Crossover between Monopole and Multipole Plasmon of Cs Monolayers on Si(111) Individually Resolved in Energy and Momentum

The evolution of the plasmon spectrum of the Si(111) ($7\times 7$)-Cs surface has been studied by energy loss spectroscopy individually resolved in energy and momentum during the transition from substrate to Cs overlayer metallization. The multipole plasmon is identified by an extremely narrow angular distribution of the inelastic electron scattering, unaccounted for by standard dipole scattering theory. A crossover between multipole and monopole surface plasmon is observed at finite surface wave vectors $q_{\parallel }$, depending on Cs coverage, and reveals a high sensitivity of the short-wavelength multipole components on surface morphology.

Figure 1

Sheet conductance of Cs deposited on Si(111)-($7\times 7$) at 210 K. Insets are meant as schematic representation of surface structures expected for 0.75 PL (Cs islands on faulted and unfaulted halves of the DAS substrate reconstruction), 2.5 PL (continuous Cs wetting layer), and coverages above 2.5 PL (Cs clusters on top of wetting layer), respectively.

Figure 2

Electron energy loss spectra for $\Delta K_{\parallel }=0$ for three different amounts of Cs deposited on Si(111)-($7\times 7$) at 210 K. The arrows mark loss energies due to surface plasmon scattering for which scattering profiles have been measured (see Fig. 3).

Figure 3

Scattering profiles with respect to $\Delta K_{\parallel }$ for inelastic scattering at $\mathrm{Cs}/\mathrm{Si}(111)$. The scattering vector $\Delta K_{\parallel }$ is given in % of the distance between the (00) and the (10) diffraction spot of the Si(111) surface. 10% SBz corresponds to $0.188{\AA }^{-1}$.

Figure 4

Plasmon energy loss spectra of a Cs wetting layer (2.8 PL) on Si(111) ($7\times 7$) for different wave vectors $q_{\parallel }$. Inset shows the energies of the surface plasmons (MP, SP) and the bulk plasmon (BP) of the Cs layer, as determined from fits to the measured spectra. The line indicating dispersion of the combined MP-SP modes is meant as guide to the eyes.

Figure 5

FWHM of the multipole plasmon peak during Cs deposition at 210 K. The solid lines are meant as guide to the eyes. The dashed curve shows the FWHM of the elastic specular Bragg peak.