Diploma - Max Planck Institute for Solid State Research

17
3 Experimental foundations
3.1 Photoemission
Having already regarded the principles of photoemission (see ch. 2.2) this paragraph
concentrates on experimental issues. At least, one has to note the following effects and
differences:
• To obtain angle-resolved spectra high-quality monocrystalline samples are needed
because the translational invariance is a requirement for momentum conservation
in photoemission.
• Since the incident photon beam has a finite spot size, we have spatial as well
as temporal integration for the outgoing particle wave functions in addition to
the intrinsic interference contribution of different PE channels. Furthermore, the
sample’s surface is not homogeneous (breaking exactly at one well defined layer)
and exhibits terraces and single atoms adsorbed at the surface. Additionally, the
sample can have different oriented domains and therefore one usually integrates
(spatially) over a non-homogeneous surface region (we selected samples whith
large single-domain regions for PE).
• Having a semi-infinite solid, we face in principle three different states (cf. fig. 2.6)
which are crucial for photoemission: bulk states, surface resonances and surfaces
states (the last two will be subsumed as surface states). Given that the
Figure 3.1: manipulator including sample holder inside the preparation chamber: left: raw
sample with lever stick; right: cleaved sample