Multichannel resonances in complex energy R-matrix theory

A method for determining the positions and widths of resonances occurring in collisions of electrons with atoms and molecules is developed using an analytic continuation of R-matrix theory into the complex energy plane. The method is particularly effective for the case of multi-channel scattering and has the important advantage of allowing the number of resonances within a given region to be determined unambiguously. Illustrations of the versatility of the technique are provided by considering multiphoton ionization of hydrogen atoms and the scattering of electrons by single-charged oxygen molecular ions. The results are compared to values obtained by fitting the eigenphase sum to a Breit-Wigner form. The advantages of the new approach are discussed. These include the ability to locate very narrow resonances and to examine the analytic structure of the scattering matrix for cases involving many scattering channels.