Photoexcited collective modes in thin alkali layers adsorbed on Al

Photons axe an alternative probe to electrons or ions for the detection of surface and interface collective modes. We have used synchrotron radiation in conjunction with angle resolved photoyield measurements to study the evo lution of overlayer excitation modes in thin alkali layers. Our experimenta l results show clear evidence of collective modes such as the multipole pla smon, bulk overlayer plasmon and threshold excitation. Both the energy and lineshape of these photoyield resonances show strong thickness dependence. The response of the hidden substrate is rather different. The Al multipole peak shows no energy shift as a function of coverage confirming the absence of an intermediate charge density. We discuss our results in terms of prev ious dynamical calculations of the surface response. (C) 2001 Elsevier Scie nce B.V. All rights reserved.