REANALYSIS AND EXTENSION OF THE MNH A7-PI-X7-SIGMA+ (0, 0) BAND - FINE-STRUCTURE AND HYPERFINE-INDUCED ROTATIONAL BRANCHES

The A7Π-X7Σ+ (0, 0) band of MnH at 568 nm has been recorded by laser fluorescence excitation spectroscopy. The original rotational analysis of Nevin [Proc. R. Irish Acad.48A, 1-45 (1942); 50A, 123-137 (1945)] has been extended with some corrections at low J. Systematic internal hyperfine perturbations in the X7Σ+ state, caused by the ΔN = 0, ΔJ = ±1 matrix elements of the 55Mn hyperfine term in the Hamiltonian, have been observed in all seven electron spin components over the entire range of N″ studied. These perturbations destroy the "goodness" of J″ as a quantum number, giving rise to hyperfine-induced ΔJ = ±2 rotational branches and to observable energy shifts of the most severely affected levels. The A7Π state, with A = 40.5 cm-1 and B = 6.35 cm-1, evolves rapidly from Hund's case (a) to case (b) coupling, which produces anomalous branch patterns at low J. A total of 156 rotational branches have been identified and fitted by least squares to an effective Hamiltonian, providing precise values for the rotational and fine structure constants. Values of the principal constants determined in the fit are (1σ errors in units of the last digit are listed in parentheses): The fine structures of the A7Π and X7Σ+ states confirm the assignment of the A ← X transition as Mn 4pπ ← 4sσ in the presence of a spectator, nonbonding Mn 3d5 (6S) open core. © 1992.