Mott detector

The magnetic coil shown in figs. 2.15 and 2.16 was used to orient the electron polarisation vector P parallel to the axis of the analysing target—Mott detector system. The deflection system is part of the differential pumping stage which is necessary for the maintenance of the required ultra-high vacuum in the source chamber. 

The experimental arrangement used by Granitza et al. (1993) in their study of spin effects in (e,2e) collisions on xenon is shown in fig. 2.17. The polarisation analysis of the incident beam was in this case carried out with a spherical mini-Mott detector, which is also shown in the figure. 

The first prerequisite for measurement of photoelectron spin-polarization is the ability to separately detect the photoelectrons ejected from the different fine-structure levels (e.g., 2n3/2 and 2n1/2 for HX+ X2n). When the molecule contains a heavy atom (e.g., large spin-orbit splitting), it becomes easier to use the electron kinetic energy to distinguish the photoelectrons ejected from the different fine structure channels. For spin-polarization analysis, the accelerated electron beam (20-120 keV) can be scattered by a thin gold foil in a Mott-detector. The spin-polarization is determined from the left-right (or up-down) asymmetry in the intensities of the scattered electrons (Heinzmann, 1978). Spin polarization experiments, however, are difficult because the differential spin-up/spin-down flux of photoelectrons is typically one thousandth that obtained when recording a total photoionization spectrum. 

When secondary electrons are emitted from a magnetic material they become polarised and so by using a polarisation sensitive detector such as a Mott detector to collect the secondary electrons an image can be obtained that has magnetic contrast, allowing magnetic domain structures to be studied. This technique is known as scanning electron microscopy with polarization analysis (SEMPA). 

Electrons from a polarised primary electron beam are directed on to a surface and the spin state of the electrons that are reflected from the surface is determined. Typically, a polarised electron source consists of a GaAs crystal upon which polarised light is projected. The emerging photoelectrons from the GaAs crystal have up to 50 % polarisation. The electrons that are detected are first passed through an electron energy analyser and then through a Mott detector to determine the spin state of the electrons. 

The Mott detector for determining the spin state of electrons has a very low efficiency, which results in long collection times for SPEELS spectra. 

By combining the electron energy analyser with a polarization sensitive Mott Detector separate spin-up and spin-down valence band density of states spectra can be produced for magnetic samples. This technique is known as spin polarised ultraviolet photoelectron spectroscopy (SPUPS). 

Magnetic Measurements. Our method of choice to measure the coupling behavior was spin-polarized secondary electron emission (SPSEE). After bombardment with 1 keV electrons, spin-polarized low-energy electrons are emitted. The spin analysis is carried out by a Mott detector. The secondary electrons are emitted from the first few layers only and the method is thus surface sensitive. 

By using four detectors in a standard Mott detector, a single gold foil can be used for spin analysis along two orthogonal directions. The polarimeter depicted schematically in Figure 3.2.2.32 combines two Mott detectors, one oriented at 90° with respect to the other, allowing for complete three-dimensional spin polarimetry, that is, with quantization axes =x, y, and z [39]. 

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