Lowest unoccupied molecular orbital photochemistry

Two orbitals play a special role and are known as the Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Molecular Orbital (LUMO), these labels referring of course to the situation in the ground state. The transition of an electron between these orbitals corresponds to the lowest excited state(s), e.g. triplet Tj and singlet S1 which are of special importance in the photophysics and photochemistry of polyatomic molecules. 

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