The cathode-luminescence of ZnSe single crystals, grown from vapour phase and heat treated in Bi melts [ZnSe(Bi)] at 1200 K for 120h, and also in Bi melts doped by Al [ZnSe(Bi, Al)] were investigated at 4.2 K. The LO-phonon replicas of free excitons emission line dominate in cathode-luminescence spectra of the samples heat treated in Bi followed by water quenching. The lines of bound excitons series, I₁^s,d - nLO , were observed in the emission of all samples, including the original ones. It was found that plasmon replicas are not proper only for I₁^s - nLO lines, but, also, for I₁^d - nLO lines. The weak exciton-plasmon coupling is manifested in one plasmon Stokes side band shoulders of I₁^d - nLO lines. It is developed the theory of bound excitons multi-quantum optical transitions with participation of mixed plasmon-phonon oscillation modes. This theory allows for the calculation of the emission spectra form-function, without using any model. The theoretical spectra are in agreement with the experimental data.