L A Morgan

GTOBAS: fitting continuum functions with Gaussian-type orbitals☆

GTOBAS is a program for fitting Gaussian-type orbitals (GTOs) to Bessel and Coulomb functions over a finite range. The exponents of the GTOs are optimized using the method of Nestmann and Peyerimhoff [J. Phys. B 23 (1990) L773]. The appended module NUMCBAS provides the numerical Bessel and Coulomb functions required as input for the program. The use of GTO continuum basis sets is particularly important in electron–molecule scattering calculations when polyatomic targets are involved. Sample results for such calculations are also discussed.

The UK molecular R-matrix codes

Abstract The R-matrix method has been used with great success in recent years to model low energy scattering of electrons and positrons by molecules. The codes, developed by a consortium of UK research groups as part of the Collaborative Computational Project 2 (CCP2), have now reached a high of robustness and stability. Their overall design is described in this paper.

R-matrix calculations for polyatomic molecules: electron scattering by

A new computer program has been developed which allows us to use the R-matrix method to study electron scattering by polyatomic molecules. Our first application is to scattering by in its linear, equilibrium geometry for energies up to 10 eV. We confirm the earlier assignment of symmetry to the resonance near 2 eV but we are unable to locate any resonance having symmetry in this energy range. We present integral and differential cross sections which are generally in excellent agreement with experiment.

Electron impact excitation cross sections for CO

The R-matrix method has been used to calculate electron impact excitation cross sections for the lowest seven electronically excited states of CO in the energy range 6-18 eV. These states are represented using configuration interaction (CI) expansions and an algorithm for treating long CI expansions in scattering calculations is presented. The calculations are carried out for a range of internuclear distances 1.8<or=R<or=3.0 a0. Several new resonance structures are identified. Good agreement is obtained with the experimental results of Furlong and Newell (1993) for the excitation of the a 3 Pi state.

Electron impact dissociative excitation of water within the adiabatic nuclei approximation

The R-matrix method is used to calculate dissociative excitation cross sections for the four lowest-lying electronically excited states of H2O in the energy range 5-15 eV. For the first time calculations are performed taking into account the nuclear motion by means of an adaptation of the adiabatic nuclei approximation. Cross sections are compared with previous and new fixed-nuclei results and also experiments. Resonance positions and widths are calculated for different geometries of the water molecule.

ELECTRON IMPACT EXCITATION OF WATER

The R-matrix method is used to calculate electron scattering cross sections for the excitation of four low-lying electronically excited states of the water molecule. These states, of symmetry , , and , have vertical excitation energies in the range 7-10 eV and are known to be dissociative. Resonances, corresponding to dissociative states of the anion, are found in the , and symmetries at energies in excellent agreement with the results of dissociative attachment experiments. Integrated cross sections are given for scattering energies up to 20 eV

Electronic excitation of molecular hydrogen using the R-matrix method

Calculations are presented for electron-H2 collisions for energies up to 25 eV. Excitation from the ground, X 1 Sigma g+, to the lowest six excited electronic states, b 3 Sigma u+, a 3 Sigma g+, c 3 Pi u, B 1 Sigma u+, E, F 1 Sigma g+ and C 1 Pi u, is explicitly considered for all total symmetries up to and including 2 Phi g. The target states are represented using a full configuration interaction treatment within a basis of Slater-type orbitals optimized to give accurate vertical excitation energies. Results are presented for eigenphase sums, in which the resonant and other features are analysed, as well as total electronic excitation cross sections. Extensive comparison is made with the available experimental data. In contrast to many previous ab initio calculations the results are in good agreement with the observed electronic excitation cross sections for all the processes considered.

Bound states using the R-matrix method: Rydberg states of HeH

A method is presented for adapting scattering calculations performed with the molecular R-matrix method to find bound states based on the atomic method of Seaton. Quantum defect theory is used to determine initial energy grids and to determine whether all the bound states have been located. This method is particularly suited to the Rydberg states of electron plus molecular ion systems. The authors calculate and assign the lowest 33 electronic states of the HeH molecule. Previously on 14 of the lowest (n<5) bound states have been fully characterized, with several states omitted. They suggest that the omitted states give rise to some of the observed but previously unexplained weak transitions. Vibrational motion is included in their calculations within the adiabatic approximations. Effects arising from short-range correlations and nuclear motion are shown to be very significant for the lowest electronic states. Transition energies amongst the excited states agree with accurate spectroscopic determinations to better than 50 cm.

Differential cross sections for electronic excitation of molecular hydrogen using the R-matrix method

Differential cross sections are calculated for electron-H2 collisions for scattering energies up to 20 eV. Elastic scattering and excitation from the ground, X 1 Sigma g+, to the lowest six excited electronic states, b3 Sigma u+, a3 Sigma g+, c3 Pi u, B1 Sigma u+, E, F 1 Sigma g+ and C 1 Pi u, is explicitly considered for all total symmetries up to and including 2 Phi g for a single fixed H2 geometry. The target states are represented using a full configuration interaction treatment within a basis of Slater-type orbitals optimized to give accurate vertical excitation energies. Results are presented for both resonant and non-resonant differential cross sections. Comparison is made with the available experimental and theoretical data. Excellent agreement is obtained for elastic differential cross sections; agreement for the inelastic differential cross sections is only moderate. Possible improvements to these calculations are discussed.

Electron impact excitation of H and He+. I. 1s to ns transitions

A polarized orbital distorted wave model is used to calculate total and differential cross sections for 1s to ns(n=2,3,4,5) electron impact induced transitions in H and He+ from threshold to 20 Ryd. The total cross sections for the 1s to 2s transitions are in good agreement with experiment, but cannot in this model show resonances. The differential cross sections at energies above 100 eV agree closely with those of Geltman and Hidalgo (1971). The model reduces to the Born-Oppenheimer (z=0) or CBO I(z=0) when the polarization and static potentials are omitted.