Electron Dynamics at Surfaces

The theoretical description can specifically address the various scattering processes by the development of appropriate models, which are presented in Section 6.2. In experiment, the combined effect of all scattering processes is usually observed, and only by extensive and careful analysis, the individual contributions can be disentangled. Therefore, the presentation of the experimental methods is organized according to the physical observables as listed in Table 6.1. The electron dynamics in the time domain is accessible only in time-resolved 2-PPE and is discussed in Section 6.3. Photoelectron spectroscopies as well as STS access the electron dynamics via the spectral line shape. These aspects are presented in Section 6.4. Real space imaging of electron waves is possible only in STM, which is discussed in Section 6.5. The final Section 6.6 compares the information obtained by the various experimental methods with theoretical calculations and presents the current understanding of electron dynamics at surfaces. 

The most detailed and direct information on the electron dynamics at surfaces stems from time-resolved 2-PPE experiments. The method is explained in Section 3.2.4.3, and this section presents important examples for the electron dynamics at surfaces. 

The previous sections presented the available methods to obtain information about the electron dynamics at surfaces. This section compares the results obtained by different techniques according to the various scattering processes. More extensive tabulations of results and detailed discussions may be found in recent reviews [1,32]. 

Excitation of the surface is done by photons, and the dipole selection rales can also be apphed to the first step in two-photon photoemission. The experimental parameter space is expanded compared to regular photoemission by the option to choose different photon energies and polarizations for the two photons employed. For short photon pulses generated by femtosecond lasers, the time delay between the two photons is an additional experimental parameter that allows the time-resolved sampling of the population in the excited state. This last feature in particular is unique to two-photon photoemission and permits the detailed investigation of the electron dynamics at surfaces, which is the topic of Chapter 6. 

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