Electron impact ionization efficiency curves of argon clusters and argon/methanol heteroclusters

In a previous paper (J. Chem. Phys. 1991, 94,1850) we reported electron impact appearance energies for Ar_n(CH₃OH)_m heteroclusters for small cluster sizes and concluded that at threshold the dominant ionization mechanism was intracluster Penning ionization involving the electronically excited 4s state of an argon atom. In this paper we have extended these measurements to a wider range of stagnation pressures and electron energies and observe that the appearance energies of argon-methanol heterocluster ions shift to higher values which we interpret as consistent with exciton-induced Penning ionization. We have observed additional high-energy structure in the electron ionization efficiency curves of certain cluster ions around 27 eV, only under expansion conditions which favor the formation of large clusters. Possible explanations for this high-energy structure are discussed.