1. Department
of Physics, Faculty of Science, The University of Ngaoundere, P.O. Box 454, Ngaoundere, Cameroon
2. Laboratoire
de Spectroscopie Atomique et Moléculaire, Faculté des
Sciences de Tunis, Université de Tunis El Manar, Campus Universitaire 1060, Tunis, Tunisia
3. University of Maroua, P.O. Box, 46, Maroua, Cameroon