Rotational and hyperfine structure in the A⁶Σ⁺–X⁶Σ⁺ electronic transition of MnO

The (0,0), (0,1), and (1,0) bands of the A 6 Σ + –X 6 Σ + transition of MnO near 5500 Å have been photographed at high dispersion, and partial rotational analyses carried out. The spectrum is unusually complicated because of the high electron multiplicity, the manganese hyperfine structure, and extensive rotational perturbations. It is found that hyperfine perturbations frequently accompany the rotational perturbations. These are a new type of perturbation, resulting from a mixing of the F 3 and F 4 electron spin components in the upper state through hyperfine matrix elements of the type ΔN = 0, ΔJ ± 1, ΔF = 0 as a result of rotational perturbations. Extra lines, obeying the selection rules ΔJ = 0, ± 2, are induced: these give the energy separations of the F 3 and F 4 electron spin components of the ground state. As a result it has been possible to determine the electron spin fine structure parameters for the two states despite the parallel polarization of the electronic transition. The reason for the great complexity of the A 6 Σ + ν = 0 level is the occurrence of a large avoided crossing near N = 26 with a level which is possibly a highly excited vibrational level of the ground state. The ground state Mn—O bond length is r 0 = 1.647 7 Å, and ΔG 1/2 (X 6 Σ + ) = 832.41 cm −1 .