Cold cathodes for sealed-off CO2lasers

Cold cathodes for CO
2
lasers should possess a very low sputtering rate, have a minimum of negative ions in the sputtering products, and consume no oxygen. These conditions can be satisfied by selecting cathode materials from those elements of group I in the periodic table which form semiconducting oxide layers and then exposing these elements to a reducing gas mixture. Experimental results show that silver, internally oxidized silver-copper alloys, and pure copper satisfy most of these requirements in He-CO
2
-CO-Xe gas mixtures. Infrared CO and CO
2
gas analysis performed on simple gas discharge tubes using such cathodes indicate that laser lives of 10 000 h can be expected. In our laboratory a 1-W sealed-off CO
2
laser using a gas volume of only 50 cm
³
had an initial power output of 0.72 W which rose to 1.1 W after 3000 h and still gave 0.7 W after 12 000 hours; the laser used an internally oxidized silver-copper alloy cathode.