Photoemission Observation of Surface-Plasmon Excitation in the Alkali Metals

Measured energy distributions of photoemitted electrons from K, Rb, and Cs show a high-energy peak identified as unscattered (primary) electrons, and an intermediate peak at somewhat lower energies which is believed to be due to a plasmon energy loss. This intermediate structure becomes more pronounced and closer to the primary peak on proceeding from K to Rb to Cs. This systematic trend, together with the absence of any clearly discernible intermediate structure in Na, is consistent with the variation of the corresponding plasmon energies. In Rb and Cs the intermediate peak is sufficiently pronounced to permit its identification as a surface rather than a volume-plasmon excitation.