Another defect center has been observed in micro-Raman and photoluminescence (PL) studies of diamond films deposited on silicon substrates by microwave plasma and hot filament chemical-vapor deposition (HFCVD). The strong, to our knowledge, previously unreported PL emission at 775 nm (1.60 eV) is observed from the HFCVD film only. This impurity/defect center is shown to be distinct from the well-known Si-related defect at 735 nm (1.69 eV) which was observed in both films. The possibility of an interfacial SiC layer being the source of the PL emission is also investigated. Possible impurity/defect complexes responsible for this optical defect center are discussed. © 1994 The American Physical Society.