- We report on a detailed gain measurements and analysis of the extremely efficient supersonic chemical oxygen-iodine laser (COIL) recently developed in our laboratory (Appl. Phys. Lett., 85, 5851 (2004)). The power and spatial distributions of the gain and temperature across the flow were measured for different supersonic nozzles with both staggered and non-staggered iodine injection holes, different injection locations along the flow and nozzle throat-heights. 40.0% efficiency was measured for 1 s at the early stage of operation, followed by a sustained 35.5% chemical efficiency for 20 s. By carefully studying and optimizing the operation of the chemical generator, 0.73 yield of singlet oxygen was obtained for conditions corresponding to the highest efficiency.