MULTICHANNEL RESONANCES IN COMPLEX ENERGY R-MATRIX THEORY

A method for determining the positions and widths of resonances occurr ing in collisions of electrons with atoms and molecules is developed u sing an analytic continuation of R-matrix theory into the complex ener gy plane. The method is particularly effective for the case of multi-c hannel scattering and has the important advantage of allowing the numb er of resonances within a given region to be determined unambiguously Illustrations of the versatility of the technique are provided by cons idering multiphoton ionization of hydrogen atoms and the scattering of electrons by singly-charged oxygen molecular ions. The results are co mpared to values obtained by fitting the eigenphase sum to a Breit-Wig ner form. The advantages of the new approach are discussed. These incl ude the ability to locate very narrow resonances and to examine the an alytic structure of the scattering matrix for cases involving many sca ttering channels.