Resonant low-energy electron attachment to O2 impurities in dense neon gas

We report measurements of resonant low-energy electron attachment to O2 molecular impurities in neon gas in the temperature range 46.5 K T 101 K. The reduced attachment frequency shows a well defined peak as a function of the gas density N when the electron energy is resonant with the 4th vibrational level of O_2-. For 46.5K T8.4K a second peak has been detected at a much higher density, which is due to the formation of ions in the 5th vibrational level. The temperature dependence of the first peak position can be explained within an ionic bubble model by computing the electron excess free energy as a function of T and N. The peak shape is rationalized by taking into account the density dependent shift of the electron energy distribution function and the density of states of excess electrons in a disordered medium, and by assuming that electrons sample the gas density over a region of the order of the ionic bubble radius.