A STUDY OF NbCr AND NbCr$^{-}$ BY ANION PHOTOELECTRON SPECTROSCOPY

Abstract

We report the 488 nm photoelectron spectrum of the NbCr$^{-}$ anion. For the $^{2}\Delta$ ground state of neutral NbCr, the short bond length (1.894 \AA) and high bond energy (D$_0$ 3.0263(6) eV) measured by R2PI spectroscopy indicate high order multiple bonding. We find that the NbCr$^{-}$ anion has a $^{1}\Sigma^{+}$ ground state, in which the "extra" electron occupies the (4d)$\delta$ bonding orbital, giving a $1\sigma^{2}1\pi^{4}1\delta^{4}2\sigma^{2}$ valence electron configuration and a formal bond order of 6. Low-lying excited states of NbCr (assigned as two $^{2}\Sigma^{+}$ states) and NbCr$^{-}$ ($^{3}\Delta$) are also observed. The spectra provide the electron affinity of NbCr, energies of the $^{2}\Sigma^{+}$ and the $^{3}\Delta$ excited states, vibrational frequencies for the NbCr and NbCr$^{-}$ ground states and for the $^{2}\Sigma^{+}$ excited states, and (from Franck-Condon analyses) differences among the bond lengths of the observed states. These results are compared with our previous data for the Group 5/6 congeners NbMo, VCr, and VMo, and with DFT predictions. We also report results for ongoing experiments on the flow tube reactions of the Group V metals Nb and Ta with butadiene, and the vibrationally-resolved photoelectron spectra of some of the organometallic reaction product anions.