Singlet oxygen is the common name used for the two metastable states of molecular oxygen (O2) with higher energy than the ground state triplet oxygen [1]. The energy difference between the lowest energy of O2 in the singlet state and the lowest energy in the triplet state is about 3625 kelvin (Te (a¹Δg <- X³Σg-) = 7918.1 cm-1.)

http://en.wikipedia.org/wiki/Singlet_oxygen