This transition is strictly forbidden by spin, symmetry, and parity selection rules; hence, direct excitation of ground state oxygen by light to form singlet oxygen is very improbable.
2.
They are not observed due to strong but not absolute suppression, by spin-parity selection rules ( for beta decays and isomeric transitions ) or by the thickness of the potential barrier ( for alpha and cluster decays and spontaneous fission ) .-- ->
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