The first optically detected magnetic resonance experiment wasdone using the ³P₁ state of gas phase mercury atoms. {J. Brossel and F.Bitter, Phys.Rev., 1952, 86, 308}. The sensitivity for a CW X-Band EPR experiment is 10¹⁰ to 10¹¹ spins, using optical detection single molecule experiments are possible. {Nature, 1993, 363. 242}. Basic principles.