Electron Interactions with Excited Atoms and Molecules

Excited species, particularly those in long-lived metastable states, can have a profound effect on the behaviour of low temperature gas discharges. They often present a considerably different atomic or molecular structure to their ground state ``parent'' atom or molecule. In the case of rare gas atoms, several of their lowest lying excited states have structures resembling loosely bound, one-electron systems, similar to their nearest alkali neighbor in the periodic table. They have large dipole polarizabilities and, as a consequence, extremely large scattering cross sections for low energy electrons. Combined with their long lifetimes, large internal energy and reasonably high excitation probability, they become an important component of a discharge environment. This talk will review some of the work in studying these important excited states -- their role in low temperature discharges was always a fascination for Art Phelps and he was a strong advocate for their detailed study.