Introduction

Abstract

A detailed understanding of electronic structures of materials is very important from the view points of fundamental science as well as application. Among various means to probe electronic structures, the photon-in and electron-out measurements seem to be very useful since the energies, wave vectors, polarizations and time structures are well defined for photons and electrons. Historically speaking, the photoelectric effects were first observed more than a century ago [1] when the spark between two electrodes was facilitated by illumination of the negative electrode by ultraviolet light.