<sod>Two less stable, higher energy [[excited state]]s are readily accessible from this ground state, again in accordance with [[Hund's rule of maximum multiplicity|Hund's first rule]];<ref>{{cite book | last1 = Frimer | first1 = Aryeh A. | title = Singlet Oxygen: Volume I, Physical-Chemical Aspects | date = 1985 | publisher = CRC Press | location = Boca Raton, Fla. | isbn = 9780849364396 | name-list-format = vanc | pages = 4–7 }}</ref> the first moves one of the high energy unpaired ground state electrons from one degenerate orbital to the other, where it "flips" and pairs the other, and creates a new state, a singlet state referred to as the <sup>1</sup>Δ<sub>g</sub> state (a [[term symbol]], where the preceding superscripted "1" indicates it as a singlet state).<ref name="Klán_2009"/><ref name="Atkins_2006"/> Alternatively, both electrons can remain in their degenerate ground state orbitals, but the spin of one can "flip" so that it is now opposite to the second (i.e., it is still in a separate degenerate orbital, but no longer of like spin); this also creates a new state, a singlet state referred to as the <sup>1</sup>Σ{{su|b=g|p=+}} state.<ref name="Klán_2009"/><ref name="Atkins_2006"/> The ground and first two singlet excited states of oxygen can be described by the simple scheme in the figure below<ref>For triplet ground state on right side of diagram, see C.E.Housecroft and A.G.Sharpe ''Inorganic Chemistry'', 2nd ed. (Pearson Prentice-Hall 2005), p.35 ISBN 0130-39913-2</ref><ref>For changes in singlet states on left and in centre, see [[F. Albert Cotton]] and [[Geoffrey Wilkinson]], ''Advanced Inorganic Chemistry'', 5th ed. (John Wiley 1988), p.452 ISBN 0-471-84997-9</ref> (a theoretical presentation and a simplification, as the experimentally observed excited states of oxygen are actually made up of combinations of electronic states).{{citation needed|date=August 2015}}<eod>
<sod>Two less stable, higher energy [[excited state]]s are readily accessible from this ground state, again in accordance with [[Hund's rule of maximum multiplicity|Hund's first rule]];<ref>{{cite book | last1 = Frimer | first1 = Aryeh A. | title = Singlet Oxygen: Volume I, Physical-Chemical Aspects | date = 1985 | publisher = CRC Press | location = Boca Raton, Fla. | isbn = 9780849364396 | name-list-format = vanc | pages = 4–7 }}</ref> the first moves one of the high energy unpaired ground state electrons from one degenerate orbital to the other, where it "flips" and pairs the other, and creates a new state, a singlet state referred to as the <sup>1</sup>Δ<sub>g</sub> state (a [[term symbol]], where the preceding superscripted "1" indicates it as a singlet state).<ref name="Klán_2009"/><ref name="Atkins_2006"/> Alternatively, both electrons can remain in their degenerate ground state orbitals, but the spin of one can "flip" so that it is now opposite to the second (i.e., it is still in a separate degenerate orbital, but no longer of like spin); this also creates a new state, a singlet state referred to as the <sup>1</sup>Σ{{su|b=g|p=+}} state.<ref name="Klán_2009"/><ref name="Atkins_2006"/> The ground and first two singlet excited states of oxygen can be described by the simple scheme in the figure below.<ref>For triplet ground state on right side of diagram, see C.E.Housecroft and A.G.Sharpe ''Inorganic Chemistry'', 2nd ed. (Pearson Prentice-Hall 2005), p.35 ISBN 0130-39913-2</ref><ref>For changes in singlet states on left and in centre, see [[F. Albert Cotton]] and [[Geoffrey Wilkinson]]. ''Advanced Inorganic Chemistry'', 5th ed. (John Wiley 1988), p.452 ISBN 0-471-84997-9</ref><eod>