Bernd M. Nestmann

Calculation of the discrete component of resonance states in negative ions by variation of nuclear charges

A method for obtaining the discrete component of resonance states in negative molecular ions is described. It is based on standard MRD-CI type calculations and especially designed for treating larger molecular systems. The resonance is thereby stabilised by increased nuclear charge. The method is applied to the calculation of N2- in its 2 Pi g( pi u4 pi g) and 2 Pi u( pi u3Ss2) resonance states.

CI method for determining the location and width of resonances in electron-molecule collision processes

CI calculations are employed to evaluate the location and width of resonance states in electron-molecule collision processes. A diabatic negative-ion state is obtained by increasing the nuclear charge of the system and is interpreted as the compact component in Feshbachs projection operator formalism. Results are presented for the 2 Pi g state of N2-. Scattering cross sections for vibrational excitations to the lowest six energy levels are evaluated within the boomerang model.

R-matrix calculation for electron-methane scattering cross sections

A recently developed technique for calculating cross sections in electron-molecule collision processes based on variational R-matrix theory is discussed. One of the main advantages of this method is its applicability to polyatomic systems. As a first example, integral and partial cross sections for the e-+CH4 system below the ionization limit and in the energy range of the Ramsauer minimum are presented. In addition, the corresponding eigenphases are shown. All calculations are based on the fixed-nuclei approximation. The paper is concluded by an analysis of the Ramsauer minimum with respect to the angular dependency of the differential cross sections in that energy region.

Ab initio study of the resonant electron attachment to the F2 molecule

Dissociative attachment to and vibrational excitation of diatomic molecules by electron impact is discussed within the projection operator approach. The present method lifts the assumption of separability of the discrete-continuum coupling term Vde(R), i.e., it is no longer required to write it as a product of a function depending on coordinate R and energy e separately. The method is applied to the 2Σu resonant dissociative electron attachment to and vibrational electron excitation of the F2 molecule. The required ab initio data have been computed using a recently developed algorithm. This algorithm is based on the Feshbach–Fano partitioning technique and the R-matrix method (FFR). The FFR method is discussed in the context of this particular application.

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. .

Study of electron polarization and correlation effects in resonant and background electron scattering off CF3Cl

Abstract We report on ab initio calculations for electron–CF 3 Cl scattering based on the R-matrix approach. Eigenphases and cross-sections are presented for collision energies up to 15 eV in static exchange and static exchange plus polarization (SEP) approximation. The SEP results for integral cross-sections agree satisfactorily with experimental total cross-sections. Moreover, the partial cross-sections confirm the symmetry assignment of the lowest three resonances as has been found in experiments carried out by Mann and Linder. Electron polarization and correlation effects are studied separately for resonant and background scattering. For this separation, the Feshbach projection operator technique formulated within the R-matrix framework is applied. We found that the description of resonant scattering is considerably affected by configuration interaction. The background scattering appears to be appropriately described on the SEP level.

Ab initio calculation of transition rates for autoionization: the Auger spectra of HF and F−

Abstract A method for the quantitative computation angular integrated molecular Auger spectra is proposed. It combines quantum chemistry procedures for the description of the bound states involved with scattering theory for the representation of the scattering states. The bound states are described by configuration-interaction (CI) wavefunctions; the scattering problem is solved by a Green operator formalism. The method is illustrated by computing the Auger spectra of hydrogen fluoride and because we want to investigate the molecular effects, the isoelectronic fluoride ion.

An R-matrix approach for electron scattering off polyatomic molecules

A variational R-matrix approach from first principles-the selected states R-matrix (SSRM)-is proposed for the study of electron scattering off polyatomic, non-linear molecules. Cartesian Gaussian-type orbitals are used exclusively to represent both valence and continuum orbitals. The two-electron integrals along with the nuclear-attraction integrals are evaluated over the entire space. In a subsequent step these integrals are effectively confined to the R-matrix sphere by subtracting their contributions from outside the sphere using a multipole expansion which approximates the projectile-target interaction in the outer region. This approach allows the use of efficient integral codes available for the calculation of molecular bound states. Correlation and polarization effects are only taken into account for selected, energetically low-lying (n+1)-particle CI roots, corresponding to the R-matrix poles. Higher-lying roots are approximated by a static exchange calculation. In order to test this method, electron scattering off N2 is considered in detail for both resonant (2 Pi g) and non-resonant (2 Sigma g+) scattering symmetries. Good accordance is found between the eigenphase sums from the SSRM calculations and results obtained with a standard R-matrix package for linear molecules as well as with reference data from the literature.

Optimized Gaussian basis sets for representation of continuum wavefunctions

Exponents of Gaussian-type basis functions have been optimized for electron-molecule scattering purposes. The criterion for optimization was to obtain the best least-squares fit to six Bessel functions jl(kh(l)*r) representing the continuum functions. The values for the radial momentum kh(l) are defined by the boundary conditions for the Bessel functions to have vanishing radial derivatives at r=20 au. For each l=0, 1 and 2, Gaussian basis sets of eight functions have been optimized. The results are of excellent quality. It is therefore concluded that usual atomic Gaussian basis sets, augmented by these functions, can be sufficient in electron-molecule scattering calculations, such as R-matrix calculations, for example.

Characterization of metastable anionic states within the R-matrix approach

Metastable anionic states are very useful for describing resonances in electron-molecule scattering processes. However, a rigorous definition of such metastable states does not exist and their numerical representation, in particular in cases of very broad resonances, is difficult. In the present paper, the formalism of the R-matrix theory is employed to develop a general concept for constructing metastable electronic states. The discretization of the scattering continuum by the R-matrix formalism allows us to define metastable anionic states similar to diabatic states in cases of avoided crossings of potential curves in molecules. This present technique also allows the solution of the background scattering problem and the determination of the energy-dependent decay width of resonances. The concept is demonstrated for the state of , the state of and the state of .