Yttrium oxide upconverting phosphors. 3. Upconversion luminescent emission from europium-doped yttrium oxide under 632.8 nm light excitation

Anti-Stokes and Stokes emissions have been observed from cubic Y2O3:EU3+ under 632.8 nm excitation. All the emission features exhibited a marked thermal dependence, decreasing in intensity as the temperature was lowered. Arrhenius plots of this thermal behavior indicated that the EU3+ ions were thermally excited to a low-lying level ca. 1300 cm(-1) above the ground state; this low-lying excited state was assigned to the F-7(2) level. Subsequent absorption of a 632.8 nm photon by the thermally excited EU3+ ion promoted it to its D-5(0) level. Anti-Stokes emission bands at wavelengths longer than 580 nm exhibited a one photon dependence on the 632.8 nm exciting light and are assigned to the D-5(0) --> F-7(0), D-5(0) --> F-7(1), and D-5(0) --> F-7(2), transitions of the Eu3+ ion. Stokes emission bands also showing a one photon dependence were assigned to the D-5(0) --> F-7(3) and D-5(0) --> F-7(4) transitions. Upconversion anti-Stokes emission bands were observed at wavelengths shorter than 580 nm and showed a two photon dependence on the 632.8 nm exciting light. These emission bands are assigned to Eu3+ ions on the C-2 sites of cubic Y2O3 with the only exception being a band at 582.2 nm. Although this emission band showed a thermal behavior similar to that of the other emission bands, it is assigned to the Eu3+ ion on the S-6 site of cubic Y2O3, in keeping with previous assignments of others.