Resonances in the electronic excitation of molecular hydrogen.

Ab initio R-matrix scattering calculations are presented for electron-Hp collisions for energies up to 30 eV. Excitation from the ground state, X 'Xg+, to five accurately represented, low-lying electronic states, b 'Z " +, a 'Zg+, B 'Z,+, c'H " and C 'H " is explicitly considered. An attempt is made to rationalize the wealth of experimental data. Resonance structures similar to those assigned experimentally to the a and c series are found as well as a number of other resonancelike features. The scattering of electrons by molecular hydrogen is one of the most fundamental processes in the field of molecular physics. Despite this there are many areas of uncertainty concerning a range of processes including electronic excitation, dissociative recombination, and differential cross sections. In particular, even for this apparently simple system, theoretical calculations on the electronic-excitation process have been restricted to two states (see, for instance, Refs. 2-7). All of these studies used single-configuration wave functions to represent the electronic states involved. Hydrogen, in common with most molecules, has a number of nearby low-lying electronic states. In particular , there are experimental data on excitation to the lowest excited states, ' all of which have vertical exci-tation thresholds in the range 10-13 eV. These studies yield a wealth of data including features which have been associated with several series of resonances. '0' Theoretically these resonances have been studied using nonscattering (Feshbach or stabilization) techniques. ' ' However, despite these, the exact prove-nance of the resonances is still controversial' and many questions remain to be answered on the electronic-excitation cross sections. ' Clearly, a thorough theoretical treatment of low-energy electronic-excitation processes in Hz must take account of the low-lying states. To do this successfully, it is also necessary to have accurate electronic-excitation thresholds for each state included in the calculation. This cannot be achieved by using simple single-configuration wave functions. In this Letter we report electronic-excitation calculations on Hz using up to six states in the close-coupled expansion. These calculations are performed using a target basis optimized to give a good representation of all the target states involved. These states were calculated using a full-configuration interaction within the basis of Slater-type orbitals (STO's) specified below. To our knowledge this is the first time a sophisticated and reliable target wave function has been used to study the electron-impact electronic excitation of any molecule.