A method of investigating the mechanism of extremely rapid photochemical reactions involving the formation of free radicals (both inorganic and organic) by irradiating a given reaction mixture with a flash of high-intensity light, thus producing the short-lived radicals that activate photochemical reactions. These products are instantaneously analyzed spectroscopically, which permits identification of the radical species from the spectra obtained. The time lapses involved in this technique are approximately 1/100,000 second. It has also been applied to study of the exceedingly fast reactions occurring in flames and explosions.See Photochemistry; Free Radical; Photolysis.