Plasmon Damping and Surface Interband Transitions on Ag(011) and (001)

The dependence of surface-plasmon damping versus plasmon momentum and energy has been measured for the Ag(011) surface with angle-resolved electron energy loss spectroscopy. The data are compared with previous results obtained for Ag(001). At small transferred momenta, q parallel-to, we observe a nearly linear dependence of the energy loss width with q parallel-to due to the growing phase space available for e-h pair creation. However the very efficient damping mechanism observed for q parallel-to beyond 0.10 angstrom-1 is not present for Ag(011). We demonstrate that it is due the onset of interband transitions between filled and empty surface states. Two such states are indeed present near the XBAR point of Ag(001) as demonstrated by inverse photoemission experiments and electronic band structure calculations. The energy gap between the two bands can thus be determined by EELS with a greater precision than by photoemission.