Cross Sections For He Research Articles

Electron impact double ionization cross sections of light elements

A simple user-friendly semiempirical model is proposed to calculate electron impact double ionization cross sections of He, Li, Li+, B+, C+, C3+, O, O2+, O3+, Ne, Ne+, Ne2+, Na, Mg, Al3+, S, and Arq+ (q=0–7) targets for the incident electron energies from threshold to 106 eV. The contributions in the total double ionization cross sections from the direct double ionization and inner-shell ionization processes are taken into account on the basis of experimental data considered. The results of the present analysis are compared with the available experimental data and theoretical calculations. The model is found successful for the description of experimental cross sections. Since, this model may be a prudent selection to meet the demand level in plasma modeling due to its simple inherent structure.

4He-induced L X-ray production cross sections in Pt and Bi

L shell X-ray production cross sections for 4He on Pt and Bi are measured at 2.0, 2.3 and 3.0 MeV. Good agreement is found with the available data of Balsamo et al. [A. Balsamo, N. De Cesare, F. Murolo, E. Perillo, G. Spadaccini, M. Vigilante, J. Phys. B: Atom. Mol. Opt. Phys. 32 (1999) 5699]. The results are compared with those of theoretical calculations using the ECPSSR model [W. Brandt, G. Lapicki, Phys. Rev. A 23 (1981) 1717]. The difference already observed at low incident ion energy between ECPSSR calculations and measured data for the L β and L γ lines clearly appears in this work.

Comparative analysis of Herman‐Wallis factors for uses in coherent anti‐Stokes Raman spectra of light molecules

AbstractThe cross section of Raman processes involving light molecules takes into account the unavoidable coupling between rotational and vibrational modes. This coupling is quantified by the Herman‐Wallis (HW) factor and its use was recently criticized in view of the known thermometric application of vibrational spectra of coherent antiStokes Raman scattering (CARS) of hydrogen. Following this line of research, this paper addresses the physical limitations of the HW factors employed ordinarily in Q‐branch CARS measurements. Furthermore, new determinations of the vibration–rotation coupling are considered. These are based on a more realistic scenario consisting of anharmonic oscillations and polarizability variations during vibration. Numerical examples for fundamental diatomics, namely H2 and N2, are given throughout the text. It is found that, as expected, the Raman cross section of H2 is strongly dependent on the HW factor and the consequent correction to CARS line intensities is of the order of 10% or greater. Surprisingly, the analysis suggests that N2 spectral data are also affected, to a certain extent, by the new HW factors. Usually, this correction is completely disregarded, but under the new understanding of the vibration–rotation coupling, Q‐branch CARS lines of N2 are predicted to undergo variations between 1 and 5% for rotational levels that are sufficiently populated at temperatures of interest for CARS studies. Oxygen is also prone to vibration–rotation corrections and is finally shown as an example between the two limits of hydrogen and nitrogen. Copyright © 2009 John Wiley & Sons, Ltd.

Differential Cross Sections for the Ionization of OrientedH2Molecules by Electron Impact

A nonperturbative close-coupling technique is used to calculate differential cross sections for the electron-impact ionization of H2 at an energy of 35.4 eV. Our approach allows cross sections for any orientation of the molecule with respect to the incident electron beam to be analyzed. New features in the resulting cross sections are found compared with the case where the molecular orientation is averaged, and also with cross sections for He at equivalent electron kinematics. When averaged over all possible molecular orientations, good agreement is found with recent experimental results.

Photoionization cross sections of H2+ and H2with complex Gaussian-type basis functions optimized for the frequency-dependent polarizabilities

Within the framework of the complex basis function method, the photoionization cross sections of H(2)(+) and H(2) were calculated based on the variational principle for the frequency-dependent polarizabilities. In these calculations, complex orbital exponents of Gaussian-type basis functions for the final state continuum wavefunctions were fully optimized for each photon energy with the numerical Newton-Raphson method. In most cases, the use of only one or two complex Gaussian-type basis functions was enough to obtain excellent agreement with previous high precision calculations and available experimental results. However, there were a few cases, in which the use of complex basis functions having various angular momentum quantum numbers was crucial to obtain the accurate results. The behavior of the complex orbital exponents as a function of photon energy was discussed in relation to the scaling relation and the effective charge for photoelectron. The success of this method implies the effectiveness of the optimization of orbital exponents to reduce the number of basis functions and shows the possibility to calculate photoionization cross sections of general molecules using only Gaussian-type basis functions.

Neutrino‐Nucleus Reaction Cross Sections for Light Element Synthesis in Supernova Explosions

The neutrino-nucleus reaction cross sections of 4He and 12C are evaluated using new shell model Hamiltonians. Branching ratios of various decay channels are calculated to evaluate the yields of Li, Be, and B produced through the ν-process in supernova explosions. The new cross sections enhance the yields of 7Li and 11B produced during the supernova explosion of a 16.2 M☉ star model compared to the case using the conventional cross sections by about 10%. On the other hand, the yield of 10B decreases by a factor of 2. The yields of 6Li,9Be, and the radioactive nucleus 10Be are found at a level of ~10−11 M☉. The temperature of νμ ,τ- and -neutrinos inferred from the supernova contribution of 11B in Galactic chemical evolution models is constrained to the 4.3-6.5 MeV range. The increase in the 7Li and 11B yields due to neutrino oscillations is demonstrated with the new cross sections.

Simultaneous ionization of both collision partners in the strong perturbative energy range(20–500keV∕amu)

Recently, we presented a classical-over-barrier ionization (COBI) model [J. X. Shao et al., Phys. Rev. A 77, 042711 (2008)] to calculate the pure single- and double-ionization cross sections of He. We extend the COBI model to calculate the simultaneous ionization of the projectile and target in the energy range of $20--500\phantom{\rule{0.3em}{0ex}}\mathrm{keV}∕\mathrm{amu}$. The results of the model are in excellent agreement with the experimental data. The mechanism of the simultaneous ionization is also discussed.

Signature of Ericson fluctuations in helium inelastic scattering cross sections near the double ionization threshold

We calculated the inelastic electron impact excitation cross sections of He+ by electrons for a model helium atom to examine the onset of the signature of quantum chaotic scattering in this simple system. We find Ericson fluctuations EF in the calculated inelastic scattering cross sections only when the impact energies lie within about 0.21 eV below the double ionization threshold. We also discuss the stringent requirements and the proper methods for analyzing the inelastic scattering cross sections in order to observe EF experimentally.

Polarization effect and post-collisional interaction in (e, 2e) reaction process for He and Ar in coplanar asymmetric geometry

The (e, 2e) triple differential cross sections for He(1s2)，Ar(3p6)and Ar(2p6)have been calculated using the modified distorted wave Born approximation (DWBA) in coplanar asymmetric geometry. The kinematics employs large energy transfer and is close to minimum momentum transfer. The theoretical results have been compared with those of the experiment, the Brauner, Briggs and Klar (BBK) method and the standard DWBA calculation. It is shown that the polarization and post-collisional interaction effects are very important in coplanar asymmetric (e, 2e)reaction with special kinematics for He(1s2), Ar(3p6) and Ar(2p6).

The evolution of the triple differential cross sections for the double photoionization of He and H2

We use the time-dependent close-coupling method to explore the triple differential cross sections produced by double photoionization of He and H2. Recent extensions of our technique have allowed extraction of the triple differential cross section as the double photoionization process evolves, allowing further exploration of the underlying mechanisms. We demonstrate that the angular distributions of the two outgoing electrons typically reach a converged shape after 5 or 6 field periods. We also further compare our method with recent experimental measurements, as well as demonstrating some similarities in the triple differential cross sections for He and for H2 at certain molecular orientations.

Two center Electron Emission in fast Collisions of Bare C and F Ions with He and H2 and CDW-EIS Mode

We report the energy and angular distributions of the electron double differential cross sections (DDCS) for two collision systems: 6 MeV/u C6+ on H2 and 4 MeV/u F9+ ions on He. The electrons having energies between 1 and 500 eV are detected at about ten different emission angles between 30° and 150°. The measured data is compared with the state-of-the art continuum distorted wave-eikonal initial state (CDW-EIS) and the first Born (B1) models. In case of molecular H2 target a molecular wave function as been used for the calculations of the cross section of H2. A comparative study has been presented for the spectral shape for the atomic (He) and molecular (H2) target.

The 4He total photo-absorption cross section with two- plus three-nucleon interactions from chiral effective field theory

The total photo-absorption cross section of 4He is evaluated microscopically using two- (NN) and three-nucleon (NNN) interactions based upon chiral effective field theory (χEFT). The calculation is performed using the Lorentz integral transform method along with the ab initio no-core shell model approach. An important feature of the present study is the consistency of the NN and NNN interactions and also, through the Siegert theorem, of the two- and three-body current operators. This is due to the application of the χEFT framework. The inclusion of the NNN interaction produces a suppression of the low-energy peak and enhancement of the high-energy tail of the cross section. We compare to calculations obtained using other interactions and to representative experiments. The rather confused experimental situation in the giant resonance region prevents discrimination among different interaction models.

Photodetachment of a Negative Hydrogen Molecular Ion near anInterface

Photodetachment of a negative hydrogen molecular ion near an interfaceis studied by using the two-centre model and the closed orbit theory.The calculation results show that the photodetachment cross section isrelated to the distance between the two centres in theH2− and different molecular ion-interface distances. Thecomparison between the cross section of H2− near aninterface with the section of H− shows that at the equilibriumdistance of two centres and at low photon energy, the photodetachmentcross section of H2− is about twice the cross section ofH−, which shows that the interference of the two nuclei is verystrong; when the distance between the two centres is large, the sectionof H2− is almost the same as the cross section of H− nearone interface, which indicates that the interference effect of the twocentres vanishes.

ColdHe+H2collisions near dissociation

Cross sections for He+H{sub 2} collisions are reported for rovibrational states near dissociation at translational energies less than 1000 cm{sup -1}. In contrast to our previously reported cross sections for lower-lying initial rotational states, the excited rotational states near dissociation give rise to shape resonances for energies between 0.001 and 1 cm{sup -1}. The emergence of these resonances with increasing rotational level is opposite to the trend found for He+CO where the shape resonance strength decreases with rotational level for low-lying states. Quasiresonant energy transfer and low energy excitation thresholds are also discussed and it is found that there are 11 rovibrationally excited states that are stable against collision at ultracold temperatures.

Partial wave scattering cross sections for He–HBr collision

A new anisotropic potential is fitted to ab initio data. The close-coupling approach is utilized to calculate state-to-state rotational excitation partial wave cross sections for elastic and inelastic collisions of He atom with HBr molecule based on the fitted potential. The calculation is performed separately at the incident energies: 75, 100 and 200 meV.The tendency of the elastic and inelastic rotational excitation partial wave cross sections varying with total angular quantum number J is obtained.

The study of state-selected ion-molecule reactions using the vacuum ultraviolet pulsed field ionization-photoion technique

This paper presents the methodology to generate beams of ions in single quantum states for bimolecular ion-molecule reaction dynamics studies using pulsed field ionization (PFI) of atoms or molecules in high-n Rydberg states produced by vacuum ultraviolet (VUV) synchrotron or laser photoexcitation. Employing the pseudocontinuum high-resolution VUV synchrotron radiation at the Advanced Light Source as the photoionization source, PFI photoions (PFI-PIs) in selected rovibrational states have been generated for ion-molecule reaction studies using a fast-ion gate to pass the PFI-PIs at a fixed delay with respect to the detection of the PFI photoelectrons (PFI-PEs). The fast ion gate provided by a novel interleaved comb wire gate lens is the key for achieving the optimal signal-to-noise ratio in state-selected ion-molecule collision studies using the VUV synchrotron based PFI-PE secondary ion coincidence (PFI-PESICO) method. The most recent development of the VUV laser PFI-PI scheme for state-selected ion-molecule collision studies is also described. Absolute integral cross sections for state-selected H2+ ions ranging from v+ = 0 to 17 in collisions with Ar, Ne, and He at controlled translational energies have been obtained by employing the VUV synchrotron based PFI-PESICO scheme. The comparison between PFI-PESICO cross sections for the H2+(HD+)+Ne and H2+(HD+)+He proton-transfer reactions and theoretical cross sections based on quasiclassical trajectory (QCT) calculations and three-dimensional quantum scattering calculations performed on the most recently available ab initio potential energy surfaces is highlighted. In both reaction systems, quantum scattering resonances enhance the integral cross sections significantly above QCT predictions at low translational and vibrational energies. At higher energies, the agreement between experiment and quasiclassical theory is very good. The profile and magnitude of the kinetic energy dependence of the absolute integral cross sections for the H2+(v+ = 0-2,N+ = 1)+He proton-transfer reaction unambiguously show that the inclusion of Coriolis coupling is important in quantum dynamics scattering calculations of ion-molecule collisions.

State-to-state quantum reactive scattering for four-atom chemical reactions: Differential cross section for the H+H2O→H2+OH abstraction reaction

The time-dependent wave packet method was extended to calculate the state-to-state differential cross section for the title four-atom abstraction reaction with H2O in the ground rovibrational state. One spectator OH bond length was fixed in the study, but the remaining five degrees of freedom were treated exactly. It was found that (a) the differential cross section changes from being strongly backward peaked at low collision energy to sideward scattering at E = 1.4 eV, and (b) the rotational state-resolved differential cross section for H2 differs substantially from that for OH.

On mechanisms of He I collisional excitation in He-He system

The present work is devoted to study of He I excitation in low energy He-He collisions. The purpose of this work is the investigation of mechanisms of atomic levels excitation at collision energies, corresponding the conditions of adiabatic approximation. The results of the experimental investigation of the excitation cross sections of He I 31P, 33P, 31D, 33D levels in the collision energy range from threshold up to 500 eV (c.m. system) and degree of polarization for 21S-31P, 23S-33P, 21P-31D and 23P-33D transitions in this energy range are represented. Also the degree of polarization for 21P-n 1D transitions as function of the main quantum number (3 ≤ n ≤ 12) at the fixed collision energy is represented.

Test ofJ-matrix inverse scattering potentials on electromagnetic reactions of few-nucleon systems

The J-matrix inverse scattering nucleon-nucleon potentials (JISP), describing both two-nucleon data and bound and resonant states of light nuclei to high accuracy, are tested on the total photoabsorption cross sections of Deuteron, Triton, 3He and 4He. The calculations in the three- and four-body systems are carried out via the Lorentz integral transform method and the hyperspherical harmonics (HH) technique. To this end the HH formalism has been adapted to accommodate non-local potentials. The cross sections calculated with the JISP are compared to those obtained with more traditional realistic interactions, which include two- and three-nucleon forces. While the results of the two kinds of potential models do not differ significantly at lower energies, beyond the resonance peak they show fairly large discrepancies, which increase with the nuclear mass. We argue that these discrepancies may be due to a probably incorrect long range behavior of the JISP, since the one pion exchange is not manifestly implemented there.

State-to-state cross sections for H2 and its isotopic variants

State-resolved cross sections for electron–H2 and H–H2 collision-induced processes have been calculated, using semiclassical and quasiclassical approaches, respectively. Corresponding results for the deuterium system are presented, introducing mass-scaling relations.