JET-COOLED ELECTRONIC SPECTROSCOPY OF ZrC

Zirconium monocarbide is produced in high yield from the reaction of laser- ablated Zr metal with a helium/methane (1%) gas mixture under supersonic free jet conditions. The ground state is $X^{1}\\Sigma^{+}$, but the low-lying $a^{3}\\Sigma^{+}$ state has also been identified by high-resolution laser-induced fluorescence; this has $T_{0} = 690 cm^{-1}$ and $w_{e} = 885 cm^{-1}$. Molecular constants have been derived for $^{90}ZrC$ (51.45% natural abundance) for both states; the corresponding constants of the minor isotopomers $^{92}$ZrC (17.15%) and $^{94}$ZrC (17.38%) have also been determined for $a^{3}\\Sigma^{+}$, and scale isotopically within experimental error. The principal electronic bands in the visible region form a strongly perturbed $^{3}\\Pi_{r} - a^{3}\\Sigma^{+}$ system, of which the origin band has been analysed in detail. The perturbations in the $^{3}\\Pi_{r}$ state appear to be caused by singlet states.