ELECTRONIC SPECTROSCOPY OF MOLYBDENUM MONOCARBIDE
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Date
1997
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Ohio State University
Abstract
In a continuation of our study 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 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. Because the R2PI experiment is mass selective we have been able to record rotationally resolved spectra for each of the seven major isotopes of MoC independently for nearly every band. 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 $11\sigma^{2} 2\delta^{1}, ^{2}\Delta_{r}$. 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 in 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 $0.55404 \pm 0.00012\;cm^{-1}$. This corresponds to a ground state bond length of $1.68711 \pm 0.00019$ {\AA}. In very recent and uncompleted work we have discovered two vibronic transitions in isoelectronic CrC near $12,000\;cm^{-1}$. This represents the third $3d$ transition metal monocarbide to be identified by its electronic gas phase spectrum.
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$^{a}$ B. Simard, P. I. Presunka, H. P. Loock, A B\'erces, and O. Launila, submitted to J. Chem. Phys. (1997). $^{b}$ J. Langenberg, R. Dabell, L. Shao, D. Dreessen, M. D. Morse, in preparation.
Author Institution: University of Utah
Author Institution: University of Utah