Vacuum ultraviolet (VUV) photolysis of N2O immersed in high‐pressure Ar has been used to produce O(1S0) at high density. A two‐chamber apparatus was used to prevent the effects of electron and ion collision in the experimental volume, thereby providing a much clearer kinetic situation. The gain profile and stimulated emission cross section near 558 nm and the upper‐state lifetime were determined for ArO.