Energy Transfer in MAGNESIUM-OXIDE:CHROMIUM(+3)

Available from UMI in association with The British Library. Requires signed TDF. Using steady state and time resolved luminescence spectroscopy energy transfer between Cr^ {+3} ions in MgO was investigated. Five sharp zero-phonon lines observed in the excitation spectrum of the < 100> (Cr _sp{Mg}{cdot } V_sp{Mg} {''} Cr_sp {rm Mg}{cdot}) centre are shown to originate from three distinct donor centres from which excitation is transferred to the acceptor < 100> (Cr_sp {Mg}{cdot} V _sp{Mg}{' '} Cr_sp{Mg}{cdot}) centre. The energy transfer process is non resonant involving either emission of a single phonon or the Raman process. For each donor -acceptor pair, the energy transfer rate is measured by analysing the time dependence of luminescence from the acceptor. The magnitude of the electric quadrupole-quadrupole and the electric dipole-quadrupole donor-acceptor interactions, calculated using group and perturbation theories, is insufficient to account for the observed energy transfer rate. An antiferromagnetic superexchange donor-acceptor interaction governs the transfer process. On the basis of such a mechanism, geometric models are proposed which illustrate the relative positions of the donor and acceptor ions in each of the three systems. In addition to this localised process, energy transfer between randomly distributed Cr^{+3} ions in MgO is investigated. Spectroscopic measurements show that energy is transferred from Cr^ {+3} ions in octahedral symmetry sites to tetragonal and orthorhombic symmetry sites, the extent of transfer is found to be small. Transient grating spectroscopic measurements show that spatial energy migration among resonant Cr^{+3} ion is negligible at low Cr^{+3} concentrations but occurs when the Cr^{+3} concentration is increased.