Photoemission Electron Microscopy (PEEM)

In the development of PEEM, the Oregon microscope project, lead by Griffith and Rempfer, played an important role. Griffith and Rempfer built an ultrahigh vacuum UV-PEEM with a spatial resolution of 10 nm and produced a steady stream of beautiful micrographs of biological samples (Griffith and Rempfer, 1987; Rempfer and Griffith, 1989). Rempfer also investigated the spatial resolution of PEEM and the properties of electron lenses of different shape and using different operation voltages, both in experiment and in theory (Rempfer, 1985; Rempfer and Griffith, 1989). Bauer and Telieps built an electron microscope with both LEEM and PEEM modes (Telieps and Bauer, 1987). Instead of photoelectrons, the LEEM employs electron diffraction of low-energy electrons and excels at the investigation of crystalline surfaces, epitaxial films, and film growth. A relatively recent development is the use of X-rays instead of UV radiation. X-ray photoemission electron microscopy (X-PEEM) was demonstrated for the first time in 1988 by Tonner and Harp (1988). In 1993 Stöhr et al. (1993) showed that X-PEEM can image magnetic domains at high resolution. X-PEEM instrumentation has evolved rapidly during the past decade, and almost every major synchrotron facility is now home to a PEEM instrument (Anders et al., 1999; Heyderman et al., 2003; Kuch et al., 2000; Schneider and Schönhense, 2002; Wei et al., 2003). PEEM is limited in resolution by the chromatic and spherical aberrations of the electron lenses. It has been shown that an aberration corrector can improve the resolution down to 1 nm (Fink et al., 1997). Currently, two aberration-corrected X-PEEMs are under construction, the “SMART” PEEM at BESSY II (Hartel et al., 2002) and the PEEM-3 at the Advanced Light Source (Wan et al., 2004). In Section 2 we will discuss the basic layout of an X-PEEM experiment. We will also describe the image generation due to X-ray absorption and discuss contrast mechanisms with the aid of some examples. Section 3 will address the electron optics of uncorrected PEEM, and Section 4 will extend the discussion to aberration correction. Section 5 will deal with a very important application of X-PEEM: imaging of magnetic domains. The last section will introduce time-resolved X-PEEM, sometimes called TRPEEM, which is a very recent development. TR-PEEM is used to image magnetic processes on the nanoscale with picosecond time resolution.