Table of contents

Hydrogen is the simplest atom, comprising only a proton and an electron. This simplicity means its properties can be calculated theoretically with impressive accuracy, currently of order 10^-11. At the same time, rapid advances in the techniques of laser spectroscopy have paved the way for experimental measurements at a comparable level of precision. This article begins with an outline of the problems encountered in the theoretical calculations and explains where the current uncertainties lie. Next, methods of Doppler-free laser spectroscopy are described and recent key experiments based on them reviewed. In the latest measurements, the Rydberg constant R_infinity , the scaling factor for all transition frequencies in hydrogen, has been determined with a precision approaching 10^-11. This means that all other transitions in hydrogen, ranging from the microwave to the ultraviolet, can be considered as metrological standards to this same level of precision.

Low energy electron microscopy (LEEM) is a new technique for surface imaging, based on the wave nature of the electron. Although it makes use of electron lenses as in conventional electron microscopes it differs from them in that the electrons have energies of the order 1-100 eV when they interact with the surface. As a consequence, the method is very surface-sensitive and the probing depth may be tuned by varying the energy. Contrast is mainly produced by diffraction. Resolution is determined mainly by the chromatic and spherical aberration of the decelerating/accelerating field in front of the specimen. LEEM is easily combined with low energy electron diffraction (LEED), photoemission electron microscopy (PEEM) and other emission microscopies. The specimen is easily accessible which allows a wide variety of in situ studies over a wide temperature range. The review gives an introduction to the basic electron-specimen interaction processes, to the instrumentation and to the factors which govern contrast and resolution. The remaining, larger part of the review deals with applications to clean surfaces, atom-surface interactions and thin films. Topics discussed include topography, phase transitions, adsorption, reaction, segregation, growth, sublimation and magnetic microstructure. Related techniques are also discussed briefly.