Knowledge Bank

Measurements of the hyperfine parameters of four excited electronic states of NbN have been carried out. Including previous work on the $B3-X3\Delta system1$. hyperfine parameters have now been measured for a total of six electronic states, namely $\Delta \pi \mathrm{<mrow\; data-mjx-texclass="ORD">1,3</mrow>},\Phi ,\sigma 0a1\Delta$ and $X3\Delta$. It is not simple to interpret these parameters without considering higher order spin-orbit effects. For example, the $\Delta \pi \mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}\Pi$ state lies within the spin-orbit structure of the $\delta \pi \mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Pi$ state in zero order, and so strong is the interaction between them that the energy order of the two lowest spin components of the $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Pi$ state is reversed. Extensive wavelength-resolved fluorescence studies were needed to understand these interactions. These wavelength-resolved fluorescence studies have also led to the discovery of three new low-lying electronic states, $\delta 2\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Sigma ,\delta 2\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}\Gamma$ and $\sigma 2\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}\Sigma +$, and to the location of $\sigma \delta \mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}\Delta$. Above $19,000cm-1$ the energy level patterns are severely distorted by the onset of charge transfer transitions: the spectra become very perturbed and complicated in this region. The pattern of states found in NbN is likely to be typical of the early diatomic 4d metal compounds; fairly well-behaved low-lying states can be associated with unpaired electrons in the metal orbitals, but the regularity of the structure will break down totally when a ligand electron is promoted to the metal orbitals.