In e-beam sustained diffuse discharges in gas mixtures which contain small additives of electronegative gases, the discharge characteristic (current density versus reduced field strength) may exhibit negative differential conductivity (NDC) depending on the source function and the concentration of attacher gas. In discharges exhibiting negative differential conductivity, electron depleted domains of high electric field intensity are formed in the discharge. The results of electrical and optical measurements performed on an e-beam sustained diffuse discharge in a gas mixture of 2% C2F6 in 1 atm Ar are presented and compared with theoretical predictions. The steady state current density (J) versus reduced field strength (E/N) exhibits a strong negative differential conductivity in an E/N range of 2.5 Td < E/N < 5 Td. Time resolved photographs taken in this E/N range show distinct luminous layers perpendicular to the discharge axis.