Determination of allowed transitions types and the optical parameters of Se–Ge–Ag chalcogenide films Amira M. Shakra and Essam G. El-Metwally Eur. Phys. J. B, 91 10 (2018) 245 Published online: 10 October 2018 DOI: 10.1140/epjb/e2018-90273-7