CaGa 2 S 4 compounds doped with rare-earth elements (REE) exhibit luminescence with various colors. The emission process between the transition levels of REE ions has ever been extensively studied. We have measured the photoconductivity (PC) to confirm the locations of the 4f ground levels of the Eu 2+ and Ce 3+ions in the band gap of the host compounds of CaGa 2 S 4 for the first time.

1 Introduction Alkaline-earth thiogallates doped with rare earth elements are well known as good photoluminescent materials [1-3]. These have a rather large band gap of above 4 eV giving advantages of light emission of various colors associated with the de-excitation process between inner-level transitions of the rare earth ions of the Lanthanide series (hereafter abbreviated as RE ions).Application-oriented work as phosphor materials aside, most basic research work has been devoted to the determination of the electronic energy level ladders of the RE ions in the different host materials of this family of compounds. Dorenbos published a series of work in 2003 to present the electronic energy levels of the RE ions in different host materials through bibliographic study of ever published data [4]. He concluded that the relative relation of energy levels of the RE ions has a similarity in many different host materials and may be expressed in a simple form by only taking into account the red shifts in the host [4]. Actually the exact energy level of each RE ion in the band gap of a