SFB 616

 

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 Project B8:
Hot electron relaxation in layers and at interfaces


 Elementary scattering processes responsible for dissipation and transport processes at interfaces occur on ultrafast timescales and photoelectron spectroscopy in the time domain represents an excellent tool for their analysis. Conventional photoemission accesses binding energies of electronic states and their momentum dispersion. The use of femtosecond laser pulses in pump-probe experiments allows obtaining direct insights to the energy and momentum dependence of ultrafast dynamics.

Fig. 1: Scheme of two photon photoelectron spectroscopy (2PPE). Emission of a photoelectron requires two photons which each have a photon energy below the work function Φ=Evac-EF. Thereby unoccupied electronic states can be probed as an intermediate state in the 2PPE process (right panel). If the two laser pulses are delayed in time, a pump-probe experiment is performed and the population dynamics of the probed state are monitored at a well defined energy and momentum parallel to the surface.

This project investigates the elementary interaction processes and emphasizes work performed on quantized electronic states in thin metal and organic films. Here we address decay processes in the limit of low optical excitation densities, where electrons decay according to their interaction with carriers in equilibrium. Hot electron relaxation in epitaxial metallic films like Pb/Si(111) and organic structures on pthalocyanine on single crystal metal substrates are investigated.

 As a femtosecond light source a commercial tuneable Ti:sapphire laser system, which is based on the regenerative amplifier RegA 9040 manufactured by Coherent. The beam paths of our setup and the obtained typical pulse parameters are given in Fig. 2, top. We emphasize the tuneable high repetition rate of this laser system up to 300 kHz and stable cw pump lasers. Both are essential to obtain an excellent signal to noise ratio. Pairs of pump and probe laser pulses are selected for a specific experiment and focused into the UHV chamber onto the sample surface, see Fig. 2, bottom panel. The UHV chamber is equipped with various tools for film preparation and characterization like dosers and evaporators, Auger electron spectrometer, and optics for low energy electron diffraction. Using a liquid He cryostat the sample can be cooled to study the influence of thermal excitations on the decay processes of interest. For angle-resolved photoelectron analysis we employ a hemispherical photoelectron spectrometer optimized for detection of low energy electrons emitted by laser pulses.

Fig. 2: The experimental setup for time-resolved photoelectron spectroscopy from surfaces and interfaces combines a tuneable Ti:Sapphire laser system operating at several 100 kHz repetition rate (top panel) and an ultrahigh vacuum (UHV) chamber (bottom panel). The laser system is based on a cw-pumped regenerative amplifier (Coherent RegA 9040). Tuning from the near IR into the UV is facilitated by the use of optical parametric amplifiers. The UHV chamber, in which samples are prepared in situ, contains the electron time-of-flight spectrometer (e-TOF) and various surface science preparation and characterization tools like evaporators, gas dosers, Auger electron spectrometer, and an instrument for imaging diffraction patterns of low energy electrons. Spatial overlap of pump and probe pulses is verified using a CCD camera positioned outside the vacuum chamber.

Fig. 3: Time-resolved 2PPE results on a 15 ML Pb film grown on Si(111). The main panel depicts in a false color representatio the detected time-resolved 2PPE intensity. Vertical cuts give 2PPE spectra as shown in the right panel. In the lower diagram the 2PPE intensity is shown as a function of pump-probe delay for the three electronic states of interest. Furthermore, the cross correlation of both laser pulses is detected by electrons of no specfic intermediate state at the highest obtained energies.

 In metals absorption of a photon generates an electron-hole pair. The large electron density results in an efficient and extremely fast screening of the photohole. Exciton formation does not occur and the decay of the excited (or hot) electron can be studied directly in the time domain. Typical results are given in Fig. 3. From the 2PPE spectra shown at right for fixed time delay three electronic states are distinguished, the n=1 image potential state (IPS) in front of the metal film and two electronic quantum well states residing in the metal film. We denote the lowest unoccupied quantum well state by luQWS and the subsequent state by luQWS+1. Since the photoelectrons are analyzed regarding their kinetic energy the decay can be energy resolved and in the lower panel the decay for the IPS and the QWS states are shown. The decay of the quantum well states is described by coupled rate equations, see the solid lines, which describe the observed intensity in full detail. The decay of luQWS+1 exhibits a two component decay which occurs due to a competion of decay in the metal film by electron-electron scattering described by tA and transfer of photo-excited carriers from the Si substrate in the Pb film leading to tB. For more information see Kirchmann et al., Phys. Rev. B 78, 035437 (2008).


   
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