Argon mediated electron impact ionization of Ar_n(CH₃ OH)_m heteroclusters

In this paper we have contrasted electron impact cluster mass spectra of a methanol/argon expansion with a methanol/helium expansion and have measured appearance potentials for several of the prominent ions in each case. We have observed a number of ionic species in the argon expansion (i.e., CH_x O⁺ where x = 0-4) which are not observed in the helium expansion. Appearance potentials for these species fall in the region 11.3-11.8 eV indicating that the threshold ionization of Ar_n(CH₃ OH)_m heteroclusters is mediated by the Ar 4s excited states ( ³P_2,0; 11.55 and 11.72 eV). This represents the first time that intracluster Penning ionization has been positively identified using electron impact measurements.