This phenomenon is used to cleave a ketone and abstract a gamma hydrogen to make a substituted cyclobutane in the Yang reaction. This phenomenon must be utilized to form cyclobutadiene (“cyclo-buta-di-ene”) in a pericyclic manner, since it’s otherwise (10[1])spin-forbidden per the (10[1])Woodward–Hoffmann rules. Either this phenomenon (10[1])or another represented by a capital delta are used to promote homolysis (“huh-MALL-uh-sis”) of small molecules, such as in the first step of allylic bromination. Reaction diagrams will typically represent this phenomenon with the letters “h nu” above (10[1])the reaction arrow. (10[3]-5[1])Compounds that decompose due to this phenomenon, such as hydrogen peroxide, are typically kept (10[1])in opaque or brown bottles. For 10 points, reactions involving what phenomenon are studied in photochemistry? ■END■ (10[1])