https://doi.org/10.1351/goldbook.S05695
Oxygen molecule (dioxygen), O2, in an excited
singlet state
. The
ground state
of O2 is a triplet (3Σg). The two
metastable
singlet states derived from the ground-state configuration are 1Δg and 1Σg+ (the latter with the higher energy).
Notes:
  1. Use of the term singlet oxygen alone, without mention of the chemical species is discouraged since it can also refer to an oxygen atom in a 1S or 1D
    excited state
    . The oxygen atom
    ground state
    is a triplet 3P state and the 1S and 1D states are also derived from the ground-state configuration.
  2. Triplet state
    quenching
    by triplet dioxygen (i.e., a process of
    photosensitization
    ) is the most common procedure for the production of singlet molecular dioxygen in solution. For many chemical species, the efficiency with which the
    triplet state
    is quenched by triplet dioxygen and, independently, the efficiency with singlet molecular dioxygen is produced, is controlled by the spin-
    statistical factor
    .
Source:
PAC, 2007, 79, 293. (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 420 [Terms] [Paper]