◽  
Matrix Theory ◽  
Impact Excitation ◽  
Electron Impact Excitation ◽  
Excitation Threshold ◽  
Plasma Modeling ◽  
Excitation Cross Sections ◽  
Density Matrix Theory

Electron-impact excitation of tungsten ions [Formula: see text] has been studied using the fully relativistic distorted wave (RDW) theory. The excitation cross-sections of selected electric dipole allowed transitions of these ions to be determined for the incident electron energies from the excitation threshold to 20 keV. Further, the calculated cross-section results are fitted with an analytical expression for direct applications in plasma modeling. In addition, using the density matrix theory, the linear polarization of the photons emitted from the excited tungsten ions when decay to their respective ground states is presented.

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