RADIATIVE DAMPING AND INTERFERENCE IN THE RESONANCE STRUCTURE OF THE ELECTRON-IMPACT EXCITATION CROSS-SECTIONS FOR TI20+

We have performed R-matrix calculations for the electron-impact excitation of a highly charged ion in which an optical potential is included to allow for radiative damping of intermediate resonance states. The present case considered is that of electrons incident on Ti20+ ions. Comparisons with undamped R-matrix calculations, distorted-wave calculations, and experimental measurement are made. The radiative optical potential accounts for significant damping of high-n resonances. Configuration interaction between resonances, which is automatically incorporated in the R-matrix method but usually omitted in the implementation of distorted-wave techniques, was found to increase the resonance contributions considerably. Our radiative-damped R-matrix cross sections show fairly good agreement with the resonance profile obtained from electron beam ion trap measurements [S. Chantrenne et al., Phys. Rev. Lett. 69, 265 (1992)]. This system demonstrates the necessity for simultaneously including radiative-damping and interfering-resonance effects within electron-highly charged ion scattering calculations.