
Project
B5:
Time-Resolved Photoelectron Microscopy
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.

Surface Plasmonics
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.

Electrons in Organic Thin Films
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.


Poster:

Internationaler
Workshop 2008
B5 Buckanie et al.
PDF (0.6 MB) |

SFB616
Int. Workshop 2006
B5 Buckanie et al.
PDF (3.8 MB) |

SFB616
Int. Workshop 2005
B5 Thien et al.
PDF (2.0 MB) |

Publications: