Two-step processes according to the R-matrix theory

A method is developed which describes in R-matrix terms the quasi-resonant processes which consists of resonant and direct transitions. The method appears potentially useful in the study of many quasi-resonant two-step processes and it was illustrated by describing threshold effects which originate in resonant scattering near threshold and also resonant structures in residual nuclei excited in transfer reactions. The method displays formally the direct interaction effect on the spectroscopical widths of a multichannel resonance. It results into another parametrization of partial widths; these have, in addition to the R-matrix term, another one connected with quasi-resonant two-step processes. The total width of a multichannel resonance becomes smaller than the width of original single channel resonance as a consequence of direct channel's coupling. Similar results have been obtained in a K-matrix approach to quasi-resonant processes. (Author) 5 Refs