1. Department of Physics, Addis Ababa University, 1176 Addis Ababa, Ethiopia
2. Department of Physics, Kotebe University of Education, 31248 Addis Ababa, Ethiopia
3. Department of Chemistry, Debre Berhan University, P.O. Box 405, Debre Berhan, Ethiopia
4. Department of Chemistry and Chemical Engineering, Chalmers University of Technology, SE412 96 Goteborg, Sweden
5. Department of Physics, Laser Research Institute, Stellenbosch University, Private Bag
X1, Stellenbosch 7602, South Africa
6. School of Chemistry and Physics, University of Kwazulu Natal, Pietermaritzburg Campus, Private Bag X01, Scottsville 3209, South Africa
7. Department of Chemistry, Addis Ababa University, 33658 Addis Ababa, Ethiopia