Neutral Cross Sections Research Articles

Inert gas ion beam formation at the tandem accelerator in Alma-Ata (abstract)

At the Cockroft–Walton-type 2×1 MV tandem accelerator in Alma-Ata, a method of obtaining inert gas ion beams by means of injection of neutral particles into a first accelerating tube has been developed. The procedure of injected beam intensity control is described. The neutralization cross section of He+ and Ar+ ions on the various gaseous targets has been estimated. Recently obtained results in comparison with analogous data obtained elsewhere are presented.

Charge exchange collisions between alkaline-earth ions and alkali atoms

The ratio of the charge exchange cross sections for Ba+ (5d) and Ba+(6s) on Na, Rb and Cs, for Sr+(4d) and Sr+(5s) on Na and Rb, and for Ca+(3d) and Ca-(4s) on Na have been measured for ion impact energies between 0.5 keV and 30 keV. For the collisions with Na, absolute charge exchange cross section values are derived using data for the resonant process Na+Na+ as a reference. The results are compared to theoretical near-resonance predictions. A method is proposed to study the polarization dependence of the neutralization cross sections for reactions involving ionic D states.

Organic neutralization agents for neutralization-reionization mass spectrometry

Porter has shown that excited neutrals of specified internal energies can be prepared by neutralization of an ion beam with metal vapors of low ionization potential (IP). For specific problems in neutralization-reionization mass spectrometry, a metal with the desired IP value may not be available, or it may present experimental problems such as a high vaporization temperature, instrument contamination, or detector instability. The use of organic neutralization agents such as tetra- p-anisylethylene (IP = 6.0 eV) can minimize these problems (although cross sections for neutralization with these are a factor of 5 lower than those with metals), and can provide a much wider range of IP values. Their utility is demonstrated in the neutralization of C 4H + 4 and C 4H + 8 ions to produce C 4H 4 and C 4H 8 of selected internal energies. However, for CH + 4 neutralization, the CH 4 neutrals formed have a much lower internal energy than predicted, indicating that electron transfer from the neutralization agent predominantly produces its ions in excited states.

Dissociative Ionisation and Neutral Dissociation: CF4, a Case Study

New results for the total neutral dissociation cross section and an estimate of the gross cross section for the production of neutral fluorine by electron impact on CF4 have been obtained by combining previously reported results for the total dissociation cross section and the counting cross section for the total dissociative ionisation. This advancement was made possible by recently reported results for multication formation obtained from coincidence experiments. The estimate of the neutral fluorine production cross section is of the same order of magnitude as the total dissociation cross section itself, a similarity which may explain why CF4 is such an effective etching gas. The available cross section data for electron impact energies between 5 and 200 e V are reviewed.

Collisional plasma sheath model

The effects of ion collisionality on the plasma sheath are revealed by a two-fluid model. In contrast to previous work, the ion–neutral collision cross section is modeled using a power law dependence on ion energy. Exact numerical solutions of the model are used to determine the collisional dependence of the sheath width and the ion impact energy at the wall. Approximate analytical solutions appropriate for the collisionless and collisionally dominated regimes are derived. These approximate solutions are used to find the amount of collisionality at the center of the transition regime separating the collisionless and collisional regimes. For the constant ion mean-free-path case, the center of the transition regime for the sheath width is at a sheath width of five mean-free paths. The center of the transition regime for the ion impact energy is at a sheath width of about one-half of a mean-free path.

Angular distributions and charge exchange in Na scattering from clean and Cs covered Ag(111)

An experimental study of 35 eV Na scattering at normal incidence from a clean and Cs covered Ag(111) surface is presented. The measured ion and neutral yield are in quite good agreement with model calculations by Nakanishi et al. using the time-dependent Newns-Anderson model taking into account the Coulomb interaction between electrons of opposite spin occupying the ionization and affinity level. For a complete description local work function effects cannot be neglected. With the help of classical trajectory calculations most of the scattering phenomena can he understood, both for the clean and for the cesiated surface. From the measurements a neutralization cross section of 175 ± 40 Å 2 is extracted for each adsorbed Cs atom. The pronounced rainbow scattering observed for the clean surface is masked but does not disappear upon Cs adsorption.

Coulombic and neutral trapping centers in silicon dioxide.

Metal-oxide-semiconductor structures incorporating thermally grown silicon dioxide films were implanted with arsenic ions and then annealed at high temperatures. The subsequent trapping sites produced are amphoteric. Coulombic-attractive traps (for electrons) were produced with the avalanche injection of holes from the silicon substrate and the subsequent capture of some of these holes on the arsenic-related sites. During internal photoemission of electrons from a thin aluminum gate, the voltage shifts due to hole annihilation by electrons were recorded and the effective capture cross section was determined. This capture cross section was found to vary from \ensuremath{\sim}${10}^{\mathrm{\ensuremath{-}}12}$ to 3\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}15}$ ${\mathrm{cm}}^{2}$ for average electric fields ranging from 2\ifmmode\times\else\texttimes\fi{}${10}^{5}$ to 3\ifmmode\times\else\texttimes\fi{}${10}^{6}$ V/cm. An average field threshold (\ensuremath{\sim}1.2\ifmmode\times\else\texttimes\fi{}${10}^{6}$ V/cm) was found, below which the capture-cross-section--average-field dependence follows a power law with an exponent of approximately -1.5. Above the average field threshold, the power-law exponent was found to be approximately -3.0.Also, when the amphoteric arsenic-related sites are empty, they can form neutral trapping sites for electrons. For these trapping centers, it is found that the neutral capture cross section is relatively independent of the average electric field. For average fields ranging from 5\ifmmode\times\else\texttimes\fi{}${10}^{5}$ to 6\ifmmode\times\else\texttimes\fi{}${10}^{6}$ V/cm, the neutral cross section is found to be approximately constant at (1--2)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}15}$ ${\mathrm{cm}}^{2}$. For the Coulombic electron traps, classical and quantum-mechanical Monte Carlo simulations agree qualitatively with the experimental results. These simulations suggest that the heating of the electron-energy distribution and tunnel detrapping are the primary cause of the decrease in the effective capture cross section in the high-field regime. For the neutral traps in the low-field regime, the classical Monte Carlo simulation also agrees with the experimental results. However, for fields above the electron-heating threshold, the simulation predicts an increase in the capture cross section not found in the experimental data. We suggest that this discrepancy arises since the classical simulation does not account for tunnel detrapping, which would lower the effective cross section.

Velocity-dependent neutralization cross sections of Ba+ ground and metastable states by Na.

A combined optical-pumping--laser-induced-fluorescence method was developed to determine the fraction of metastable-state ions in a ${\mathrm{Ba}}^{+}$ beam. This allowed us to measure the ratio of the charge-exchange cross sections for collisions of ${\mathrm{Ba}}^{+}$(6s) and ${\mathrm{Ba}}^{+}$(5d) ions with a sodium vapor for impact energies ranging from 3 to 29 keV. For a collision energy of 5 keV, the ratio of the cross sections for ${\mathrm{Ba}}^{+}$(5d $^{2}$${\mathit{D}}_{3/2}$) and ${\mathrm{Ba}}^{+}$(5d $^{2}$${\mathit{D}}_{5/2}$) was measured. All results are compared to theoretical near-resonance predictions.

Mutual neutralization in H +-H − collision

Mutual neutralization in collisions of protons with negative hydrogen ions has been studied using the standard multistate impact parameter model. The total wave function of the system was expanded in travelling two electron atomic orbitals centered on the two nuclei. The recently proposed configuration interaction wave function for the negative hydrogen ion [1] was used. Several bound and continuum pseudostates in conjunction with exact n = 1–3 states of the hydrogen atom were used. The two electron exchanged interaction has been treated exactly. The neutralization cross section has been calculated for impact energies in the range 0.15–50.0 keV. The computed values of the total neutralization cross section are in good agreement with the available experimental data.

Neutral pion production cross sections in heavy ion collisions from 80 to 219 MeV/nucleon.

Neutral pion production cross sections in heavy ion collisions from 80 to 219 MeV/nucleon.

Local work function effects in the neutralization of alkali ions scattered from cesiated surfaces

The charge state of Li, K and Cs ions scattered from cesium covered W(110) is measured as a function of the surface work function. The particle energy ranges from 100 to 2000 eV. It is shown that the local electrostatic potential has a significant influence on the neutralization process. In the case of potassium and cesium scattering the neutralization is due to a collective effect of the adsorbed cesium ions. In the case of lithium scattering it is found that the adsorbed cesium ions act as independent neutralization centres. The corresponding neutralization cross section is of the order of 600 a 0 2 .

Mutual neutralization and chemiionization in collisions of alkali metal and halogen atoms

A semiempirical valence bond method is used to obtain realistic potential curves for the three lowest 1∑+ curves of NaF, KF, LiCl, NaCl, KCl, Lil, and Nal. They are used together with a model functional form for the diabiatic coupling to compute quantal chemiionization and mutual neutralization cross sections. The accuracy of one-electron estimates of the interaction energies in the crossing regions is explored. The Landau–Zener method is found to be very reliable except for collisions involving iodine, for which modifications are necessary.

Nonadiabatic ionic–covalent transitions. Exponential-linear model for the charge exchange and neutralization reactions Na+H ⇄ Na++H−

A previous study of charge exchange processes taking place through ionic–covalent transitions is extended to the case of Na+H and Na++H− collisions. A five-state molecular expansion, with the inclusion of two-electron translation factors, is employed to calculate the charge exchange and neutralization cross sections. Transitions at the first two pseudocrossings between the energy curves, practically determine the cross sections in the energy range 0.16–5 keV amu−1. We also show that the widely used multichannel Landau–Zener theory is totally inadequate, to treat these transitions.

The Photoconductivity Exponent For Recombination At Dangling Bonds In a-Si:H

ABSTRACTA theoretical model has been developed for recombination at dangling bonds which explains the γ variations between 0.5 and 1 depending on the Fermi level position. The occupation probabilities of the T3+, T3° and T3- states under illumination have been calculated using the statistics of correlated levels. The γ exponent is derived through a parametric representation of the equations of detailed balance and charge conservation. A good agreement with experiment is obtained with a dangling bond density of 5×1015 cm-3, a placing of the T3° level at 0.95 eV below Ec, an effective correlation energy of 0.4 eV and a charged to neutral capture cross section ratio of 50.

Molecular-state cross-section calculations for H+Na

Pseudopotential molecular-structure calculations have been used to obtain the seven lowest $^{1}\ensuremath{\Sigma}$ and $^{1}\ensuremath{\Pi}$ states of CsH. These states and their associated radial and rotational coupling terms have been used to calculate the cross sections for ${\mathrm{H}}^{\ensuremath{-}}$+${\mathrm{Cs}}^{+}$ ion-pair production in $\mathrm{H}+\mathrm{Cs}(6s)$ and $\mathrm{H}+{\mathrm{Cs}}^{*}(6p)$ collisions at energies from 0.1 to 10 keV. The ion-ion mutual neutralization cross section, ${\mathrm{H}}^{\ensuremath{-}}$+${\mathrm{Cs}}^{+}$ \ensuremath{\rightarrow} H+Cs, is also presented. The cross-section calculations were done with the perturbed-stationary-state method, modified to include two-electron translation factors. The ion-pair production cross section for ground-state reactants is in good agreement with experiment; collisions of H with excited ${\mathrm{Cs}}^{*}(6p)$ show an order-of-magnitude enhancement of the ion-pair production cross section at 100 eV. The ion-ion mutual neutralization cross section is found to be large, attaining a value of 1.3 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}14}$ ${\mathrm{cm}}^{2}$ at 0.1 keV.

Subthreshold pion production.

The Boltzmann master equation as adopted for treating equilibration in heavy ion reactions has been modified to include pion production rates from (n,n), (n,p), and (p,p) collisions. We calculate neutral pion production cross sections for reactions of 35 MeV/nucleon $^{14}\mathrm{N}$ with $^{27}\mathrm{Al}$, $^{58}\mathrm{Ni}$, and $^{184}\mathrm{W}$ targets. We also calculate ${\ensuremath{\pi}}^{0}$ production cross sections for reactions of $^{12}$C${+\mathrm{}}^{12}$C, $^{58}\mathrm{Ni}$, $^{238}\mathrm{U}$ at beam energies of 60, 74, and 84 MeV/nucleon, and of 44 MeV/nucleon $^{40}\mathrm{Ar}$ with $^{40}\mathrm{Ca}$, $^{119}\mathrm{Sn}$, and $^{238}\mathrm{U}$. Results of these calculations are generally within a factor of 2 of experimental values. Compound nucleus excitations predicted for the equilibrated nuclei for the above reactions are also estimated for implications in a compound nucleus interpretation of the pion production mechanism.

Computational chemistry and aeroassisted orbital transfer vehicles

An analysis of the radiative heating phenomena encountered during a typical aeroassisted orbital transfer vehicle (AOTV) trajectory was made to determine the potential impact of computational chemistry on AOTV design technology. Both equilibrium and nonequilibrium radiation mechanisms were considered. This analysis showed that computational chemistry can be used to predict (1) radiative intensity factors and spectroscopic data; (2) the excitation rates of both atoms and molecules; (3) high-temperature reaction rate constants for metathesis and charge exchange reactions; (4) particle ionization and neutralization rates and cross sections; and (5) spectral line widths.

Molecular-state cross-section calculations for H+Cs⇄H−+Cs+

Pseudopotential molecular-structure calculations have been used to obtain the seven lowest $^{1}\ensuremath{\Sigma}$ and $^{1}\ensuremath{\Pi}$ states of CsH. These states and their associated radial and rotational coupling terms have been used to calculate the cross sections for ${\mathrm{H}}^{\ensuremath{-}}$+${\mathrm{Cs}}^{+}$ ion-pair production in $\mathrm{H}+\mathrm{Cs}(6s)$ and $\mathrm{H}+{\mathrm{Cs}}^{*}(6p)$ collisions at energies from 0.1 to 10 keV. The ion-ion mutual neutralization cross section, ${\mathrm{H}}^{\ensuremath{-}}$+${\mathrm{Cs}}^{+}$ \ensuremath{\rightarrow} H+Cs, is also presented. The cross-section calculations were done with the perturbed-stationary-state method, modified to include two-electron translation factors. The ion-pair production cross section for ground-state reactants is in good agreement with experiment; collisions of H with excited ${\mathrm{Cs}}^{*}(6p)$ show an order-of-magnitude enhancement of the ion-pair production cross section at 100 eV. The ion-ion mutual neutralization cross section is found to be large, attaining a value of 1.3 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}14}$ ${\mathrm{cm}}^{2}$ at 0.1 keV.

Absolute emission cross sections for detection of plasma impurity ions with active neutral lithium beam diagnostics

For detection of impurity ions in magnetically confined plasmas by means of active neutral Li beam diagnostics emission cross sections for electron capture by C q+ ( q = 3, 4, 5, 6) and O q+ ( q = 4, 5, 6, 7) ions have been measured. The ion impact energies correspond to injection of 7Li (2s) with 20 and 30 KeV respectively.

Estimation of charged strange particle cross sections of antiproton-proton interactions ats 1/2=3.6?8.8 GeV

The inclusive and semi-inclusive neutral strange particle cross sections are used to estimate semi-inclusive charged strange particle cross sections in antiproton-proton interactions of the pairsK0 or\(\Lambda /\bar \Lambda \) with a charged strange particle and of double charged strange particles. Their sums are given as well.