The results reviewed in this article were obtained by means of the ESCA technique at the Institute of Physics, University of Uppsala, Uppsala, Sweden and at the Department of Physics, Vanderbilt University, Nashville, Tennessee, USA.The ESCA technique is basically the study of induced emission of photo and Auger electrons from a sample irradiated with x-rays. If the incident radiation is monochromatic (e.g. an x-ray emission line) the spectrum of these electrons gives precise information about the energy states of the electrons in the sample. To extract this information, high resolution electron spectroscopy is necessary. Instruments for such spectroscopy have therefore been extensively developed during the last decade.