PHOTOABSORPTION AND IONIZATION ENERGIES OF NONSTOICHIOMETRIC CSI CLUSTERS - METALLIZATION OF A SALT

Cs and CsI vapors were mixed to produce clusters having compositions v arying from pure metal to the ionic salt. Ionization potential measure ments on these clusters were performed using photoionization and a tim e-of-flight mass spectrometer. Results are reported for two metallizat ion sequences, Cs14In for n = 1 - 13 and CsmIn for m + n = 26, 27 and m>n. The ionization potentials show a good qualitative agreement with recent computations and experiments carried out on nonstoichiometric N aF. Photoabsorption spectra are presented for CsmIn clusters for m + n =26, 27. The spectra were obtained by heating mass selected clusters in a beam by means of photoabsorption to induce evaporation of atoms. The resulting mass loss was observed in a time-of-flight mass spectrom eter. The spectra of metal rich clusters with m - n > 8 are very simil ar, showing one broad absorption peak around 1.42 eV, the energy of th e plasmon resonance of pure Cs clusters of this size. CsmIn+ clusters with less than 7 excess valence electrons show a clear peak splitting. The shape and width of the peaks is not consistent with the picture o f a homogeneous metallic cluster. In this range, a ''demetallization'' seems to take place. (C) 1997 American Institute of Physics.