S. M. SICKAFOOSE, M. D. MORSE, Department of Chemistry, University of Utah, Salt Lake City, UT 84112; AND D. A. HALES, Department of Chemistry, Hendrix College, Conway, AR 72032.

Resonant two-photon ionization spectroscopy has been used to provide the first rotationally resolved work involving multiply-bonded mixed early-late (or Brewer-Engel) transition metal dimers. The mixed metal dimers TiCo and ZrCo are shown to possess 2 Sigma+ ground states, in agreement with previous matrix isolation ESR studies. Two previously observed vibronic bands in ZrCo were rotationally resolved, as was one of several newly discovered vibronic transitions in TiCo. Ground state rotational constants for 48Ti59Co and 90Zr59Co were measured to be B0 '' = 0.18615(8) cm-1 and B0 '' = 0.11909(7) cm-1, giving r0 '' = 1.8508(4) Å and r0 '' = 1.9941(6) Å, respectively. In both molecules the ground state conforms to the Hund's case b beta S coupling scheme, with Fermi contact interactions between the unpaired sigma electron and the 59Co (I = 7/2) nucleus of bF '' = 0.0468(7) cm-1 for 48Ti 59Co and bF '' = 0.0551(7) cm-1 for 90Zr 59Co. These values indicate that the unpaired electron occupies a sigma orbital having 32% and 37.5% Co 4s character in TiCo and ZrCo, respectively. All three of the rotationally resolved bands are 2 Pi 3/2 leftarrow X2 Sigma + transitions. For 90Zr59Co the 0 - 0 and 1 - 0 bands were resolved at nu0 = 10496.1690(44) cm-1 and 10884.4266(24) cm-1, giving B0 ' = 0.11922(8) cm-1 and B1' = 0.11962(6) cm-1. For 48Ti59Co the rotationally resolved band had nu0 = 9873.8342(19) cm-1 and B' = 0.19107(6) cm-1. These results will be discussed in the context of prior work on these and related molecules.