Photoemission hemispherical analyzer

For time-resolved 2PPE spectroscopy, a combined set-up of an ultrafast laser system and an ultrahigh-vacuum photoemission spectroscopic system is indispensable. Typical electron energy analyzers have been used as the spectrometer, such as a cylindrical mirror analyzer, a hemispherical analyzer and a time-of-flight (TOF) analyzer. The TOF analyzer is mainly used for low repetition rate (<1 kFlz) laser sources, and the others are used for the lasers with multi-kldz or MHz repetition rates [11-14]. 

In our x-ray photoemission studies, a monochromatized photon beam (A1 Ka, hv = 1486.6 eV) was focused onto the sample surface, and the emitted electrons were energy analyzed with a Surface Sciences Instruments hemispherical analyzer. The take-off angle of the photoelectrons relative to the surface normal was 60° unless otherwise specified. A position-sensitive detector with 128 channels was used with a dedicated HP9836C computer to facilitate data acquisition. (22.) ... 

Quantitative spectra with optimized resolution are recorded in the FAT mode with optimized pass energies. Figure 6 exemplifies the drastic effects of the analyzer convolution function on a photoemission spectriun for a valence band UPS experiment on graphite. The spectra characterize the very same surface and are both recorded with Hel radiation. The characteristic differences of the two operation modes of the hemispherical analyzer (LH EA 200) can be clearly seen as well as the different suitability of the relevant spectra for qualitative and quantitative analysis. In XPS data, the same differences occur for the two modes of operation. The intrinsically more symmetric line profile of core level lines tends, however, to obscure the effects of the analyzer in narrow scan spectra. 

There are many different designs of electron energy analyzers but the preferred option for photoemission experiments is a Concentric Hemispherical Analyzer (CHA), which uses an electric field between two hemispherical surfaces to disperse the electrons according to their kinetic energy (Fig. 5.9). 

In Section 3.2.2.5.1, an outline is given of how the spin polarization of the photoelectrons can be measured after their energies and emission angles have been preselected by passing through a hemispherical sector analyzer, thus enabling spin-resolved photoemission experiments (see also Chapter 7). 

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