Origin of the line shapes from intramolecular Penning ionization in benzene/argon clusters

A supersonic beam of benzene (Bz) seeded in argon was ionized by VUV radiation from the Berlin electron storage ring (BESSY). Resonance structures in the photoionization efficiency curves of Bz and Bz⋅Ar_n (n=1,...,4) are attributed to intramolecular Penning ionization after excitation of argon within a mixed cluster. The dependence of the shapes of the resonance lines on the seed ratio and on the cluster size is reported. It is concluded that formation of large mixed clusters causes broad resonances which are blue shifted relative to the monomer absorption line, whereas the excitation of a single argon atom bound to benzene causes narrow and red shifted lines.