ELECTRONIC SPECTROSCOPY OF MOLYBDENUM MONOCARBIDE

Abstract

In a continuing effort to understand the nature of the metal carbon bond in transition metal monocarbides we have undertaken the first spectroscopic investigation of MoC by resonant two-photon ionization spectroscopy. Molybdenum carbide was produced by laser vaporization of a Mo sample disk in a supersonic expansion of He and $3\% CH_{4}$. Using 6.42 eV photons for photoionization we have observed a number of transitions between $17 700 cm^{-1}$ and $24 000 cm^{-1}$. Twenty four of the observed bands have been studied at sufficient resolution ($0.04 cm^{-1}$) to allow the rotational structure of each transition to be resolved. An analysis of the isotope shifts has allowed a tentative identification of three band systems among the 24 observed transitions. A number of the observed bands have remained ungrouped. Every rotationally resolved band appears to be an $\Omega = 1 \leftarrow 0$ transition. Given that the ground state of $NbC^{a}$ is known to be $11 \sigma^{2}2 \delta^{1}, {^{2}}\Delta_{***}$ and that of $RuC^{b}$ is known to be $11 \sigma^{2}2 \delta^{4}, {^{1}}\Sigma_{+}$, it seems likely that the addition of another electron on moving from NbC to MoC yields for a ground state the $\Omega = 0^{+}$ component of a $^{3}\Sigma^{-}$ term. This is consistent with the observation that every transition in this study originates from an $\Omega = 0$ state. The $X^{3}\Sigma^{-}$ rotational constant for the most abundant isotope, $^{98}Mo^{12}C$, has been determined to be $0.553640 \pm 0.000055 cm^{-1}$. This corresponds to a ground state bond length of $1.687719 \pm 0.000084$ {\AA}.