A CO.sub.2 slab laser is disclosed having a pair of spaced apart electrodes defining a rectangular discharge region. RF energy is fed through the electrodes to excite the CO.sub.2 gas. A pair of mirrors are located adjacent the electrodes to define the resonant cavity. A recombinant surface is placed between the ends of the electrodes and the mirrors to quench oxidizing species generated by the discharge before they reach the mirrors. In this manner, the degradation of the mirrors is reduced so that the high power performance of the laser can be maintained. The recombinant surfaces can be defined by forming extension regions at the end of the electrodes between which the discharge is minimized. Alternatively, a mirror shield having a beam transmitting aperture can be used to quench the oxidizing species.