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

New experimental results on attachment reactions involving free electr ons at sub-meV resolution allow for the first time a conclusive compar ison of measured rate coefficients for Rydberg electron attachment wit h those calculated from the measured free electron cross sections on t he basis of the quasi-free electron model for Rydberg electron collisi on. Using classical velocity distributions for the high n Rydberg elec trons and our measured free electron attachment cross sections, we cal culate 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. W e 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 qua litative agreement with our high n Rydberg data (n greater-than-or-sim ilar-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 signif icantly larger than free electron rate coefficients k(e) (E = \E(n\), while for circular orbits (l = n - 1) they agree. On average, attachme nt reactions of Rydberg electrons in low l orbits proceed with an effe ctive collision energy substantially smaller than the binding energy \ E(n)\.