Neutral Cross Sections Research Articles

Electron-impact dissociation cross sections for CHF3 and C3F8

Absolute total dissociation cross sections, σt,diss, by electron-impact are reported for CHF3 and C3F8 from 10 to 300 eV using the chemical gettering technique described by Winters and Inokuti (1982 Phys. Rev. A 25 1420). Data are concentrated in the near-threshold region (10–30 eV). The thresholds for dissociation of CHF3 and C3F8 are determined to be 10.4 eV and 11.9 eV, respectively. Ionization thresholds occur at 16 eV for CHF3 and 16.2 eV for C3F8. Neutral dissociation cross sections of both CHF3 and C3F8 are obtained by subtracting the ionization cross sections, σt,ion, from the total dissociation cross sections, σt,diss.

Neutralization and Detachment in H+–H− Collisions

The cross sections for neutralization and detachment inH+–H− collisions in the energy range from 1.0 to100 keV/u are calculated using the two-centre atomic orbitalclose-coupling (TC-AOCC) method. The results are compared with theavailable experimental and theoretical data. It is found that theneutralization cross section agrees well with the experimental data bySchön et al. [J. Phys. B 20 (1987) L759] and Melchert et al., [J. Phys.B 32 (1999) L139] especially at low energies. However, for thedetachment process, our calculated cross section lies between theexperimental data by Melchert et al. and by Peart et al. [J. Phys. B 9(1976) 3047] for the energy below 15 keV/u. Above this energy, ourresult is smaller than the two experimental data. It is worth pointingout that there exists a large difference between these two experimentaldata and it is difficult to judge which data is more accurate.Therefore, a high-precision measurement for detachment cross sectionsis expected to resolve this discrepancy and to test the theoreticalcalculations.

Electron impact dissociation cross sections for C2F6

Absolute total dissociation cross sections for electron impact, σt,diss, from 8 to 700 eV are reported for C2F6. A dense set of data was obtained in the technologically important 8–30 eV energy range relevant to modelling the type of plasmas used in both fundamental and applied scientific investigations. The threshold for dissociation was found to be 12.0 ± 0.2 eV and appears to be associated with a Rydberg state. Estimated values for the total neutral dissociation cross section, σneut,diss, were obtained by subtracting the ionization cross sections (all ionizing events cause dissociation) from the total dissociation cross section. It is shown that a calibration error in a paper by one of us (HFW) caused a distortion of several previous investigations. When the data from the present work are used to recalibrate data from swarm experiments, agreement becomes quite reasonable. There is now a consistent set of data obtained from several investigators which describe the dissociation of C2F6. Neutral dissociation cross sections are obtained from electron-impact excitation calculations and found to be in reasonable agreement with measurements over most of the energy range.

The empirical dependence of radiation-induced charge neutralization on negative bias in dosimeters based on the metal-oxide-semiconductor field-effect transistor

The dependence of radiation-induced charge neutralization (RICN) has been studied in metal-oxide-semiconductor field-effect transistor (MOSFET) dosimeters. These devices were first exposed to x rays under positive bias and then to further dose increments at a selection of reverse bias levels. A nonlinear empirical trend has been established that is consistent with that identified in the data obtained in this work. Estimates for the reverse bias level corresponding to the maximum rate of RICN have been extracted from the data. These optimum bias levels appear to be independent of the level of initial absorbed dose under positive bias. The established models for threshold voltage change have been considered and indicate a related nonlinear trend for neutralization cross section σN as a function of oxide field. These data are discussed in the context of dose measurement with MOSFETs and within the framework of statistical mechanics associated with neutral traps and their field dependence.

Mutual neutralization in slow H+2−H− collisions

The process of mutual neutralization in slow H+2+H− collisions is considered within the multi-channel Landau–Zener model. The cross sections for electron capture from H− to specific gerade (singlet and triplet) states of H2 molecule are calculated in the CM energy range from 0.05 keV/u to 5 keV/u. The calculated total mutual neutralization cross section is in satisfactory agreement with the experimental data available in the CM energy range 20–2000 eV.

Pulsed CsI-coated tufted carbon fiber cathode plasma

This paper discusses the out-gassing behavior of a repetitively pulsed electron beam diode with a CsI-coated tufted carbon fiber cathode and a carbon fiber anode. Through this paper, a cross section model was developed that utilizes experimental data including pressure, temperature and voltage (energy) for determination. Mass and optical spectroscopy identified neutral and ion species likely responsible for the pressure pulses. The identified neutral and ionization cross sections from the published literature and databases were compared with the experimentally determined cross sections. The types of cross sections were electron-neutral and was single impact leading to single, double and triple ionization as well as elastic scattering. This comparison leads to the conclusion that initial pressure bursts consist of plasma with constituents of Cs, Cs/sup ++/, Cs/sup +++/, I, I/sup ++/, N/sub 2/, N/sub 2//sup +/, H/sub 2/O/sup +/, H, H/sup +/, and H/sub 2//sup +/. Larger pressure bursts after the initial ones also include Cs/sup +/ and I/sup +/. Smaller bursts observed during steady state operations fluctuate between plasmas that contain H and H/sup +/, and those that contain H, H/sup +/, and H/sub 2//sup +/. Consistently, the initial pressure pulses exceeded those found under steady-state operations. The optical spectroscopy timing and results are self-consistent with what is obtained from the cross section model. The cross section model coupled with the experimental data determines the transient plasma makeup responsible for the observed pressure pulses.

Capture of antiprotons by some radioactive atoms and ions

Cross sections for antiproton capture are calculated using the fermion molecular dynamics method for ions of current interest in experiments determining nuclear structure of the radioactive nuclei ${}^{8}\mathrm{He},$ ${}^{11}\mathrm{Li},$ ${}^{11}\mathrm{Be},$ and ${}^{21}\mathrm{Mg}.$ The cross sections for the corresponding neutral atoms are also calculated. It is found that, except for helium, the cross sections for the ion and neutral atom at usual capture energies are similar, i.e., neither the enhanced trajectory curvature nor the absence of the most weakly bound electron have great effect. The behavior of the cross sections is also analyzed at very low collision energies, where the ion cross sections go as $1/E$ and the neutral cross sections as $1/\sqrt{E}.$

The Nature of Damped Lyα Absorbing Galaxies atz≤ 1: A Photometric Redshift Survey of Damped Lyα Absorbers

We study the nature of damped Lyα absorption (DLA) systems at z ≤ 1 using a sample of 11 DLA galaxies, for which accurate redshift measurements are available. Five of the 11 systems are identified in our ongoing photometric redshift survey of DLA galaxies, while the remaining six systems are identified by previous groups using either spectroscopic or photometric redshift techniques. Absolute B-band magnitudes of the galaxies range from MAB(B) = -15.3 to -20.3. Impact parameter separations of the galaxy and absorber pairs range from ρ = 0.31 to 25.4 h-1 kpc. We first demonstrate that the precision of photometric redshifts is sufficient for identifying DLA galaxies because DLA systems are rare, and their intrinsically high column density implies a small impact parameter of the host galaxy to the QSO line of sight. We then adopt this first large DLA galaxy sample to study the neutral gas cross section of intermediate-redshift galaxies and examine the optical properties of DLA galaxies at z ≤ 1. The results of our study are as follows: (1) the extent of neutral gas around intermediate-redshift galaxies scales with B-band luminosity as R/R* = β with R* = 24-30 h-1 kpc and β = 0.26 at N(H ) = 1020 cm-2; (2) the observed incidence of the DLA systems versus the B-band luminosity of the DLA galaxies is consistent with models derived from adopting a known galaxy B-band luminosity function and the best-fit scaling relation of the neutral gas cross section at MAB(B) - 5 log h ≤ -17; (3) comparison of the observed and predicted number density of DLA systems supports that luminous galaxies can explain most of the DLA systems found in QSO absorption line surveys and a large contribution of dwarfs (MAB(B) - 5 log h ≥ -17) to the total neutral gas cross section is not necessary; (4) of the 11 DLA systems studied, 45% are disk dominated, 22% are bulge dominated, 11% are irregular, and 22% are in galaxy groups, indicating that galaxies that give rise to the DLA systems span a wide range of morphological types and arise in a variety of galaxy environments; and (5) galaxies that contain the bulk of neutral gas in the universe do not appear to exhibit a substantial luminosity evolution between z = 0 and 1.

Neutral pion measurements in polarized proton collisions from PHENIX at RHIC

The Relativistic Heavy Ion Collider (RHIC) started operation as a polarized proton collider in December, 2001 with transverse-spin beams. This report presents the preliminary result for the absolute neutral pion production cross section at s =200 GeV in the mid-rapidity region by the PHENIX experiment. We compare this result to the prediction of a next-to-leading order perturbative QCD calculation. The measurement of the neutral pion asymmetries in this run and in future runs are also discussed.

Evaluation of the density of states parameters of a-Si:H by AC photoconductivity measurements and numerical simulation

In this study, we discuss the results of AC-photoconductivity (AC-PC) experiments on intrinsic a-Si:H, and the results of a numerical model developed for AC-PC. High quality samples were illuminated with a monochromatic light in the photon energy region between 1.0 and 1.7 eV. The light was chopped with frequencies ranging from 10 to 600 Hz, and incident photon flux could be varied between 1×10 13 and 3×10 15 ph cm −2 s −1. Within these ranges, the magnitude of the photoconductivity and its phase difference were measured as a function of the photon energy, the chopper frequency and the photon flux using a lock-in amplifier. The numerical model has been developed from Simmons–Taylor statistics [Phys. Rev. B4 (1976) 502]. By means of simulation, we studied the influence of the excitation chopper frequency, the photon flux and the photon energy. The choice of the parameters strongly influences the energy and frequency dependence of the phase shift, as well as its absolute value. The most important parameters that affect the absolute value of the phase are charged to neutral capture cross section ratio, c. Application of the simulation model to the considered set of samples allows us to determine both the order of magnitude of the capture cross section and the ratio of the capture cross section of the charged and neutral states.

Measurement of the neutral pion cross section in proton-proton collisions at √s=200 GeV with PHENIX

The inclusive cross section for neutral pion production in the range 1< p T <13 GeV/c in ∣η∣<0.35 has been measured by the PHENIX experiment in proton-proton collisions at √s=200 GeV. An NLO pQCD calculation is, within the experimental and theoretical uncertainties, consistent with the measurement.

Plasma molding over surface topography: Energy and angular distribution of ions extracted out of large holes

Plasma molding over surface topography was investigated by measuring the energy and angular distribution of ions extracted from a hole in contact with a high density plasma. Holes with diameter larger than as well as smaller than the local sheath thickness were studied in argon or deuterium gas. When the hole diameter (10 μm) was much less than the sheath thickness, the plasma was not perturbed by the presence of the hole. The ion energy distribution (IED) had multiple peaks due to ions sampling the time-varying potential while crossing the sheath. The ion angular distribution (IAD) was Gaussian, peaking at zero angle with respect to the surface normal. These results agree with reported studies. At the other extreme, when the hole diameter (1270 μm) was larger than the sheath thickness, plasma “leaked” into the hole. The IED had a single peak since ions now experience an average sheath potential. The IAD was quite broad extending beyond 30° off normal. When the hole diameter (508 μm) was comparable to the sheath thickness, the shape of the IED and IAD was in-between the two extremes mentioned above. The IAD became more isotropic with increasing power, suggesting that the plasma leaked only partly through the hole (the plasma–sheath meniscus was located inside the hole). For all cases, increasing pressure resulted in lower ion energy in argon plasmas due to ion–neutral collisions. Increasing pressure had little effect on the ion energy for deuterium plasmas, for hole diameter less than 508 μm. This is due to the smaller ion–neutral collision cross section for deuterium.

Parametrizations of inclusive cross sections for pion production in proton-proton collisions

Accurate knowledge of cross sections for pion production in proton-proton collisions finds wide application in particle physics, astrophysics, cosmic ray physics, and space radiation problems, especially in situations where an incident proton is transported through some medium, and one requires knowledge of the output particle spectrum given the input spectrum. In such cases accurate parametrizations of the cross sections are desired. In this paper we review much of the experimental data and compare it with a wide variety of different cross section parametrizations. In so doing, we provide parametrizations of neutral and charged pion cross sections which provide a very accurate description of the experimental data. Lorentz invariant differential cross sections, spectral distributions, and total cross section parametrizations are presented.

Absolute cross section for the formation of Si(1S) atoms following electron impact dissociation of SiH4

A combination of electron scattering and laser-induced fluorescence (LIF) techniques was used in the experimental determination of the absolute cross section for the formation of Si(1S) ground-state atoms following the neutral molecular dissociation of SiH4 by electron impact for energies from 20 eV to 100 eV. Electron impact on SiH4 produces—among other species—Si(1S) ground-state atoms which are detected by pumping the Si(3p)2 1S→(3p)(4s)1P transition at 390 nm with a tunable dye laser and recording the subsequent Si(3p)(4s)1P→(3p)2 1D fluorescence at 288 nm. We found a peak cross section for the formation of Si(1S) atoms from SiH4 of 4.5×10−17 cm2 at an impact energy of 60 eV. When compared to the previously determined total SiH4 neutral dissociation cross section obtained from measurements in a constant-flow plasma reactor [Perrin et al., Chem. Phys. 73, 383 (1982)], we find a branching ratio of about 0.037 for the formation of Si(1S) atoms in the electron-impact induced neutral dissociation of SiH4. The absolute calibration of our measured dissociation cross section was made relative to the cross section for the formation of N2+(X) ground-state ions produced by electron impact on N2 which was previously measured in the same apparatus using the same experimental technique. This cross section is known to within ±10% and can serve as a benchmark for the calibration of neutral dissociation cross sections as discussed previously [Abramzon et al., J. Phys. B 32, L247 (1999)]. © 2000 American Institute of Physics.

Neutral bremsstrahlung measurement in an atmospheric-pressure radio frequency discharge

Neutral bremsstrahlung emission spectrum is measured in an atmospheric-pressure radio frequency (rf) capacitive discharge for a gas mixture of helium (99.5%) and oxygen (0.5%) using a high resolution triple monochromator between 450 and 1000 nm. Good agreement is obtained for spectral variation and absolute intensity between the observed neutral bremsstrahlung and theoretical emissivity calculated using electron–neutral momentum cross sections. Based on a theoretical fitting, the discharge is characterized by a time averaged electron density of 2.9×1011 cm−3 and an electron temperature of 1.9 eV for an input power density of 28 W/cm3.

Theoretical investigation of B- neutralization in energetic collisions with charged bare projectiles

The neutralization of B- ions in energetic collisions with bare charged projectiles is studied for two different ranges of collision parameters: (a) Z<<v; (b) Z v, where Z and v are the projectile charge and velocity, respectively. For both ranges analytical formulae are derived for the cross section of B- neutralization. These formulae are generalized for collisions with relativistic velocities. Scaling relations are proposed for the dependence of the neutralization cross section on the electron affinities in weakly bound negative ions with an active p- or s-electron. With known cross sections for the neutralization of B- and H- the scaling relations allow us to estimate neutralization cross sections for other negative ions with loosely bound s- and p-electrons. We also discuss the possibility of applying our results to estimate the neutralization of B- in collisions with electrons.

Neutralization and charge transfer in H+-H- and H-H collisions

Cross sections for charge transfer, neutralization and excitation in H-H and H--H+ collisions are presented in the energy range 1-30 keV. The semiclassical close-coupling approach is used with a two-centre two-electron atomic state expansion and frozen total spin. All details of the two-centre atomic state expansion scheme are given. The basis is expanded onto the bound state of H- and the two-centre H(1s)-H(nlm) states, where n covers the K, L and M shells. A good agreement with experiments is achieved for charge transfer, neutralization and excitation to H(2s) in H-H collisions. Disagreement with previously published close-coupling calculations and experimental data is noted, especially for two-electron charge transfer in H--H+.

An ab initio study of mutual neutralization in Na++H- collisions

First principles calculations of mutual neutralization in Na++H- collisions have been performed for collision energies between 0.01 and 100 eV. A ten-state molecular expansion has been employed with a quantal description of the relative motion and a diabatic ab initio calculation for the two active electrons. While results for the total neutralization cross section are generally within 15% of those obtained by Janev and Radulovic (1978), cross sections into the less populated final states differ much more, by in excess of an order of magnitude for the 3p level. At 100 eV satisfactory agreement is obtained with the results of Olson and Kimura (1985). The low-temperature rate coefficient for mutual neutralization is calculated to be 3.0 × 10-7 (T/300 K)-1/2 cm3 s-1. Cross sections for ion-pair production in Na(3l) + H collisions have also been determined, showing substantial differences from the results of Janev and Radulovic (1978), particularly for production from 3s at low energy. Differences of 5% and 35% from the calculations of Olson and Kimura (1985) for 3s and 3p, respectively, at 100 eV are obtained.

Neutralization of H− ions in collisions with fast, multiply charged ions

The neutralization of H− ions in collisions with fast, multiply charged ions is considered in the parameter region where the Born approximation applies. An analytical formula is obtained for the H− neutralization cross section in such collisions.

Neutralization of negative hydrogen ions in collisions with multicharged fast ions

We study the neutralization of negative hydrogen ions in collisions with multicharged fast ions (including relativistic ions) by using an approach that allows a simple expression for the neutralization cross section to be derived over a range of collision parameters where the standard Born approximation breaks down.