JET-COOLED ELECTRONIC SPECTROSCOPY OF SEVERAL MONOLIGATED TRANSITION METALS: CrCCH, $NiCH_{3}$, $CrCCH_{3}$, AND $NiC_{2}H_{4}$

Abstract

In attempts to find electronic Transitions in the diatomic metal carbides NiC, CrC, and HfC via R2PI spectroscopy by vaporizing the pure metal in a supersonic expansion of He seeded with 3% methane we have discovered optical transitions in a number of metal ligand complexes, some with more than one carbon atom. The polyatomic species for which we have recorded spectra are CrCCH, $NiCH_{3}, CrCH_{3}, Ni-C_{2}H_{2}, CrC_{3}H_{2} CrCH_{2}$, and $HfC_{2}$. The First Four of these have yielded by far the most interpretable spectra. In its ground state CrCCH is almost certainly linear with considerable ionic $(Cr^{\prime}CCH^{-})$ character. All five {d} electron are most likely high spin coupled to give $\tilde{X}^{6}\Sigma^{+}$. The optical spectrum of CrCCH between $11,000 cm^{-1}$ and $13,400 cm^{-1}$ displays a progression in the Cr-CCH stretch with $\omega_{e} = 426.4\pm 1.1 cm^{-1}$ and $\omega_{e} X_{e} = 0.70\pm 0.17 cm^{-1}$. All of the main bands are split into three sub-bands with a spacing of approximately $10 cm^{-1}$ and $17 cm^{-1}$. This is suggestive of the expected zero field splitting of a $6\Sigma^{+}$ state into its $\omega = 1/2, 3/2$, and 5/2 components and leads us to believe we are observing a $^{6}\Sigma^{-} \leftarrow {^{6}}\Sigma^{+}$ transition. Three of the six observed bands have been rotationally resolved, and analysis is under way. In the case of $CrCH_{3}$, no upper state progression has been identified. The spectrum is dominated by sex bands within $500 cm^{-1}$ of the origin, which may be indicative of a sextet to sextet transection in which one of the states has residual orbital angular momentum that is causing a spin-orbit splitting into six spin components. These six bands are repeated about $510 cm^{-1}$ to the red when the region between $11,500 cm^{-1}$ and $12,500 cm^{-1}$ is scanned under conditions that make the molecular beam hot. We have deduced a ground state Cr-CH3 stretching frequency of $509.2\pm 1.4 cm^{-1}$ from these features. Nickel methyl displays sharper features than $CrCH_{3}$ as well as an upper state progression with $\omega_{e} =455.3 cm^{-1}$, which is presumably a $Ni-CH_{3}$, stretching frequency. Finally, the vibrational spectrum of $Ni-C_{2}H_{4}$, is quite complicated, but the rotationally resolved spectrum of several bands is very simple.