Photoluminescence activity in natural silica excited in the vacuum-UV range

We report an experimental study on the optical absorption and photoluminesc ence detected in samples of natural silica. Our results show that the two e mission bands, beta (similar to 3.1 eV) and alpha(E) (similar to 4.3 eV), h ave an excitation profile in the Vacuum ultraviolet region with a maximum a t similar to 7.5 eV. This excitation profile indicates that, in terms of en ergy levels of the luminescent defect, there is a transition from a ground state, S-0, to a second excited state, S-2 able to excite PL emission, in a ddition to the well known transition corresponding to the optical absorptio n band, B-2 beta Our data are in a quantitative agreement with 'ab initio' calculations carried out for a two-fold coordinated Ge defect in SiO2 and e xclude a correlation between the S-0 --> S-2 transition and the 7.6 eV E ba nd. (C) 1999 Elsevier Science B.V. All rights reserved.