INVESTIGATION IN THE VUV RANGE OF THE EXCITATION EFFICIENCY OF THE TB3-3(AL-X,GA-Y)(5)O-12 HOST LATTICES( ION LUMINESCENCE IN Y)

VUV excitation spectra of the visible luminescence of Tb3+ activated Y -3(Al-x,Ga-y)(5)O-12 compounds and their diffuse reflectance spectra a re presented. The crystal field strength, the absorption threshold of the matrix and the external quantum efficiency (Q(ext)) associated wit h the different excitation mechanisms (4f(8) --> 4f(7)5d transitions i n the activator or interband transitions in the host) decrease when th e gallium concentration increases. Such variations are ascribed to an increase of the oxidizing character of the host matrix along with the gallium concentration. The decrease of the external quantum efficiency (Q(ext)) is discussed in terms of a trivalent rare-earth ion (RE(3+)) configurational coordinate model and related to the intervention of t he photoionized state [RE(3+) --> RE(4+) + e(-)]. The non radiative re laxation rate increases when this state is stabilized.