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

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

A numerical study of time-dependent schrödinger equation for multiphoton vibrational interaction of no molecule, modelled as morse oscillator, with intense far-infrared femtosecond lasers

For the NO molecule, modelled as a Morse oscillator, time-dependent (TD) nuclear Schrödinger equation has been numerically solved for the multiphoton vibrational dynamics of the molecule under a far-infrared laser of wavelength 10503 nm, and four different intensities, I = 1 × 108, 1 × 1013, 5 × 1016, and 5 × 1018 W cm-2 respectively. Starting from the vibrational ground state at zero time, various TD quantities such as the norm, dissociation probability, potential energy curve and dipole moment are examined. Rich high-harmonics generation (HHG) spectra and above-threshold dissociation (ATD) spectra, due to the multiphoton interaction of vibrational motions with the laser field, and consequent elevation to the vibrational continuum, have been obtained and analysed

Large momentum transfer limit of some matrix elements

The matrix element εfi(K), or ε, that appears in the study of elastic and inelastic electron-atom scattering from an initial state i to a final state f in the first Born approximation depends explicitly on the momentum transfer ℏK⃗ . The uncertainty in the value of the calculated cross sections arises not only from the application of the Born approximation but also from the approximate nature of the wave functions used. For the 1 S1−2 P1 transition in helium, we present an analytic expression in terms of the 1 S1 and 2 P1 wave functions for the leading coefficient C1 in the asymptotic expansion of ε as a power series in 1K; C1 is defined by ε∼C1K5 as K∼∞. An accurate numerical value of C1 is obtained by using a sequence of better and better 1 S1 and 2 P1 wave functions. An accurate value of C1 can be useful in obtaining an approximate analytic form for the matrix element. We also present analytic expressions, in terms of the 1 S1 wave function, for the coefficients of the two leading terms of ε for the diagonal case, that is, for the atomic form factor, and we obtain accurate estimates of those coefficients. The procedure is easily generalizable to other matrix elements of helium, but it would be difficult in practice to apply the procedure to matrix elements of other atoms. We also give a very simple approximate result, valid for a number of matrix elements of heavy atoms, for the ratios of the coefficients of successive terms (in the asymptotically high-K domain) in a power series in 1K. Finally, we plot ε for 1 S1 to 1 S1 and for 1 S1 to 2 P1, with the known low-K and high-K dependence extracted. One might hope that each plot would show little variation, but the 1 S1 to 1 S1 plot varies considerably as one goes to high K, and the 1 S1 to 2 P1 plot shows a very rapid variation for K∼∞, strongly suggesting that at least one element of physics —perhaps a pole outside of but close to the domain of convergence—has been omitted

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

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