Accurate Hartree-Fock vibrational branching ratios in 3σg photoionisation of N2

The authors report vibrational branching ratios for resonant photoionisation of N2 leading to the X2 Sigma g+ state of N2+. Their theoretical values are obtained from an accurate solution of the adiabatic-nuclei frozen-core Hartree-Fock model of molecular photoionisation. In contrast to other theoretical results the present results are in very good agreement with experimental measurements. Differences between the present and previous calculations are discussed

Application of the Schwinger variational principle to electron scattering

The authors present the results of the first rigorous application of the Schwinger variational principle to electron scattering with the inclusion of exchange. The results of this application to e-He scattering in the static-exchange approximation show that the Schwinger method provides accurate solutions of the scattering problem with small basis set expansions

Variational treatment of electron-polyatomic molecule scattering calculations using adaptive overset grids

The Complex Kohn variational method for electron-polyatomic molecule scattering is formulated using an overset grid representation of the scattering wave function. The overset grid consists of a central grid and multiple dense, atom-centered subgrids that allow the simultaneous spherical expansions of the wave function about multiple centers. Scattering boundary conditions are enforced by using a basis formed by the repeated application of the free particle Green's function and potential, $\hat{G}^+_0\hat{V}$ on the overset grid in a "Born-Arnoldi" solution of the working equations. The theory is shown to be equivalent to a specific Pad\'e approximant to the $T$-matrix, and has rapid convergence properties, both in the number of numerical basis functions employed and the number of partial waves employed in the spherical expansions. The method is demonstrated in calculations on methane and CF$_4$ in the static-exchange approximation, and compared in detail with calculations performed with the numerical Schwinger variational approach based on single center expansions. An efficient procedure for operating with the free-particle Green's function and exchange operators (to which no approximation is made) is also described

Iterative approach to the Schwinger variational principle applied to electron—molecular-ion collisions

We present a study of electron—molecular-ion collisions. The scattering equations are solved using an iterative approach to the Schwinger variational principle. These equations are formulated using the Coulomb Green's function to properly treat the long-range Coulomb tail of the molecular-ion potential. We apply this approach to electron—hydrogen-molecular-ion collisions in the static-exchange approximation. We obtain elastic differential cross sections, and also use the continuum states from these calculations to compute the photoionization cross section of the hydrogen molecule. The iterative method used here converged rapidly in all calculations performed

Padé-approximant corrections to general variational expressions of scattering theory: Application to 5σ photoionization of carbon monoxide

We discuss a method for systematically correcting results obtained using variational expressions in scattering theory. The approach taken is to compute a sequence of Padé approximants of the form [N/N] for the error in an initial variational estimate obtained using a basis-set expansion. The relationship between the Padé-approximant approach and the iterative Schwinger method for correcting variational estimates is also examined. We discuss a large class of general variational expressions to which the Padé-approximant approach can be applied. The variational expressions considered include those for the wave function, for photoionization transition matrix elements, as well as for scattering matrix (K-matrix) elements. We have applied this approach to the 5σ photoionization of CO using the frozen-core Hartree-Fock and fixed-nuclei approximations. We find that the Padé-approximant method converges rapidly and reliably. Both total photoionization cross sections and photoelectron angular distributions from threshold to 40 eV are presented and compared to previous experimental and theoretical results. We find major quantitative discrepancies between the present results for the total cross section and previous theoretical results

Studies of differential and total photoionization cross sections of carbon dioxide

The photoionization of CO2 has been studied using accurate frozen-core Hartree-Fock final-state wave functions. The Hartree-Fock continuum equations were solved using the iterative Schwinger variational method. We present differential and total cross sections for photoionization leading to the X 2Πg, A 2Πu, B 2Σu+, and C 2Σg+, states of CO2+ as well as for oxygen and carbon K-shell photoionization. The present cross sections are compared to experimental data and are found to be in generally good agreement. The theoretical cross sections exhibit features due to a narrow shape resonance in those channels where the continuum wave functions have σu symmetry. The relation between these results and experimental cross sections is discussed. The present fixed-nuclei results have also been compared to published theoretical results obtained using the Stieltjes-Tchebycheff moment theory approach and the continuum multiple-scattering method

Studies of differential and total photoionization cross sections of molecular nitrogen

The photoionization of molecular nitrogen has been studied using a frozen-core Hartree-Fock final-state wave function with a correlated intitial-state wave function. The final-state wave function was obtained using the iterative Schwinger variational method. The effects of initial-state correlation were studied by comparing cross sections obtained using a configuration-interaction-type initial-state wave function with those obtained using a Hartree-Fock initial-state wave function. In this paper we compare our accurate single-center expansion results with other theoretical results. We find that earlier single-center cross sections were not well converged with respect to their expansion parameters. The results of the continuum multiple-scattering method and the Stieltjes-Tchebycheff moment-theory approach are found to be in qualitative but not quantitative agreement with the present results. We also compare our computed total cross sections as well as integrated target angular distributions with experimental results for photoionization leading to the X 2Σg+, A 2Πu, and B 2Σu+ states of N2+. We find generally good agreement, which is improved by the inclusion of initial-state correlation effects, especially in the resonant photoionization channel leading to the X 2Σg+ state of N2+. We also report integrated detector angular distributions for these three channels

Nanoscopic models for radiobiological damage: metastable precursors of dissociative electron attachment to formic acid

The HCOOH molecule represents the simplest organic acid which is also supposed to play a role in the interstellar formation of more complicated biomolecules. Its interaction with slow electrons in the gas phase is analysed in the present work with the view of providing specific structural and dynamical information on those resonant states which lead to different transient negative ions (TNIs) formation. The latter resonant states in turn guide molecular fragmentation along different pathways, forming HCOO−, O− and OH− fragments as experimentally observed. The present calculations, carried out at the equilibrium molecular geometry, indeed support the presence of two main resonances within the expected energy range and further indicate the presence of antibonding nodal planes in the excess electron resonant wave function features which could explain the observed fragmentation products formed during the subsequent dissociative break-up

Iterative approach to the Schwinger variational principle for electron-molecule collisions

We present an iterative approach which uses the Schwinger variational principle to solve the Lippmann-Schwinger equation for electron-molecule scattering. This method combines the use of discrete basis functions to describe the effects of the noncentral molecular potential with an iterative procedure which provides systematic convergence of the scattering solutions. Results for electron-H2 scattering in the static-exchange approximation show that the method converges rapidly and gives very accurate results