A versatile instrument for structural studies of interfaces in ultrahigh vacuum (abstract)

tructural studies of interfaces with synchrotron x‐radiation (SXR) are presently carried out with several different methods which were developed in connection with SXR over the past decade: dynamical x‐ray diffraction using standing wavefields (XSW), kinematical surface diffraction (KSD), Fresnel reflection (FR), and absorption spectroscopy (SEXAFS, SXANES). These methods can determine different, and often complementary parameters of a system such as short‐/long‐range order, relaxation relative to bulk, structure normal/parallel to surface, valence state of surface atoms, roughness of a buried interface, etc. Accordingly, these different principles have also been combined to form new methods such as diffraction of evanescent x rays during total external reflection (DEXTER) and reflection EXAFS (REFLEXAFS). It is, therefore, highly desirable to have the opportunity to study the same interface system with different methods. On the other hand, preparation of a system in ultrahigh vacuum requires in most cases expensive bulky equipment and time consuming techniques. These considerations call for a setup which provides different experimental options. We have, therefore, installed at the beamline ROEMO I at HASYLAB a versatile UHV system for interface studies. A sample can either be prepared inside a chamber with three Knudsen cells for MBE growth and standard equipment for cleaning, heating and surface characterization by RHEED, or inside a chamber for aggressive gases. The first one is installed outside the interlock area providing access also when the ROEMO beam is on and the second one inside the interlock area.From either chamber the sample can be transferred into an analysis chamber at the ROEMO I station or into a transportable baby chamber. The analysis chamber can be used for measurements with standing waves, absorption spectroscopy and reflectivity measurements. Electrons are detected with a spherical analyzer and photons outside the vacuum through Be windows with a NaI detector for reflectivity and with a Si(Li) for scattered radiation. The baby chamber can be transported to other stations at HASYLAB which are equipped for kinematical surface diffraction and measure with high angular precision the intensity/angle correlation of many (often several hundred) different reflections. The baby chamber approach is used to avoid heavy and expensive diffractometer designs or sophisticated UHV angular feedthroughs. The baby chamber can also be used to measure with XSW, FR, and DEXTER. The performance of the system will be illustrated together with first results on different detection schemes