JET-COOLED OPTICAL SPECTROSCOPY OF YTTRIUM MONOCARBIDE - EVIDENCE FORA (4)PI(I) GROUND-STATE

The first observation of yttrium monocarbide, YC, is reported. A numbe r of vibronic bands have been observed in the 635-650 nm region. Four of these bands have been investigated at a resolution of 120 MHz, and rotational constants have been derived. The bands originate from a com mon lower Omega=5/2 state which, on the basis of recent ab initio calc ulations [Shim, Pelino and Gingerich, J. Chem. Phys. 97 ( 1992) 9240], is the lowest lying spin-orbit component of the ground (IIi)-I-4 stat e. This state arises from a...10 sigma(2)11 sigma(1)12 sigma(1)5 pi(3) electron configuration. The nature of the excited states is also disc ussed.