Resonant two-photon ionization spectroscopy of jet-cooled OsC

The optical spectrum of diatomic OsC has been investigated for the first time, with transitions recorded in the range from 17 390 to 22 990 cm?1. Six bands were rotationally resolved and analyzed to obtain ground and excited state rotational constants and bond lengths. Spectra for six OsC isotopomers, ???Os??C (40.3% natural abundance), ???Os??C(26.0%), 189Os12C(16.0%), 188Os12C(13.1%), 187Os12C(1.9%), and 186Os12C(1.6%), were recorded and rotationally analyzed. The ground state was found to be X 3?3, deriving from the 4?316?1 electronic configuration. Four bands were found to originate from the X 3?3 ground state, giving B0"=0.533 492(33) cm?1 and r0"=1.672 67(5) ? for the 192Os12C isotopomer (1? error limits); two of these, the 0-0[19.1]2?X 3?3 and 1-0[19.1]2?X 3?3 bands, form a vibrational progression with ?G'1/2=953.019 cm?1. The remaining two bands were identified as originating from an ?"=0 level that remains populated in the supersonic expansion. This level is assigned as the low-lying A 3?0+ ? state, which derives from the 4? 216?2 electronic configuration. The OsC molecule differs from the isovalent RuC molecule in having an X 3?3 ground state, rather than the X 2? 4, 1?+ ground state found in RuC. This difference in electronic structure is due to the relativistic stabilization of the 6s orbital in Os, an effect which favors occupation of the 6s-like 16? orbital. The relativistic stabilization of the 16? orbital also lowers the energy of the 4? 216?2, 3?? term, allowing this term to remain populated in the supersonically cooled molecular beam.