Photoelectron spectroscopy of argon clusters: evidence for an Ar13 ionization chromophore

Ultraviolet photoelectron spectra have been measured for argon clusters prepared in pulsed supersonic expansions of both pure and seeded mixtures of argon. The broad band spectra show structural features which relate to the degree of condensation and hence the mean cluster size obtained at different stagnation pressures. These are interpreted in terms of an ion–core hypothesis with diatomics-in-molecules calculations being used to simulate spectral features. The analysis shows that Ar+3 cores are formed in the initial ionization process of very small clusters, with the Ar+13 core becoming dominant for all larger clusters and for the condensed state of argon.