Excitation and decay of aluminum bulk plasmons at the aluminum/copper phthalocyanine interface

We present the results of an experiment aimed at studying the archetypal properties of the aluminum bulk plasmon at an organic/metal interface. Electron-electron coincidence spectroscopy is used to determine the contribution of aluminum bulk plasmon decay to the ionization of a thin copper phthalocyanine film. The latter directly depends on the amplitude of the bulk plasmon electric field (generated in the metal substrate) protruding inside the molecular overlayer. The emission of low-energy electrons from the clean substrate is dominated by plasmon-assisted ionization events. These events are not observed when the molecules are adsorbed onto the surface. Our findings suggest that, for the considered system, the bulk plasmon wave is confined within the medium in which it is generated and the interaction of the plasmon field with electrons located in the molecular overlayer does not lead to the emission of low-energy electrons.

Figure 1

Schematic of the experimental apparatus used to detect the reflected (analyzer 1) and secondary electrons (analyzer 2) in coincidence. A specular scattering geometry is adopted in which the reflected electrons have the same angle to the surface normal as the primary electrons (${\theta }_o={\theta }_i$).

Figure 2

Top: Electron-energy-loss spectrum of Al(111) (black points). The red solid line represents the best fit obtained by summing the contribution of surface (SP), bulk (BP), and multipole (MP) plasmons. Bottom: Electron-energy-loss spectrum of 4-Å CuPc/Al(111).

Figure 3

Data points with error bars: SE spectrum measured in coincidence with the aluminum bulk plasmon loss feature of Al(111) (top) and 4-Å CuPc/Al(111) (bottom). The dashed green lines represent the corresponding SE spectra measured by analyzer 2 alone.

Figure 4

Initial binding energy and momentum of LEE emitted in the decay of the Al(111) bulk plasmon (orange squares) obtained from Eqs. (1) and (2). The height and width of the rectangular boxes take into account energy and momentum resolution of the experimental setup. The kinetic energy of the emitted electron is indicated on the right axis. The background, adapted from Ref. [24], represents the calculated band structure of Al(111).

Figure 5

SE spectrum of CuPc/Al(111) measured in coincidence with the Al bulk plasmon energy-loss feature and reported with dependence on the emitted electron binding energy obtained from Eq. (1) (open circles with error bars). The green crosses represent the corresponding valence-band photoemission spectrum. An analogous photoemission spectrum acquired at a resolution comparable with the coincidence experiment is represented by the dashed red line.

Figure 6

Data points with error bars: electron-electron coincidence spectra of Al(111) (top) and 4-Å CuPc/Al(111) (bottom) obtained at $E_1+E_2=97.5$ eV (open triangles) and $E_1+E_2=92.5$ eV (open circles). The coincidence spectra are compared to the corresponding energy-loss spectra (dashed green line).