COMPARISON OF RATE COEFFICIENTS FOR RYDBERG ELECTRON AND FREE-ELECTRON ATTACHMENT

New experimental results on attachment reactions involving free electrons at sub-meV resolution allow for the first time a conclusive comparison of measured rate coefficients for Rydberg electron attachment with those calculated from the measured free electron cross sections on the basis of the quasi-free electron model for Rydberg electron collision. Using classical velocity distributions for the high n Rydberg electrons and our measured free electron attachment cross sections, we calculate Rydberg electron attachment rate coefficients K(nl) for the two cases SF6 and HI for Rydberg binding energies \E(n)\ of 0.1-40 meV. We find a significant increase in k(nl) towards lower binding energies, especially for HI, which is due to the deviation of the free electron cross section from the limiting s-wave behaviour sigma0 approximately E-1/2. The increase at \E(n)\ less-than-or-similar-to 2 meV is in qualitative agreement with our high n Rydberg data (n greater-than-or-similar-to 80) if l-mixing due to residual electric fields is taken into account. For low l, Rydberg rate coefficients k(nl)(\E(n)\) are significantly larger than free electron rate coefficients k(e) (E = \E(n\), while for circular orbits (l = n - 1) they agree. On average, attachment reactions of Rydberg electrons in low l orbits proceed with an effective collision energy substantially smaller than the binding energy \E(n)\.