Time-resolved photoemission spectroscopy has established itself as a well-recognized method to study excited electrons at surfaces. Adding spatial resolution adds the possibility to correlate the excitations with structural properties. In this Project of the Sonderforschungsbereich, we use a commercial photoemission electron microscope and a pulsed fs-laser source to study electronic excitations in self-assembled nanostructures on surfaces and in organic thin films.

If a small particle is excited by an external electric field, depending on the wavelength of the excitation, the particle size and shape, and the material of the particle, it is possible to excite a resonant collective motion of the electrons in the particle: a plasmon. In particular, the particle absorbs some energy from the exciting field in the plasmon and dissipates it in various ways. If the plasmon is excited again before the origninal energy has dissipated, the excitation is sufficient to trigger the photoemission of an electron. We use these photoemitted electrons to generate a microscopic image of the plasmon and the underlying structure. Since two photons are needed to excite one electron, this is based on a two-photon-photoemission (2PPE) process: If the plasmon can be excited resonantly, the area appears bright in the image, if the excitation is off-resonance, the area is dark.

Fig 1: Silver islands and wires on a Si (001) surface, From Ref. [1]

Figure 1 shows three images of Ag islands and wires on the surface under different illumination. Panel (a) shows an image of the surface in regular photoemission. The silver islands and wires appear bright on the dark Si substrate. Panels (b) and (c) show the same area in 2PPE under illumination with 400nm laser pulses of 40fs duration. In (b), the electric field is perpendicular on the long axis of the wire 'A'. The plasmon can be excited and the wire appears bright. In (c), the electric field is along the wire 'A': The resonance frequency for the plasmon excitation in the wire has shifted to red, the plasmon can not be excited any more, and the wire remains dark.

Thin crystalline and semiconductiong films of organic molecules gain increasing importance in the semiconductor industry. The growth properties, electronic transport mechanisms, and the decay of electronic excitations in these materials, however, are not yet understood. We use Pentacene and Anthracene thin films on Si(001) and Si(111) to correlate surface morphology with the decay of electronic excitations.

Fig 2:
Left: Single-molecule high pentacene islands on a stepped Si(111) surface in Atomic Force Microscopy (AFM). Right: Similar islands (dark) in two-photon-photoemission PEEM.

The AFM image on the left of Fig. 2 shows that Pentacene forms small dendritic islands on the surface. Thicker films are polycrystalline and rough. Our first aim is to investigate single islands with two-photon-photoemission. On the right of Fig. 2, a two photon photoemission microscopy image is shown. It can be shown, that photoemission proceeds via electronic excitation of electrons from the highest occupied molecular orbitals (HOMO) into unoccupied states, and then excitation into the vacuum through a second photon. The density of unoccupied states for our laser energy (3.1eV) is comparatively low, and accordingly the islands appear dark in the right of Fig. 2. In Anthracene, our laser energy is resonant with the HOMO-LUMO gap and as a consequence Anthracene islands appear bright in two photon photoemission microscopy.

To study the electron dynamics in the intermediate (LUMO) state, a pump-probe experiment is required. Here, a first (400nm, 40fs) pump-pulse excites the electrons in the film, while a second (similar) laser pulse probes the decay of the excitation. This allows to study the interplay of structure and the decay of the population of the excited states - energy dissipation at it's best.