Knowledge Bank

The spectra of small, unsaturated transition metal compounds are generally complicated and, despite considerable progress in modern high-resolution laser spectroscopy, for from understood. One of the reasons for the complexity of these spectra is the presence of a multitude of low-lying molecular electronic states. These emerge from the energetic proximity of the $ndi+2,ndi+1(n+1)s1$, and $ndi(n+1)s2$ configurations in transition metal atoms and the large number of ways in which their angular momenta can be coupled. Moreever, spin-orbit interaction causes heavy perturbations between the densely speed electronic sates. Electronic structure calculations including spin-orbit and rotational coupling can be of considerable help in explaining and assigning these complex spectra. By starting of in a basis of electronic $\Lambda \Sigma$ eigenstates we avoid the computation of radial non-adiabatic coupling matrix elements caused by the spin - dependent parts of the relativistic Hamiltonians. Electron correlation is included by applying general purpose electronic structure codes. Spin-orbit a nd rotational interactions between the electronic states are accounted for in a second step. After a transformation of these off-diagonal potentials to a basis of unperturbed rovibronic functions the J-dependent coupling matrices are diagonal zed. Eigenvalues obtained in this way can directly be compared with measured term values. Applications to the spectra of NiH, PdH, and PtH will be presented.