Singlet sigma: The "other" singlet oxygen in solution

The lowest excited electronic state of molecular oxygen, O-2(a(1)Delta(g)), is often called simply singlet oxygen. This singlet delta state is an ackn owledged and well-studied intermediate in many solution-phase photosystems. However er, the second excited electronic state of oxygen, O-2(b(1)Sigma(g )(+)), is also a singlet. It has recently become possible to monitor this s inglet sigma state in solution, which, in combination with studies of the s inglet delta state, contributes to a better understanding of a variety of g eneral problems in chemistry.