Electron attachment to O-2 molecules in dense helium and argon gases

The resonant attachment frequency nu(A) of excess electrons to O-2 molecules in dense helium and argon gases has been measured by the time-of-flight method. Measurements in helium have been performed in the tem temperature range 50-150 K. nu(A) shows two sharp peaks as a function of the host medium density N, as a result of the formation of O-2(-) ions in the fourth and fifth vibrational levels owing to the shift of the electron self-energy with N. The density of the maximum of the first peak exhibits a positive temperature dependence partially explained by a simple model in which a bubble is formed around the ion and the resulting free-energy variation is taken into account in the determination of the resonance energy. On the contrary, measurements carried out in argon at T = 162 K in an extended density range [(5-115) x 10(20) cm(-3)] give a completely different result, as nu(A) monotonically increases as a function of density and does not show any peak. The analysis of argon data needs a much better knowledge of the impurity-solvent interaction and of the electron self-energy since a rough estimate shows that even in argon a peak should have been observed.