Neutral Cross Sections Research Articles

Self-consistency of electron-THF cross sections using electron swarm techniques.

The drift velocity and first Townsend ionization coefficient of electrons in gaseous tetrahydrofuran are measured over the range of reduced electric fields 4-1000 Td using a pulsed-Townsend technique. The measured drift velocities and Townsend ionization coefficients are subsequently used, in conjunction with a multi-term Boltzmann equation analysis, as a further discriminative assessment on the accuracy and completeness of a recently proposed set of electron-THF vapor cross sections. In addition, the sensitivity of the transport coefficients to uncertainties in the existing cross sections is presented. As a result of that analysis, a refinement of the momentum transfer cross section for electron-THF scattering is presented, along with modifications to the neutral dissociation and dissociative electron attachment cross sections. With these changes to the cross section database, we find relatively good self-consistency between the measured and simulated drift velocities and Townsend coefficients.

Comprehensive investigation of the electronic excitation of W(CO)6 by photoabsorption and theoretical analysis in the energy region from 3.9 to 10.8 eV.

High-resolution vacuum ultraviolet photoabsorption measurements in the wavelength range of 115–320 nm (10.8–3.9 eV) have been performed together with comprehensive relativistic time-dependent density functional calculations (TDDFT) on the low-lying excited sates of tungsten hexacarbonyl, W(CO)6. The higher resolution obtained reveals previously unresolved spectral features of W(CO)6. The spectrum shows two higher-energy bands (in the energy ranges of 7.22–8.12 eV and 8.15–9.05 eV), one of them with clear vibrational structure, and a few lower-energy shoulders in addition to a couple of lower-energy metal-to-ligand charge-transfer (MLCT) bands reported in the literature before. Absolute photoabsorption cross sections are reported and, where possible, compared to previously published results. On the basis of this combined experimental/theoretical study the absorption spectrum of the complex has been totally re-assigned between 3.9 and 10.8 eV under the light of spin–orbit coupling (SOC) effects. The present comprehensive knowledge of the nature of the electronically excited states may be of relevance to estimate neutral dissociation cross sections of W(CO)6, a precursor molecule in focused electron beam induced deposition (FEBID) processes, from electron scattering measurements.

A 1D Model of Radial Ion Motion Interrupted by Ion–Neutral Interactionsin a Cometary Coma

Abstract Because ion–neutral reaction cross sections are energy dependent, the distance from a cometary nucleus within which ions remain collisionally coupled to the neutrals is dictated not only by the comet’s activity level but also by the electromagnetic fields in the coma. Here we present a 1D model simulating the outward radial motion of water group ions with radial acceleration by an ambipolar electric field interrupted primarily by charge transfer processes with H2O. We also discuss the impact of plasma waves. For a given electric field profile, the model calculates key parameters, including the total ion density, n I , the H3O+/H2O+ number density and flux ratios, R dens and R flux, and the mean ion drift speed, , as a function of cometocentric distance. We focus primarily on a coma roughly resembling that of the ESA Rosetta mission target comet 67P/Churyumov–Gerasimenko near its perihelion in 2015 August. In the presence of a weak ambipolar electric field in the radial direction the model results suggest that the neutral coma is not sufficiently dense to keep the mean ion flow speed close to that of the neutrals by the spacecraft location (∼200 km from the nucleus). In addition, for electric field profiles giving n I and within limits constrained by measurements, the R dens values are significantly higher than values typically observed. However, when including the ion motion in large-amplitude plasma waves in the model, results more compatible with observations are obtained. We suggest that the variable and often low H3O+/H2O+ number density ratios observed may reflect nonradial ion trajectories strongly influenced by electromagnetic forces and/or plasma instabilities, with energization of the ion population by plasma waves.

Theoretical study of mutual neutralization inHe++H−collisions

Total and differential cross sections for mutual neutralization in ${\mathrm{He}}^{+}$ and ${\mathrm{H}}^{\ensuremath{-}}$ collisions at low to intermediate (0.001 eV to 100 eV) are calculated ab initio and fully quantum mechanically. Atomic final-state distributions and isotope effects are investigated. The theoretical model includes dynamics on eleven coupled states of $^{2}\mathrm{\ensuremath{\Sigma}}^{+}$ symmetry, where autoionization is incorporated. The potential-energy curves, autoionization widths, and nonadiabatic couplings of electronic resonant states of HeH are computed by combining structure calculations with electron scattering calculations. The nuclear dynamics is studied using a strict diabatic representation of the resonant states. Effects of rotational couplings between $^{2}\mathrm{\ensuremath{\Sigma}}^{+}$ and $^{2}\mathrm{\ensuremath{\Pi}}$ electronic states are investigated in the pure precession approximation.

Mutual neutralization in collisions of Li + and F −

Abstract Mutual neutralization in collisions of Li+ and F− is driven by an avoided crossing between the two lowest 1 Σ + electronic states of the LiF system. These electronic states are computed using the multi-reference configuration interaction method. We investigate how the adiabatic potential energy curves and the non-adiabatic coupling element depend on the choice of the reference configurations as well as the basis set. Using diabatic states, the total and differential cross sections for mutual neutralization are computed.

Improved calculation of the contact stress in strip rolling

The contact problem for strip rolling in plane deformation is analytically solved for zones with variable frictional forces, when using Mises plasticity conditions. A method of plotting the contact-stress curves is developed. The conditions in which slip and retardation zones exist are established, and their dimensions are determined. The position of the neutral cross section is found. Replacement of the precise plasticity condition by an approximate condition in terms of primary stress when the deformation regions are short reduces the calculated rolling force by 30–40%.

Influence of the Collisions on the Dusty Plasma Sheath in the Presence of an Oblique Magnetic Field

The dust dynamics in a magnetized collisional plasma-sheath are numerically studied by using the fluid model. Isothermal electrons, cold fluid ions, cold fluid dust grains and immobile neutral particles are taken into account in the sheath. As dust can be created by detaching small pieces of the wall limiting plasma, naturally, these grains can have different sizes. Therefore, the influence of dust size on the sheath characteristics is considered. Assuming the dust–neutral collision cross section has a power law dependency on the dust velocity. The comparison of the effect of the dust radius in both specific collisional models shows that in the constant cross section model, dust size plays a more role with respect to the constant collision frequency. The effect of the dust size on dust velocity is investigated for different values of the power factor. It shows that dust velocity when reaching near the wall in constant cross section model is much less than constant mobility model, and the velocity of the smaller dust is lower on the wall. If dust density is very small, the kind of collisional model has no significant influence on the electric potential. But by increasing dust density, a little fall in the local electric potential and a little rise in the sheath thickness are seen in constant cross section model.

Impact of implementing the Meyer-Neldel behavior of carrier emission pre-factors in solar cell and optical detector modeling

The Meyer-Neldel behavior reported for the emission probabilities of electrons and holes was included in our code, replacing the gap state capture cross sections of the Shockley-Read-Hall formalisms with capture cross sections containing an exponential function of the trap energy depth. The Meyer-Neldel energies for electrons and holes are the slopes of these exponentials. Our results indicate that emission probabilities of neutral states no deeper than approximately 0.45 eV and 0.37 eV from the conduction and valence band edges, respectively, can show a Meyer-Neldel behavior only, while on the other hand, its implementation in deeper gap states makes the replication of experimental J-V curves of p-i-n solar cells and detectors impossible. The Meyer-Neldel behavior can be included in all neutral capture cross sections of acceptor-like tail states without affecting the J-V characteristics, while it cannot be included in all capture cross sections of neutral donor-like tail states and/or defect states without predicting device performances below the experimental figures, that become even lower when it is also included in charged capture cross sections. The implementation of the anti Meyer-Neldel behavior at tail states gives rise to slightly better and reasonable device performances.

Calculation of harmonic losses and ampacity in low-voltage power cables when used for feeding large LED lighting loads

A numerical investigation on the harmonic disturbances in low-voltage cables feeding large LED loads is reported. A frequency domain analysis on several commercially-available LEDs was performed to investigate the signature of the harmonic current injected into the power system. Four-core cables and four single-core cable arrangements (three phases and neutral) of small, medium, and large conductor cross sections, with the neutral conductor cross section approximately equal to the half of the phase conductors, were examined. The cables were modelled by using electromagnetic finite-element analysis software. High harmonic power losses (up to 2.5 times the value corresponding to an undistorted current of the same rms value of the first harmonic of the LED current) were found. A generalized ampacity model was employed for re-rating the cables. It was found that the cross section of the neutral conductor plays an important role in the derating of the cable ampacity due to the presence of a high-level of triplen harmonics in the distorted current. The ampacity of the cables should be derated by about 40 %, almost independent of the conductor cross sections. The calculation have shown that an incoming widespread use of LED lamps in lighting could create significant additional harmonic losses in the supplying low-voltage lines, and thus more severely harmonic emission limits should be defined for LED lamps.

Photostability studies of prebiotic molecules at the VUV region

In this work we report absolute cross section studies of prebiotic molecules measured in the VUV range using the double ion chamber technique and synchrotron radiation. Absorption, ionization and neutral decay cross sections will be presented, together with the absolute ionization quantum yield. Additionally, partial ion yield spectra were measured by a TOF mass spectrometer.

PIC-MCC simulation of electromagnetic wave attenuation in partially ionized plasmas

With the use of measured electron–neutral cross sections, the transmission properties of an electromagnetic (EM) wave in a nitrogen (N2) plasma and a helium (He) plasma are studied by means of PIC-MCC (the particle-in-cell method with collision modeling by the Monte Carlo method) simulation. The plasmas are assumed to be uniform, collisional and non-magnetized. Each type of species presented in the plasmas is treated by the PIC method and the electron–neutral collisions are treated by direct Monte Carlo simulation of particle trajectories. And then the dependence of power attenuation of the EM wave on plasma parameters and wave parameters is obtained and discussed. It is found that power attenuation of the EM wave is strongly affected by the plasma density, species of neutral gas, density of neutral gas and the frequency of the EM wave. Moreover, it is also found that the stopband (passband) of EM wave propagation turns out to be narrower (wider) in collisional plasmas both numerically and analytically.

Single-Photon Multiple Detachment in Fullerene Negative Ions: Absolute Ionization Cross Sections and the Role of the Extra Electron

We have obtained experimental photo-double- and photo-triple-detachment cross sections for the fullerene negative ion using Advanced Light Source photons of 17-90 eV. The cross sections are 2 and 2.5 times larger than those for C60 and appear to be compressed and shifted in photon energy as compared to C60. Our analysis reveals that the additional electron in C60- primarily produces screening which is responsible for the modification of the spectrum. Both screening effects, the shift and the compression, can be quantitatively accounted for by a linear transformation of the energy axis. Applying the transformation allows us to map the neutral and negative ion cross sections onto each other, pointing out the close relationship of correlated few-electron dynamics in neutral and negatively charged extended systems. In contrast, dynamics of neutral and negatively charged atoms or small molecules are typically not closely related.

Near-threshold neutral pion electroproduction at high momentum transfers and generalized form factors

We report the measurement of near threshold neutral pion electroproduction cross sections and the extraction of the associated structure functions on the proton in the kinematic range $Q^2$ from 2 to 4.5 GeV$^2$ and $W$ from 1.08 to 1.16 GeV. These measurements allow us to access the dominant pion-nucleon s-wave multipoles $E_{0+}$ and $S_{0+}$ in the near-threshold region. In the light-cone sum-rule framework (LCSR), these multipoles are related to the generalized form factors $G_1^{\pi^0 p}(Q^2)$ and $G_2^{\pi^0 p}(Q^2)$. The data are compared to these generalized form factors and the results for $G_1^{\pi^0 p}(Q^2)$ are found to be in good agreement with the LCSR predictions, but the level of agreement with $G_2^{\pi^0 p}(Q^2)$ is poor.

MSSM Neutral Higgs Production Cross Section Via Gluon Fusion and Bottom Quark Fusion at NNLO in QCD

MSSM Higgs production cross section in bottom-quark annihilation is evaluated at next-to-next-to-leading order (NNLO) in QCD. Scale dependence for both the factorization and renormalization scales for center of mass energies 7, 8 TeV was found. With the help of computer program HIGLU the neutral MSSM Higgs production cross section via gluon fusion at LHC with center of mass energies 7, 8, 14 TeV including next-to-next-to-leading order QCD corrections is presented. The result for neutral Higgs boson mass is in accordance with the last experimental data received at the LHC.

Search for neutral Higgs bosons decaying to tau pairs in pp collisions at [formula omitted

A search for neutral Higgs bosons decaying to tau pairs at a center-of-mass energy of 7 TeV is performed using a dataset corresponding to an integrated luminosity of 4.6 fb−1 recorded by the CMS experiment at the LHC. The search is sensitive to both the standard model Higgs boson and to the neutral Higgs bosons predicted by the minimal supersymmetric extension of the standard model (MSSM). No excess of events is observed in the tau-pair invariant-mass spectrum. For a standard model Higgs boson in the mass range of 110–145 GeV upper limits at 95% confidence level (CL) on the production cross section are determined. We exclude a Higgs boson with mH=115 GeV with a production cross section 3.2 times of that predicted by the standard model. In the MSSM, upper limits on the neutral Higgs boson production cross section times branching fraction to tau pairs, as a function of the pseudoscalar Higgs boson mass, mA, sets stringent new bounds in the parameter space, excluding at 95% CL values of tanβ as low as 7.1 at mA=160 GeV in the mhmax benchmark scenario.

Self-consistent particle modeling of radio frequency discharge in Ar/O2 mixtures: Effects of crossed electric and magnetic fields and partial pressure

A particle-in-cell/Monte Carlo model is developed to study and analyze the electrical characteristics of the nonequilibrium plasma created by radio frequency (RF) discharge in Ar/O2 mixtures in the presence of crossed electric and magnetic fields. The method of collision treatment is based on an optimized estimation of the free time flight. The needed basic data—more specifically, the ion–neutral cross sections—are determined first. The simulation conditions are 50 mTorr for the total gas pressure and 200 V for the peak of the RF voltage at a frequency of 13.56 MHz. The magnetic field is varied from 0 to 50 G. The effect of the partial pressure ratio of O2 in the mixture and the effect of the magnitude of the magnetic field are discussed. In particular, the results show an increase of the plasma density that is ten times higher in the presence of a magnetic field.

Regge phenomenology of pion photoproduction off the nucleon at forward angles

We present a Regge model for pion photoproduction which is basically free of parameters within the framework of the $s$-channel helicity amplitude. For completeness we take into account axial mesons ${a}_{1}(1260)$, ${b}_{1}(1235)$ and tensor meson ${a}_{2}(1320)$ in addition to the primary $\ensuremath{\pi}+\ensuremath{\rho}$ exchanges for charged pion photoproduction, while the axial meson ${h}_{1}(1170)$ exchange is added to the model of $\ensuremath{\omega}+{\ensuremath{\rho}}^{0}+{b}_{1}$ exchanges for the neutral case. The present model deals for the first time with the ${a}_{2}$ and ${h}_{1}$ Regge poles in the $s$-channel helicity amplitude. For model independence, we use coupling constants of all exchanged mesons determined from empirical decay widths or from the SU(3) relations together with consistency check with existing estimates that are widely accepted in other reaction processes. Based on these coupling constants the simultaneous description of four photoproduction channels is given. Within the Regge regime, $s\ensuremath{\gg}4{M}^{2}$ and $\ensuremath{-}t<2$ GeV${}^{2}$, cross sections and spin polarization asymmetries at various photon energies are analyzed and results are obtained in better agreement with experimental data without referring to any fitting procedure. The model confirms dominance of the nucleon Born term in the sharp rise of the charged pion cross section at very forward angles, while dominance of the $\ensuremath{\omega}$ exchange with the nonsense wrong signature zero leads to the deep dip in the neutral pion cross section. In contrast to existing models, however, our model for the charged pion case shows quite a different production mechanism due to the crucial role of the tensor meson ${a}_{2}$ exchange in the cross section and spin polarization asymmetries. Also the axial meson ${b}_{1}$ exchange is found to give a sizable contribution to the photon polarization asymmetry. In the neutral case, the role of the ${b}_{1}$ is not significant, but the isoscalar ${h}_{1}$ exchange gives an important contribution to the dip-generating mechanism in the photon polarization, showing the isoscalar nature of the process with the $\ensuremath{\omega}$. These findings demonstrate validity of the present model with the prompt use of the tensor meson ${a}_{2}$ and axial meson ${h}_{1}$ for a wider application.

Absolute differential and total cross sections for neutral fragments from dissociative collisions of triatomic hydrogen like ions on He

Neutral fragment products from dissociative collisions of triatomic hydrogen like ions incident on He atoms were studied. Absolute differential and total cross sections are reported here in the energy range of 1.00–5.00 keV and scattering angles between −5.0° and 5.0°. The differential cross sections show decreasing behaviour with a slight structure around 2.0°. The total cross sections for all triatomic molecular ions studied in this work are found to be comparable for the same velocity (E/M). The measured cross sections are between 0.7 × 10−17 cm2 and 0.9 × 10−16 cm2. The present results for the neutral total cross section correlate very well with previously measured total ions cross section for H+3, D+3 and HD+2 on He.

ChemInform Abstract: Photoabsorption, Photoionization, and Neutral Dissociation Cross Sections of Dimethyl Ether and Ethyl Methyl Ether in the Extreme- Ultraviolet Range.

The absolute photoionization and neutral dissociation cross sections of dimethyl ether (CH3OCH3) and ethyl methyl ether (CH3CH2OCH3) in the 52–92 nm range have been obtained by measuring the absolute photoabsorption cross sections and photoionization quantum yields using a multistage ionization chamber and synchrotron radiation. A large deviation in the photoionization quantum yield of both molecules from unity, namely a noticeable enhancement of the neutral fragmentation cross sections, has shown that superexcited states exist densely in this wavelength range. Fluorescence excitation spectra of neutral dissociation fragments produced in the 35–100 nm photon impact on both molecules have also been measured by observing ultraviolet and vacuum ultraviolet emissions. A general appearance of the neutral dissociation cross sections correlates with the fluorescence excitation spectra from H(n=2), CH(A 2Δ), and CH3O(A). The superexcited states of dimethyl ether observed are shown to be purely repulsive states o...

Mutual neutralization in low-energyH++H−collisions: A quantumab initiostudy

The mutual neutralization of ${\mathrm{H}}^{+}$ and ${\mathrm{H}}^{\ensuremath{-}}$ at low collision energies is studied by means of a molecular close-coupling approach. All degrees of freedom are treated at the full quantum level also taking into account the identity of the nuclei. The relevant $^{1}\ensuremath{\Sigma}_{g}^{+}$ and $^{1}\ensuremath{\Sigma}_{u}^{+}$ electronic states as well as the associated nonadiabatic radial couplings are calculated for internuclear distances between 0.5 and $50{a}_{0}$. Following a transformation into a strictly diabatic basis, these quantities enter into a set of coupled equations for the motion of the nuclei. Numerical solution of these equations allows the cross sections for neutralization into the $\mathrm{H}(1)+\mathrm{H}(n)$, $n=1,2,3$ final states to be calculated. In the present paper, results are reported for the collision energy region $0.001--100\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, with special emphasis on the important energy region below $10\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The low temperature rate coefficient is obtained from a parametrization of the calculated cross section and is estimated to be valid over the range $10--10\phantom{\rule{0.2em}{0ex}}000\phantom{\rule{0.3em}{0ex}}\mathrm{K}$.