A STUDY OF VMn AND $VMn^{-}$ BY NEGATIVE ION PHOTOELECTRON SPECTROSCOPY

Author Institution: Department of Chemistry, University of MinnesotaWe report the 476-514 nm negative ion photoelectron spectra of VMn, a mixed early-late transition metal dimer which is isoelectronic with $Cr_{2}$, The $VMn^{-}$ anions were prepared in a flowing afterglow ion-molecule reactor through the addition of $Mu_{2}(CO)_{10}$ vapor downstream of a vanadium cathode discharge source. The spectrum is dominated by two intense transitions to the ground state and to one excited electronic state of neutral VMn, whose electron affinity is measured to be $0.524 \pm 0.005 $eV. Its ground state vibrational frequency is $570 \pm 20 cm^{-1}$ vibrational spacings observed up to $v=6$ do not fit a Morse potential. An excited state observed at $14140 \pm 30 cm^{-1}$ shows a much higher frequency of $680 \pm 20 cm^{-1}$, and hot bands reveal a reduced frequency of $500 \pm 20 cm^{-1}$ in the VMn anion. The similarities of these results to those previously $obtained^{1}$ for $Cr_{2}$ suggest that VMn also has a nominally hextuply bonded $^{1}\Sigma_{g}{^{*}}(3d\sigma_{g})^{2}(3d\pi_{u})^{4}(3d\delta_{g})^{4}(4s\sigma_{g})^{2}$ ground state, with the extra electron in the $^{2}\Sigma_{u}{^{+}}$ anion occupying an sp hybridized $\sigma_{u}$ orbital. Similarly, the observed VMn excited state is assigned as a $(4s \sigma_{g})^{1} (sp\sigma_{u}{^{+}})^{1} {^{3}}\Sigma_{u}{^{+}}$ state whose increased vibrational frequency reflect a reduced 4s electron density in the $d-d$ bonding region. Additional weaker transitions observed at lower binding energies display a more complex pattern than those observed for $Cr_{2}$ and are likely due, at least in part, to d electron detachment transitions to lower-lying excited states in VMn. Hot band intensities observed in spectra obtained at different ion source temperatures indicate a surprising lack of vibrational thermal equilibration of the anions