ISOTOPE AND HYPERFINE-STRUCTURE IN THE ORANGE SYSTEM OF FEO - EVIDENCE FOR 2 5-DELTA(I) EXCITED-STATES

The orange system of FeO has been reinvestigated using low-temperature molecular beam laser-induced fluorescence spectra, obtained by supersonic jet cooling. Two new weak bands have been found, and analyses of some of the previously known bands extended. Measurements of the Fe-54-Fe-56 isotope shifts have been made for most of the bands, and the hyperfine structure of the low-J lines has been recorded for two of the strongest bands of (FeO)-Fe-57. The isotope shifts are consistent with the presence of two (5) Delta(i)-(5) Delta(i) transitions lying within 1000 cm(-1); the origins of the Omega = 4 spin components lie at 5583 and 6110 Angstrom, respectively. The hyperfine patterns and the spin-orbit structure indicate that the upper state electron configurations are (3d delta)(3) (3d pi)(2) (3d sigma)(1), (D5 Delta(i), 5583 Angstrom) and O(2p pi)(3) (4s sigma)(1) (3d delta)(3)(3d pi)(3), (D'(5) Delta(i), 6110 Angstrom). The bond length in the D' state (r(0) = 1.654 Angstrom) has been obtained from a deperturbation of the 6110 Angstrom band; it is only 0.035 Angstrom longer than in the ground state, which indicates that electron promotion between the two pi orbitals, nominally O(2p pi) and Fe(3d pi), has only a small effect on the strength of the bonding. The new isotope data still do not clarify the vibrational assignments of the higher levels, which are disorganized by extensive electronic perturbations. (C) 1995 Academic Press, Inc.