Elsevier

Surface Science

Volume 593, Issues 1–3, 20 November 2005, Pages 1-11
Surface Science

Carrier dynamics on surfaces studied by two-photon photoemission

https://doi.org/10.1016/j.susc.2005.06.040Get rights and content

Abstract

The lifetime of electronic excitations at surfaces play an important role for desorption processes induced by electronic transitions. Two-photon photoemission with time, energy, and angular resolution allows to study the scattering processes of excited electrons in detail. For electronic surface states various scattering channels can be identified, such as elastic vs. inelastic or intraband vs. interband. As examples the scattering of electrons in image-potential states by copper adatoms on Cu(0 0 1) and the decay and exciton formation in the dangling-bond surface conduction band at the Si(1 0 0) c(4 × 2) surface are presented. These studies of the carrier dynamics at surfaces imply that photochemical or electronic desorption processes are influenced by surface defects and are more efficient at semiconductor than on metal surfaces.

Introduction

The first step in desorption induced by electronic transitions is the coupling of excited electrons to adsorbate states. Only if the energy is localized on the adsorbate, desorption will occur. Excited electrons usually loose energy on a femtosecond timescale by inelastic interaction with the underlying electron system of the surface and bulk [1]. Time-resolved two-photon photoemission (2PPE) allows to study the electron dynamics at surfaces in considerable detail. The basic process is illustrated in Fig. 1. A first photon excites an electron into an unoccupied intermediate state. The second photon leads to photoemission of the excited electron. By controlling the time delay between the two photons with femtosecond resolution, the electron dynamics of the intermediate state can be studied in detail. As in normal photoelectron spectroscopy the state can be selected by choosing energy and angle of the detected electrons.

In this contribution two prototypical systems will be presented: (i) The scattering of electrons bound to a metal surface by adatoms [2]. The detailed study gives insight into the possibility or probability of trapping electrons by the adsorbates. (ii) The electron dynamics in the dangling-bond bands on a semiconductor surface [3], [4] is rather involved, but carriers end up in a bound exciton state, which lives for nanoseconds.

Section snippets

Scattering by adsorbates

The simplest model system to study is the scattering of an electron by an adsorbate atom or molecule on a metal surface. In electron energy-loss spectroscopy the electron is free whereas we will study the effect for bound electrons. The image potential attracts electrons to a metal surface. If a band gap prohibits the penetration of the electron into the bulk, a Rydberg-like series of unoccupied surface states is formed [5]. The electrons are confined several Angstroms in front of the surface,

Electron dynamics at the Si(1 0 0) surface

The Si(1 0 0) surface is technologically of utmost importance, but the properties of the clean surface have been under discussion for a long time. On the geometric structure of the Si(1 0 0) surface there is general agreement that the lower-coordinated atoms at the surface rebond to buckled dimers [18]. The dimers are ordered at low temperatures in a c(4 × 2) arrangement as shown in Fig. 7. Above 200 K the surface shows a (2 × 1) LEED pattern at room temperature which is attributed to the disordered

Summary and conclusions

We presented the detailed results for the electron dynamics at surfaces obtained by two-photon photoemission with energy, angle and time resolution. These studies use photon intensities orders of magnitude below the threshold where desorption is induced by electronic excitations. However, information is obtained on how carriers are scattered and how they they loose their energy. The results clearly show that defects may play an important role in these processes and that at least for

Acknowledgements

Klaus Boger and Michael Kutschera are credited for their excellent contributions to the work presented here. We thank Michael Rohlfing for the theoretical calculations and many stimulating discussions. Support by the Deutsche Forschungsgemeinschaft is gratefully acknowledged.

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