Plasmon excitations in thin alkali metal films on Si(111)7x7

The growth of thin Cs and K films on Si(111)7 x 7 kept at 170 K has been fo llowed by second harmonic generation (SHG) during continuous deposition. In itially, both types of alkali atoms adsorb at dangling bond sites, in a sim ilar way as at room temperature. At low temperature the completion of one o verlayer is followed by multilayer growth. This causes orders of magnitude increase of SHG due to excitations of plasmons. A fairly sharp transition t emperature below which multilayers can exist was found at 260 K for both me tals. For Cs the variations in SHG with exposure can be explained in terms of coverage variations of the resonance frequency. The variations of SHG du ring deposition of K indicate the formation of islands.