Cross Sections For He Research Articles

A semiempirical model for low energy electron–atom transport cross sections: The case of noble gases

A semiempirical approach to describe low energy electron–atom transport cross sections of easy implementation and reproduction is presented. The heart of the model is an energy independent two-parameter potential that was adjusted to reproduce the accurate total cross sections for He, Ne, Ar and Kr, measured with a threshold photoelectron source technique from meV up to 20 eV. Once the potential was conceived, the model was validated by comparing the values obtained for the calculated scattering lengths and phase shifts with the respective quantities previously reported in the literature. We close the article by presenting the momentum transfer and viscosity cross sections. Good agreement is found when compared to the similar data obtained from swarm experiments, from phase shifts according to differential cross section measurements and to the cross sections reported by sophisticated ab initio relativistic many-body calculations. Tables for the phase shifts and cross sections are provided for direct use and applications.

Ultraviolet H2 luminescence in molecular clouds induced by cosmic rays

Context. Galactic cosmic rays (CRs) play a crucial role in ionisation, dissociation, and excitation processes within dense cloud regions where UV radiation is absorbed by dust grains and gas species. CRs regulate the abundance of ions and radicals, leading to the formation of more and more complex molecular species, and determine the charge distribution on dust grains. A quantitative analysis of these effects is essential for understanding the dynamical and chemical evolution of star-forming regions. Aims. The CR-induced photon flux has a significant impact on the evolution of the dense molecular medium in its gas and dust components. This study evaluates the flux of UV photons generated by CRs to calculate the photon-induced dissociation and ionisation rates of a vast number of atomic and molecular species, as well as the integrated UV photon flux. Methods. To achieve these goals, we took advantage of recent developments in the determination of the spectra of secondary electrons, in the calculation of state-resolved excitation cross sections of H2 by electron impact, and of photodissociation and photoionisation cross sections. Results. We calculated the H2 level population of each rovibrational level of the X, B, C, B′, D, B″, D′, and a states. We then computed the UV photon spectrum of H2 in its line and continuum components between 72 and 700 nm, with unprecedented accuracy, as a function of the CR spectrum incident on a molecular cloud, the H2 column density, the isomeric H2 composition, and the dust properties. The resulting photodissociation and photoionisation rates are, on average, lower than previous determinations by a factor of about 2, with deviations of up to a factor of 5 for the photodissociation of species such as AlH, C2H2, C2H3, C3H3, LiH, N2, NaCl, NaH, O2+, S2, SiH, l-C4, and l-C5H. A special focus is given to the photoionisation rates of H2, HF, and N2, as well as to the photodissociation of H2, which we find to be orders of magnitude higher than previous estimates. We give parameterisations for both the photorates and the integrated UV photon flux as a function of the CR ionisation rate, which implicitly depends on the H2 column density, as well as the dust properties.

Cross sections in He+ + H collisions for the energy region 1–100 keV u−1

We have calculated the cross sections for the elastic, excitation, electron capture, and ionization processes in He+ + H collision using a semiclassical atomic orbital close-coupling method within two-electron treatment. The calculated data are compared with those of the experiments and previous theoretical calculations. The excitation cross sections to H(n = 2) states are in good agreements with the measurements of Geddes et al. The Balmer-alpha emission cross section of H is about half the size of the experimental data of Donnelly et al. The electron capture cross section is close to the experimental data of Lockwood et al and Olson et al. The ionization cross section is in accord with the experimental data of Hsu et al.

On the Role of Molecular Polarizability in Positron Coupling to Vibrations in Homonuclear Diatomics

In a previous work [Poveda, Varella, and Mohallem (Poveda et al., Atoms, 2021, 9: 64) it was shown that the bell-like shape of the 0 → 1 vibrational excitation cross section of H2 as a function of the incoming positron energy, with its characteristic sharp onset at threshold, can be accounted for by a simple model which couples the positron to the vibrational mode of the molecule, throught the behavior of the target polarizabitity with the internuclear bond distance. The study, carried out via time-dependent wave-packet dynamics propagation, relies on a two-dimensional potential energy surface involving just the scattering (positron-target) and vibrational (target) coordinates. Here the model is extended to the full three-dimensional configuration space of the positron-diatomic complex, with the cross sections computed within a time-independent close-coupling approach. The present results confirm the previous findings, shedding light on the mechanisms through which a low-energy positron couples to the molecular vibrations.

Non-dissociative ionization cross section of the electronically excited H2 and D2 with atomic-limit principal quantum number n[formula omitted] 3 and 4

Electron-impact cross sections for non-dissociative ionization via the ground vibrational level (v=0) of the electronically excited H2 and D2 with atomic-limit principal quantum number n= 3–4, including HH¯1Σg+, B′1Σu+, D1Πu, GK1Σg+, I1Πg, J1Δg, O1Σg+, B′′B¯1Σu+, D′1Πu, P1Σg+, R1Πg, S1Δg, h3Σg+, e3Σu+, d3Πu, g3Σg+, i3Πg, j3Δg, f3Σu+, k3Πu, r3Πg, and s3Δg, are calculated by Gryzinski method in the present study. The cross sections for these states show a significant dependence on the threshold energy. The isotope effect of the cross sections for H2 and D2 is discussed. The computed cross sections of the electronic states with n= 1–4 from threshold up to 10 keV are fitted. The corresponding Maxwellian rate coefficients are derived from those cross sections with electron temperature from 1 to 10 eV.

Study of total, absorption, and 3He and 3H production cross sections in 4He-proton collisions

Light ion breakup cross sections are important for studies of cosmic ray interactions in the inter-stellar medium or radiation protection considerations of energy deposition in shielding and tissues. Abrasion cross sections for heavy ion reactions have been modeled using the Glauber model in the large mass limit or Eikonal form of the optical potential model. Here we formulate an abrasion model for 4He fragmentation on protons using the Glauber model avoiding the large mass limit and include a model for final state interactions. Calculations of energy dependent total, absorption, elastic and breakup cross sections for 4He into 3He or 3H with proton targets are shown to be in good agreement with experiments for energies from 100 to 100,000 MeV/u. The Glauber model for light nuclei with and without a large mass limit approximation is shown to be in fair agreement above 300 MeV/u, however important differences occur at lower energies.

INVESTIGATION OF THE 16O(γ, p)3H3α REACTION AT THE ENERGIES BELOW THE MESON PRODUCTION THRESHOLD

The reaction 16O(γ, p)3H3 induced by bremsstrahlung photons of endpoint energy Emaxγ = 150 MeV has been studied by the method of a diffusion chamber in a magnetic field. The energy dependence of the total cross section has been measured in the energy range from the threshold and has been founded a broad resonance centered at 55 MeV. The rate of decrease in the cross section undergoes a change in the region around 55 MeV. A comparison was made with the cross section for reactions 4He(γ, p)3H and 12С(γ, р)3H2. The agreement between the shapes of distributions for the (γ, p)3H reactions is evident and was concluded that the mechanism of interaction of the γ-quantum with the nucleus is similar. The dependence of average kinetic energy of particles on the total kinetic energy was determined. In the whole energy interval, the distribution for a proton is more than the statistical distribution. Distribution of relative energy of the proton and 3H nucleus in their c.m.s. does not agree with the predictions of the mechanism of photon absorption by an α-particle cluster, but at energies above the maximum, it agrees with calculations within the framework of the quasi-deuteron model of photoabsorption.

Double photoionization of atomic oxygen: Feshbach resonances in the two-electron continuum

We describe a joint experimental and theoretical investigation on oxygen double photoionization—the emission of two electrons from atomic oxygen following single photon absorption. High-resolution experimental measurements were performed at the Advanced Light Source, revealing sharp resonance structure superimposed on the more familiar Wannier-like, nearly-linear background. These resonance features are attributed to ionization-plus-excitation Feshbach resonances embedded in the double ionization continuum, doubly-excited states that lie above the double-ionization threshold. Such features are absent in the double photoionization cross section of He, or other quasi-two-electron systems, for which the doubly-ionized atomic core remains inert. For a corresponding theoretical analysis, the R-matrix with pseudostates (RMPS) method was invoked by calculating final-state, two-electron resonances-plus-continua wavefunctions and corresponding single-photon absorption cross sections. Overall agreement is found in the direct, background double photoionization cross section. However, the RMPS method, using a small basis due to practical computational limitations, was unable to reproduce quantitatively the smooth background or the sharper resonance features observed in the measurements, showing instead large-scale oscillations about the experimental background, and characteristic pseudoresonance jitter, associated with an insufficient convergence of the pseudostate representation to the true two-electron infinite series of Feshbach resonances embedded in the two-electron continuum. The prominent resonance structure observed highlights the need to consider multiple excitation processes in atoms more complex than He or quasi-two-electron systems.

Collisional (de-)excitation of protonated cyanoacetylene (HC3NH+) by helium at low and moderate temperatures

ABSTRACT Protonated cyanoacetylene, HC3NH+, is detected in astrophysical media, where it plays a key role as an intermediate in the chemistries of HCN/HNC and of cyanopolyynes. We first generated a potential energy surface (PES) describing the intermonomer interaction between HC3NH+ and He in Jacobi coordinates using the highly correlated CCSD(T)-F12/aug-cc-pVTZ ab initio methodology. Then, scattering calculations based on an exact close-coupling quantum-scattering technique were done to obtain pure rotational cross-sections for the rotational (de-)excitation of HC3NH+ after collision with He for total energies up to 2500 cm−1. These cross-sections are used to deduce the collision rates in the 5–350 K temperature range for the low-lying rotational levels of HC3NH+ (up to $j\,\, = \,\,15$). In addition, we generated an average PES for the HC3NH+–H2 system. The preliminary results show that the H2($j_{\mathrm{H_2}} = 0$) and He state-to-state de-excitation cross-sections have similar magnitudes, even though the H2 cross-sections are larger by a factor of 2–2.5. This work should help with the accurate derivation of protonated cyanoacetylene abundances in non-local thermodynamical equilibrium astrophysical media. These will put more constraints on the chemical pathways involving the formation and destruction of HC3NH+ while going back to the cyanopolyyne family and more generally those parts of nitrogen-containing molecular chemistry.

The Hidden Ramsauer-Townsend Effect in Positron Scattering by Rare Gas Atoms

The aim of this work is to present and discuss the Ramsauer-Townsend effect in the context of low-energy positron scattering by rare gas atoms. Opposed to what happens in electron-atom scattering, the competition between the static repulsive and polarization attractive potentials makes the phenomenon improbable in positron case. Nonetheless, we show using a semiempirical potential formulation that, while the effect is explicitly observed in the total cross sections for He and Ne, for the heavier noble gases it becomes “hidden” or implicit. We show that as the atomic polarizability increases, the suppression of s-wave goes to higher energies due to the variation of the scattering length with the polarizability. No specific signature for the effect is found in the shape of the differential cross sections but curiously, while the effect is implicit in the total cross section for Ar, it generates a minimum structure in the momentum transfer cross section. The theme is presented for non-specialist audience with emphasis in basic atomic and scattering theories along with practical results, the main objective being to boost the traditional discussions with a new view on the subject.

Formation Cross Sections of He+ Ions in Specific Electronic States in Collisions of He2+ Ions with Hydrogen Atoms

Formation Cross Sections of He+ Ions in Specific Electronic States in Collisions of He2+ Ions with Hydrogen Atoms

Modification of the Endohedral Potential after Instant Ionization of an Inner Atom

We investigate here the variation of endohedral A@CN potential due to instant ionization of an inner atom A. Using a reasonable model to describe the fullerenes shell, we managed to calculate this variation that is a consequence of the monopole polarization of CN shell. We demonstrate that the phenomenological potentials that properly simulate the CN shell should belong to a family of potentials with a non-flat bottom, instead of very often used square well potential. As concrete example, we use the Lorentz-bubble model potential. By varying the shape of this potential, we describe the various degrees of the monopole polarization of the CN shell by positive electric charge at the center of the shell. We calculated the photoionization cross sections of He, Ar, and Xe atoms located at the center of C60 shell with and without taking into account accompanying this process monopole polarization of the fullerenes shell. Unexpectedly, we found that the monopole polarization do not affect the photoionization cross sections of these endohedral atoms, thus presenting a very seldom example of potential conspiracy.

Absolute cross sections for He+ impact single ionization of noble gases

Absolute cross sections for He+ impact single ionization of noble gases

Photoionization of H2 using the molecular R-matrix with time approach

SynopsisWe present results of the first calculations using the variational ab initio molecular R-matrix with time approach. We have calculated two and four-photon ionization cross sections for H2 and studied the effects of electron correlation and choice of the Gaussian atomic basis sets. Our results are compared with earlier calculations.

PSII-12 Comparison of fine needle aspiration and tissue sections to determine Sertoli and germ cell counts in prepubertal beef bulls

Abstract Daily sperm production in bulls is determined by the size of the testicular Sertoli cell (SC) population established before puberty. Fine needle aspiration (FNA) is currently used as a diagnostic tool, but its value to determine SC and germ cell (GC) counts remains unexplored. The objectives were to compare three sampling techniques [(smears produced via 22G fine needle aspiration (SMR) vs. tissue sections collected via 14G aspiration (FNS) vs. conventional tissue sections (HIS)] and two stains [immunohistochemistry with GATA4 (IHC) and HE] to determine SC density in prepubertal bulls. Fourteen age-matched Angus bulls were surgically castrated. Testicular parenchyma samples and smears were stained using anti-GATA4 as a specific SC marker and HE. For stain comparison, mirrored tissue cuts were produced allowing cell counts of complementary seminiferous tubule (ST) cross sections. All data were analyzed using Pearson’s product moment correlations and linear regression. Testicular weights were correlated with IHC SC density (P = 0.018). High correlations existed for SC and GC counts done on ST cross sections using IHC (P < .0001) and HE (P < .0001). Within HIS and FNS, high correlations (r = 0.955; P ≤ 0.0001) existed between SC and automated GATA4+ cell densities. However, when comparing IHC SC density across techniques, no significant correlations (P ≤ 0.587) existed. The high correlation observed between SC counts done on mirrored ST cross sections for HE and IHC underlines the value of GATA4 as a specific SC marker in prepubertal bulls. Furthermore, the high correlation established between SC and GATA4+ cell densities within IHC HIS and FNS collection methods infers SC density determination can be automated. However, lack of correlation for SC density among techniques indicates that aspiration and conventional methods lead to different results. Research supported by North Dakota EPSCoR Doctoral Dissertation Assistantship (DDA) program.

Distortion of the Ionization Cross Section of He by the Coherence Properties of a C6+ Beam

We analyze the influence of the coherence of the projectile’s beam in scattering phenomena. We focus our study in the ionization of He by C 6 + projectiles at 100 MeV/amu. We assess the influence of this effect by performing a Born initial state and continuum distorted wave final state (CDW-B1) calculation together with a rigorous procedure to account for the initial coherence properties of the projectile’s beam. These calculations, which had been previously performed for only the scattering and perpendicular collision planes and within the First Born approximation (FBA), were repeated for an ampler set of collision planes. Additionally, a more refined method to describe the applicability of the aforementioned procedure, is used. We achieve a better qualitative agreement with the experimental results.

Restricted basis set coupled-channel calculations on atom-molecule collisions in magnetic fields.

Rigorous coupled-channel quantum scattering calculations on molecular collisions in external fields are computationally demanding due to the need to account for a large number of coupled channels and multiple total angular momenta J of the collision complex. We show that by restricting the total angular momentum basis to include only the states with helicities K ≤ Kmax, it is possible to obtain accurate elastic and inelastic cross sections for low-temperature He + CaH, Li + CaH, and Li + SrOH collisions in the presence of an external magnetic field at a small fraction of the computational cost of the full coupled-channel calculations (where K is the projection of the molecular rotational angular momentum on the atom-diatom axis). The optimal size of the truncated helicity basis set depends on the mechanism of the inelastic process and on the magnitude of the external magnetic field, with the minimal basis set (Kmax = 0) producing quantitatively accurate results for, e.g., ultracold Li + CaH and Li + SrOH scattering at low magnetic fields, leading to nearly 90-fold gain in computational efficiency. Larger basis sets are required to accurately describe the resonance structure in the magnetic field dependence of Li + CaH and Li + SrOH inelastic cross sections in the few partial wave-regime as well as indirect spin relaxation in He + CaH collisions. Our calculations indicate that the resonance structure is due to an interplay of the spin-rotation and Coriolis couplings between the basis states of different K and the couplings between the rotational states of the same K induced by the anisotropy of the interaction potential.

Supernova Neutrino Process of Li and B Revisited

Abstract We reinvestigate effects of neutrino oscillations on the production of 7Li and 11B in core-collapse supernovae (SNe). During the propagation of neutrinos from the proto–neutron star, their flavors change, and the neutrino reaction rates for spallation of 12C and 4He are affected. In this work, corrected neutrino spallation cross sections for 4He and 12C are adopted. Initial abundances involving heavy s-nuclei and other physical conditions are derived in a new calculation of the SN 1987A progenitor in which the effects of the progenitor metallicity are included. A dependence of the SN nucleosynthesis and final yields of 7Li and 11B on the neutrino mass hierarchy are shown in several stellar locations. In the normal hierarchy case, the charged-current (CC) reaction rates of are enhanced, and yields of proton-rich nuclei, along with 7Be and 11C, are increased. In the inverted hierarchy case, the CC reaction rates of are enhanced, and yields of neutron-rich nuclei, along with 7Li and 11B, are increased. We find that variation of the metallicity modifies the yields of 7Li, 7Be, 11B, and 11C. This effect is caused by changes in the neutron abundance during SN nucleosynthesis. Therefore, accurate calculations of Li and B production in SNe should take into account the metallicity of progenitor stars.

Absolute total cross sections for electron-impact double ionization of He(1s2s3S) and He−(1s2s2p4P)

We report on the experimental determination of absolute, total cross sections for electron-impact double ionization of helium in its metastable 1s 2s 3 S state and of the helium negative ion He− (1s 2s 2p 4 P). Cross sections are measured for impact energies ranging from threshold to 1000 eV using the animated crossed beams technique. The fast, pure beam of metastable helium required for the experiment is produced by photodetachment of fast He− ions. Results for He(1s 2s 3 S) are compared to those for He(1s 2 1 S), and exhibit similar magnitude and behaviour. The cross section for He− is large compared to other negative ions and indicates the predominance of indirect ionization channels at large impact energies.

Electron Transitions during the Capture of an Electron by a He2+ Ion at the Argon Atom

We have measured the absolute values of the total cross section of the one-electron capture by He2+ ions in the kinetic energy range 2–30 keV at the Ar atoms. The absolute values of the differential scattering cross sections of He+ ions formed during the one-electron capture and the electron capture with ionization at energies of 2.2, 5.4, and 30 keV have been determined. The electronic states of the formed ions have been determined using collision spectroscopy based on analysis of the change in the kinetic energy of He+ after the interaction. We have measured doubly differential (with respect to the kinetic energy and the scattering angle) cross sections of the formation of free electrons. The free electron formation channels (direct ionization and electron capture with ionization) have been analyzed by calculating the electron terms of the (HeAr)2+ system. The calculated cross section of capture with ionization is in conformity with the cross section measured using collision spectroscopy.