A. N. Tripathi

Chemical binding and electron correlation effects in x‐ray and high energy electron scattering

The total, inelastic and elastic intensities for x‐ray and high energy electron scattering from the ten‐electron molecules CH4, NH3, H2O, and HF have been calculated with configuration interaction wave functions. The probability density of the interelectronic distance or the radial intracule density is extracted from the intensities. The importance of basis sets, chemical binding, and electron correlation to the scattered intensities and the radial intracule density has been examined. It has been found that scattered intensities predicted with correlated wave functions agree well with experiment. Based on the electron pair distribution, the exchange and correlation holes in molecular systems have been briefly discussed.

The influence of electron correlation on anisotropic electron intracule and extracule densities

The electron intracule and extracule densities of the neon atom, the hydrogen molecule, and lithium hydride have been studied using both correlated and self‐consistent‐field (SCF) wave functions. For the first time, the effects of electron correlation on the anisotropic intracule and extracule densities are simultaneously displayed and analyzed.

Accurate one-electron momentum-space properties for the lithium isoelectronic sequence

Spin-dependent momentum-space distributions are introduced within the density-matrix formalism and the impulse approximation. The ms components of the spherically averaged momentum densities and isotropic Compton profiles are calculated for the lithium atom and members of its isoelectronic series by using full configuration-interaction wavefunctions that include over 96% of the correlation energy and represent the final stage of a convergent sequence, both in the charge and spin densities in position space. The effects of electron correlation on momentum-space properties of the lithium atom are assessed by comparisons to those obtained from an unrestricted Hartree-Fock wavefunction. Results for the ions B2+, N4+ and F6+ do not support the observation that the sum of information entropies in momentum and position spaces enhances with the quality of the wavefunction. The conjecture that JHF(0)>Jcorr( phi ) is in complete agreement with the results of this study.

Low-energy electron-SiH4 collisions using a parameter-free spherical optical potential

The authors investigate low-energy (0.1-30 eV) electron -SiH4 scattering by employing a parameter-free optical potential which is a sum of three spherical terms, namely the static, exchange and polarisation forces. The authors demonstrate that the qualitative features of the scattering parameters (such as a Ramsauer-Townsend minimum below 1 eV and a shape resonance structure around 2-3 eV) observed in recent experiments, can be very well reproduced in the present simple model. Quantitatively, the authors results (below 10 eV) are very close to the recent close-coupling calculations of Jain and Thompson (1982) and Gianturco et al (1987) using a similar kind of (but non-spherical) model optical potential. Above 10 eV, the present results are in fair agreement with experiments and are better than the earlier calculations.

Electron correlation shifts on pair densities of first-row hydrides

The electron intracule (relative motion r=r1-r2) and extracule (centre of mass R=1/2(r1+r2)) pair densities for the first-row hydrides (LiH-FH) in their ground states have been calculated from both self-consistent-field (SCF) and correlated wavefunctions. For each molecule, the radial and the projected pair densities along (longitudinal) and perpendicular to (transverse) the molecular axis are displayed and analysed as well as the correlation shifts on these pair densities. The effects of electron correlation on the moments of the pair distributions are also discussed. For the cases studied, the pair densities show monotonic changes from LiH to FH, in contrast to the results reported by Sarasola et al. (1990) for the same SCF wavefunctions. This monotonic behaviour may be ascribed to two factors: the electronic structure of the heavier atoms and the chemical bonding in these hydrides.

X-ray and electron scattering intensities of molecules calculated using density functional theory

The elastic and total intensities for x-ray and high-energy electron scattering from the ten-electron hydride series has been calculated from Kohn–Sham orbitals using the BLYP, B3LYP and LSDA functionals, and compared to the previous Hartree–Fock and singles and doubles configuration interaction (SDCI) results of Wang [J. Wang, A. N. Tripathi, and V. H. Smith, Jr., J. Chem. Phys. 101, 4842 (1994)] in the same basis. In those cases where density functional theory (DFT) provides a significantly better electron density than Hartree–Fock, the pair density and hence total scattering intensity for x-rays is also better reproduced, especially in the low s region. The asymptotic behavior of the scattering curves from the DFT methods is poorer than Hartree–Fock due to the inability of DFT to reliably predict the density at the nucleus, the electron–electron distribution at zero-electron separation, and the second moment of the electron–electron distribution.

2 1S excitation of helium: A precise distorted wave approach

The differential and total cross sections are evaluated for 1 1S–2 1S excitation of helium atom by electron impact in distorted wave approximation. The effect of the distortion of incident electron, contribution due to polarization of target and the exchange effect are appropriately taken in both the initial and final channels. Fourier decomposition of interaction potential between the projectile and target is taken and an accurate form factor has been used to calculate the transition matrix. The present results are compared with various distorted wave models as well as other theoretical and experimental results.

Elastic scattering of electrons from Rb, Cs and Fr atoms

Differential, integrated elastic, momentum-transfer and total cross sections as well as differential S, T and U spin parameters for scattering of electrons from rubidium, caesium and francium atoms in the incident energy range up to 300 eV are calculated using a relativistic Dirac equation. The projectile electron–target atom interaction is represented by both real and complex parameter-free optical potentials for obtaining the solution of a Dirac equation for scattered electrons. The Dirac–Fock wavefunctions have been used to represent the Rb, Cs and Fr target atoms. The results of differential cross sections and spin asymmetry parameter S for e-Rb and e-Cs have been compared with the available experimental and theoretical results. Detailed results are reported for the elastic scattering of electrons from the ground states of a francium atom for the first time in the wide range of incident electron energies. The results of electron-Fr elastic scattering show the similar features to those obtained in the case of e-Rb and e-Cs elastic scattering.

Elastic and inelastic scattering of high-energy electrons and X-rays by NH3, CH4 and H2O molecules

A study of elastic and inelastic differential cross sections for high-energy electrons and X-rays has been carried out with a SCF-MO wavefunction obtained in a double-zeta-quality basis of Gaussian contracted functions of Snyder and Basch (1972). The results obtained with this wavefunction were in reasonably good agreement with the recent measurements and other theoretical calculations.

Proton impact excitation for alkali atoms

The excitation cross-sections due to proton impact for the resonance transitions of alkali metal atoms Li, Na and Cs have been calculated using Born approximation. The results have been compared with the other available theoretical calculations.