AR 2P SPECTROSCOPY OF FREE ARGON CLUSTERS

Total electron and total and partial ion yield spectra of Ar clusters (with average size up to 600 +/- 200) in the region of Ar 2p excitation have been measured using synchrotron radiation and time-of-flight mass spectrometry. As the average cluster size increases, the x-ray absorption spectrum changes systematically from that of atomic Ar to that of solid Ar. The shape of the Ar 2P3/2 --> 4s region is found to be a sensitive monitor of the cluster sizes present in a molecular beam of Ar clusters. Extended x-ray absorption fine structure (EXAFS) is detected in the spectra of the larger clusters. There is a strong correlation between the intensity of the components of the Ar 2P3/2 --> 4s signal associated with clusters and the intensity of the Fourier filtered first shell Ar 2p EXAFS signal. A low amplitude, high frequency fine structure is observed in the Ar 2p continuum of the heaviest clusters which corresponds closely to that observed in solid Ar. This signal develops with cluster size more slowly than the Ar 2p EXAFS and 4s exciton signals.