Baljit K. Sarpal

Study of electron scattering by using the polyatomic R-matrix method

We present results of ab initio calculations for electron - scattering using the polyatomic R-matrix method. We consider scattering energies from 0 to 20 eV. Our calculations demonstrate that the low-energy shape resonance just above 2 eV is due to a state of the negative ion. A broad resonance is also found at higher energy ( eV). Calculations are reported for static exchange, static exchange with polarization, and correlated wavefunction levels of approximation. Our calculations are the first to use highly correlated wavefunctions in R-matrix calculations for polyatomic targets, where such a treatment is necessary to get an accurate description of the resonant scattering process. The calculated integral elastic scattering cross section is found to be in good agreement with experimental results. .

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.

On the calculation of electron-impact rotational excitation cross sections for molecular ions

Methods of computing rotational excitation cross sections for electron collisions with diatomic molecular ions are examined for impact energies up to 5 eV. The HeH and NO ions are used as test cases and calculations are performed at various levels of approximation. Previous studies have all used the Coulomb-Born approximation assuming only dipolar potentials. This approximation is found to be unreliable in a number of aspects: short-range and threshold effects are important, and the widely made assumption that only processes need to be considered is particularly questionable. Conversely, full inclusion of vibrational motion is found to be less important.

Electronic transitions in the HeH molecule: a MQDT approach

The authors make use of multichannel quantum defect theory (MQDT) to calculate the eigenvectors of the bound states of HeH. By making use of expressions for Coulomb transition dipoles and simple angular momentum algebra they calculate transition probabilities for several states of HeH. These results are in general agreement with previous ab initio determinations where available but also show considerable structure in dipole moments arising from mixing of states near avoided crossings.

Energies and angular characteristics of the resonance important for Penning and associative ionization in He(23S)-H collisions

We report R-matrix calculations for energies and width of the lowest 2 Sigma resonance important for Penning and associative ionization in He-H collisions. The ab initio determination of the angular distribution of the He (23S)-H (12S) resonance is reported. This resonance is anisotropic with considerable d character at small internuclear separation, whereas previously it had been assumed to have an isotropic distribution.

R-Matrix Calculations of Inelastic Electron Scattering by Diatomic Molecules

The R-matrix method1 has been used extensively in recent years to obtain high quality cross sections for low energy electron scattering by atomic targets. The application of the method to molecular targets is made more complicated, especially at the computational level, by the reduced symmetry and the possibility of vibrational and rotational motion. However, we have now reached the point where accurate calculations are possible, at least for diatomic targets. This review will first summarize the basic theory and then describe some recent work, carried out in the UK, on low energy electron impact excitation, dissociative recombination and dissociative photoionization.

Electron Collision Induced Excitations and Dissociation of HeH+ Using the R-Matrix Method

The presence of HeH+ ions in the interstellar medium and particularly planetary nebulae is predicted by current models of star evolution. However a recent search of NGC 70271, looking for the v = 1 → 0 emission, has failed to detect this ion despite theoretical modelling of the formation and destruction mechanisms producing observable abundances. These observations imply that the abundance of HeH+ is overestimated and/or the population of v = 1 of HeH+ is overestimated. Recently a tentative assignment of two features as having originated from HeH+ has been made in the spectrum of supernova 1987A 2. However the evidence for such an assignment is not conclusive.