Photoionization of mass-selectedKn+ions: A test for the ionization scaling law

Photoionization of mass-selected cluster ions ${\mathrm{K}}_{\mathrm{n}}^{+}$ is obtained in the size range 19≤n≤300. Photoproducts are observed by mass spectroscopy at photon energies from 3.5 to 6.2 eV, for several laser flux densities. Mass spectra reveal a competition between photoionization and photoinduced evaporation in the parent ion. The measurements of appearance potentials of doubly and triply charged clusters combined with the first ionization potentials available in the literature allow one to distinguish clearly between the two opposing scaling laws proposed in the literature.