Photoionization of argon clusters

Abstract

Argon clusters were produced in a free supersonic molecular beam expansion of pure argon at room temperature and the photoionization efficiency curves of the trimer through hexamer were measured in the wavelength regions from threshold to 700 Å. A study of the Ar+3 photoionization efficiency curve as a function of nozzle stagnation pressure shows that fragmentation of heavier clusters can dominate the spectrum, even near threshold, and even when the nozzle conditions are such that the Ar+4 intensity is only a small fraction of the Ar+3 intensity. The Ar+3 photoionization efficiency curve, obtained using nozzle stagnation conditions such that no heavier ions were detected, exhibits several broad peaks near threshold which show similarities to bands of the dimer. At high nozzle stagnation pressures, the photoionization efficiency curves for Ar+3 to Ar+6 are nearly identical due to the effects of fragmentation. These spectra exhibit two very broad features which are similar to features observed in the solid. The threshold regions for all the positive ions show extremely gradual onsets, making it difficult to determine the appearance potentials accurately. The appearance potentials for Ar+2 and Ar+3 are 855.0±1.5 and 865.0±1.5 Å, respectively, yielding a value of 0.18±0.05 eV for the dissociation energy of Ar+3. The appearance potentials for the heavier clusters Ar+4 through Ar+6 are all approximately 870±2 Å.