Muonium formation in condensed neon and argon

We review our studies of atomic muonium formation under the influence of ap plied electric fields in condensed Ne and Ar. The comparison of muonium for mation processes in solid neon, where electrons are delocalized, liquid neo n, where the coexistence of delocalized and localized electrons was found r ecently, and condensed Ar, where electrons are delocalized in both phases, brings a new understanding of excess electron localization on a microscopic scale. (C) 2000 Elsevier Science B.V. All rights reserved.