Knowledge Bank

ZrC is readily produced in supersonic expansion via reaction of 1% methane (in helium) with laser-ablated zirconium metal. Laser-induced fluorescence excitation and dispersed emission spectra have been recorded for bands in the region $630-520$ nm $(16000-19000cm-1)$. The four lowest electronic states lie below $2500cm-1$ and appear to represent all possible arrangements of two electrons in the two nearly degenerate $11\sigma$ and $12\sigma$ orbitals (both formed from $Zr5s\sigma +C2p\sigma )$; the evidence is that the small spin-spin interaction of the $X3\Sigma +$ ground state $(\lambda 0=0.5142cm-1,r0=1.807$ {\AA} for $\mathrm{<mrow\; data-mjx-texclass="ORD">90</mrow>}Zr12C)$ is consistent with a $\sigma \sigma \prime$ configuration, while the anomalous $\mathrm{<mrow\; data-mjx-texclass="ORD">12</mrow>}C/13C$ isotope shifts and vibrational intervals of the remaining three states show that they all have the same $(1\Sigma +)$ symmetry. In particular, the tightly bound $a1\Sigma +$ state ($T0=187.83cm-1,r0=1.739$ {\AA}) arises from the $11\sigma 2$ closed shell configuration. Moreover, this molecular orbital scheme is consistent with the confused level structure above $16000cm-1$. At least four close-lying II states (two singlets and two triplets) are observed and can be explained by the promotion of either $\sigma$ electron to a $\pi$ orbital; these exhibit both isoconfigurational $(1\Pi /3\Pi 1)$ and interconfigurational $(1\Pi /1\Pi$ or $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Pi /3\Pi )$ interactions. Details of the rotational and $\mathrm{<mrow\; data-mjx-texclass="ORD">91</mrow>}Zr(I=5/2)$ hyperfine structures will be presented.