N. C. and D.-D. H. Yang discovered a reaction in which this phenomenon causes C-H functionalization. Promoting reactions involving this phenomenon is the typical use of benzophenone. The first organic reaction known to involve this phenomenon is the rearrangement of santonin. This phenomenon cleaves a carbonyl in the usual forms of both types of Norrish reactions. In one biosynthetic pathway, this phenomenon causes a ring opening in (*) 7-dehydrocholesterol. (10[1])This phenomenon causes the formation of a cyclobutane ring connecting two adjacent pyrimidines, creating thymine dimers and thus causing DNA damage. (10[1])This phenomenon causes the formation of molecular oxygen and oxygen radicals (-5[1])as part of an atmospheric cycle that absorbs this phenomenon. For 10 points, (10[1])name this component of sunlight used in the biosynthesis of vitamin D. ■END■ (10[1])