(formula presented) transitions of (formula presented) in elpasolite lattices

The (formula presented) emission spectra of (formula presented) (formula presented) and (formula presented) have been recorded at temperatures down to 10 K. The spectra of (formula presented) in the cubic host (formula presented) are the most clearly resolved, and 15 transitions to terminal crystal field levels of symmetry representations (formula presented) and (formula presented) have been observed and assigned, thereby inferring that the symmetry representation of the lowest (formula presented) crystal field level is (formula presented) Each transition is characterized by strong progressions in two totally symmetric vibrational modes. The relative displacement of the potential energy curves for the (formula presented) and (formula presented) crystal field levels, along the (formula presented) internal mode coordinate, is small, being only about 5 pm. The 10-K ultraviolet absorption spectra of (formula presented) are assigned to transitions from the (formula presented) (formula presented) electronic ground state to terminal (formula presented) crystal field levels of (formula presented) Nontotally symmetric gerade vibrational modes only provide minor intensity contributions. The large energy gap between the d-f emission and f-d absorption spectra of (formula presented) in the cubic elpasolite host is rationalized. The 8-K excitation spectra of (formula presented) and (formula presented) excited by synchrotron radiation, show that the transitions to (formula presented) fall into two groups. The energy levels and wave vectors of the (independent) (formula presented) and (formula presented) configurations of (formula presented) have been calculated using a model which includes spin-orbit coupling and crystal field and Coulomb interactions, as well as the configuration interaction of (formula presented) with (formula presented) Using the eigenvector of the predominantly high-spin, lowest excited crystal field level of (formula presented) the emission intensities are reasonably well simulated. However, the refinement of the (formula presented) absorption intensities requires a more detailed knowledge of the crystal field energy level scheme of (formula presented) The configuration interaction of (formula presented) with (formula presented) and (formula presented) is discussed.