Comparison of rate coefficients for rydberg electron and free electron attachment

Abstract

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 collisions. Using classical velocity distributions for the highn Rydberg electrons and our measured free electron attachment cross sections, we calculate Rydberg electron attachment rate coefficientsknl for the two cases SF6 and HI for Rydberg binding energies |En| of 0.1–40 meV. We find a significant increase inknl towards lower binding energies, especially for HI, which is due to the deviation of the free electron cross section from the limitings-wave behaviour σ0∼E−1/2. The increase at |En|≲2 meV is in qualitative agreement with our highn Rydberg data (n≳80) if l-mixing due to residual electric fields is taken into account. For low l, Rydberg rate coefficientsknl(|En|) are significantly larger than free electron rate coefficientske(E=|En|), 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 |En|.