Scattering of low-energy electrons and positrons by atomic beryllium: Ramsauer-Townsend effect

Total cross sections for the scattering of low-energy electrons and positrons by atomic beryllium in the energy range below the first inelastic thresholds are calculated. A Ramsauer-Townsend minimum is seen in the electron scattering cross sections, while no such effect is found in the case of positron scattering. A minimum total cross section of 0.016 a.u. at 0.0029 eV is observed for the electron case. In the limit of zero energy, the cross sections yield a scattering length of -0.61 a.u. for electron and +13.8 a.u. for positron scattering

Dissociative attachment to rovibrationally excited H2

Using a local-width resonant model, the cross sections for dissociative attachment of low-energy electrons to a rovibrationally excited H_2 molecule in its ground electronic state are obtained. There are 294 such rovibrational levels. Only the contribution of the ^2Σ_u^+ resonant state of H^−_2 to the attachment process is investigated. Assuming a Maxwellian distribution for electron energies, the dissociative attachment cross sections are converted into attachment rates for various rovibrational levels of H_2. A significant enhancement of attachment rates occurs for endoergic reactions only, and the maximum possible rate for attachment to the ground electronic state of H_2 is about 10^(−8) cm^3/sec. Using the same energy distribution for electrons, the average energy carried by the H^− ions is calculated for all possible rovibrational levels. More energetic ions are formed when the attachment process is exoergic, and even the most energetic H^− ions have energies less than 0.5 eV. Furthermore, the attachment rates and the average ion energy appear to depend roughly on the total internal energy and not on the exact fraction of internal energy in rotational or vibrational modes

Rovibrationally enhanced dissociative electron attachment to molecular lithium

We have investigated the role played by initial rovibrational excitation of Li_2 on the cross sections and rates for dissociative electron attachment to the molecule. For a given internal energy, the vibrational excitation enhances the attachment cross section more than the rotational excitation. The attachment cross sections and the attachment rates reach their maximum values when the process of dissociative attachment to rovibrationally excited molecules is still endoergic and, furthermore, these quantities stay close to their maximum values even when the process changes from being endoergic to exoergic. The upper bounds on the cross sections and the rates for dissociative electron attachment to Li_2 are 12.8 A^2 and 1.25×10^(−8) cm^3 s^(−1). At a fixed electron temperature, the kinetic energy of the negative ion formed by this process increases as the vibrational quantum number of the initial neutral molecule increases; the maximum kinetic energy of the Li− ion formed by attachment to the v=12 level of Li_2 is 0.153 eV

Low-energy collisions of D+ with D and He++ with He

Quantum-mechanical calculations for differential cross sections and various transport cross sections describing the thermal-energy collisions of D+ with D and He2+ with He are presented. Lowest-order Viehland-Mason theory is used to calculate mobility of D+ in D. The zero-field mobility at 77, 303, and 10000 K is, in units of cm2V−1s−1, 10.5, 7.0, and 2.1, respectively

Rates of dissociative attachment of electrons to excited H2 and D2

Calculations are reported of the contributions of the lowest 2Σ+ u and 2Σ+ g resonant states to the rates of dissociative attachment of electrons to H2 and D2. For all electron temperatures, the rate is significantly enhanced by vibrational and rotational excitation of the initial molecule. Typically, for an electron temperature of 1.5 eV, the attachment rates for various (v, J) levels are, in cm3 sec−1, 5.4×10−15 for (0,0), 7.2×10−11 for (0,20), and 7.8×10−9 for (8,0), for H2; and 4.5×10−17 for (0,0), 1.4×10−14 for (0,20), and 6.0×10−9 for (11,0), for D2

Contributions of higher partial waves to the elastic scattering amplitude for various long-range interactions

Relevant data is available at: http://www.astronomy.ohio-state.edu/~nahar/nahar_radiativeatomicdata/index.htmlThe contributions of higher partial waves to the elastic scattering amplitude are dominated by long-range interactions which fall off as r^-n as r → ∞. Closed-form expressions for the contributions of higher partial waves (2l > n - 3) to the scattering amplitude for various long-range interactions (n ranging from 3 to 8) are presented.This research has been supported, in part, by the Air Force Office of Scientific Research under Grant No. AFOSR-84-0143

Elastic scattering of positrons and electrons by argon

Relevant data is available at: http://www.astronomy.ohio-state.edu/~nahar/nahar_radiativeatomicdata/index.htmlDifferential and integrated cross sections for the elastic scattering of low- and intermediate-energy (3-300 eV) positrons and electrons by argon atoms are calculated. Higher transport cross sections, representing moments of 1-(cosΘ)^n, for these systems are also obtained for n = 1-4. Model potentials are used to represent the interactions between positrons or electrons and argon atoms. For each impact energy, the phase shifts of the lower partial waves are obtained exactly by numerical integration of the radial equation. The Born approximation is used to obtain the contribution of the higher partial waves to the scattering amplitude. The phase shifts of the seven lowest partial waves are tabulated for various impact energies of positrons and electrons.Support of the National Science Foundation (Grant No. PHY 83-11705) and the Air Force Office of Scientific Research (Grant No. AFOSR-84-0143) is gratefully acknowledged

Relativistic approach for e^± scattering from argon

Relevant data is available at: http://www.astronomy.ohio-state.edu/~nahar/nahar_radiativeatomicdata/index.htmlDifferential and integrated elastic, integrated total cross sections as well as various polarization parameters-the spin polarization P and the parameters T and U describing the change in the polarization vector during scattering-for the scattering of electrons and positrons from argon in the energy range of 3-300 eV are calculated using the relativistic Dirac equation. The real part of the projectile-target interaction is represented by a sum of model potentials. The phase shifts for large angular momenta ħl are calculated using the Born approximation. The relativistic calculations for the differential and integrated elastic cross sections obtained using the pure real potential show almost no improvement over those obtained nonrelativistically for positron scattering from argon while similar calculations show some effects, except at low energies (≤ 5 eV) where relativistic terms are sensitive to the form of potentials used, on the values of the differential cross sections for electron scattering from argon. The polarization parameter P for electron scattering is found to be in good agreement with various calculated and measured values. A few different models of the absorption potential for the inelastic processes are used to calculate the integrated inelastic and the integrated total cross sections for positron and electron scattering from argon. It is noticed that even though the integrated elastic and the integrated total cross sections for the scattering of positrons and electrons calculated using some complex model potential agree well with the corresponding measured values, the differential cross section curves using the same model potentials can differ considerably from each other as well as from the experimental values.Support of Air Force Office of Scientific Research (Grant No. AFOSR-87-0342) is gratefully acknowledged

Elastic scattering of protons from hydrogen atoms at energies 15-200keV

Differential and integrated cross sections for the elastic process H^+ + H(1s) → H^+ + H(1s) were calculated with the use of results of coupled-state calculations in the energy range 15-200 keV. Results are presented and, at 60 keV, compared favorably with preliminary experimental data. The asymptotic form of the elastic amplitude for b≫a_0 (where b is the impact parameter) is derived for the two cases λ≪1 and λ≫1, where λ is the ratio of the collision duration to the orbital period. The asymptotic form for λ≫1 provides a useful test on the numerical accuracy of the amplitudes

Positronium formation from Li and Na atoms by use of pseudopotentials

Relevant data is available at: http://www.astronomy.ohio-state.edu/~nahar/nahar_radiativeatomicdata/index.htmlThe differential and total cross sections for the formation of positronium in its ground state from Li and Na atoms by the impact of intermediate-energy positrons are calculated in the first Born and distorted-wave Born approximations. Hellmann-type pseudopotentials are used to represent the alkali-metal ion cores. The difference in the use of pseudopotentials and the static potential for the core representation for evaluating various rearrangement cross sections is discussed.Support of the National Science Foundation (Grant No. PHY-83-1170S) and the Air Force Office of Scientific Research (Grant No. AFOSR-84-0143) is gratefully acknowledged