Persistent Photochemical Hole-Burning in the R-Lines of Tetravalent Manganese in Submicron α-Al2O3

Persistent spectral hole-burning in the R-lines of tetravalent manganese in X-irradiated submicron corundum powder is reported for the first time. Two samples were investigated, namely, nominally 0.4 atom % Mn3+-doped and 0.4 atom % Mn4+, Mg2+-codoped α-Al2O3. The samples were produced by a simple combustion reaction method at 600 °C. Spectral hole-burning properties of Mn4+ ions burned via the R1 and R2 lines were measured either by the photoluminescence excitation mode via monitoring the luminescence signal of a vibrational sideband at ∼694 nm or in luminescence, with blue light excitation after the burn period. The hole widths were studied as a function of burn fluence and temperature in the range from 2 to 90 K. We propose a photochemical hole-burning mechanism for both samples based on photoionization and photoreduction of the tetravalent manganese ion. Interestingly, the 0.4 atom % Mn4+, Mg2+ sample displayed a higher hole-burning efficiency than the Mn3+ sample. The hole widths obtained for the persistent spectral holes in the present powder samples were approximately 300 times broader at 2.2 ± 0.1 GHz compared to the ∼6 MHz width observed in transient hole-burning in a macroscopic crystal [Riesen, H. Chem. Phys. Lett. 2011, 515(44), 241−244]. We assign this effect to the disruption of the crystal lattice upon the valence state change of the Mn4+ ion in the hole-burning process. From the hole width, we obtained a decay time of 0.72 ns for the 2A̅ → E̅ one-phonon relaxation process within the split 2E excited state. The stability of the spectral holes burned via the R1 and R2 lines was investigated for both samples, and we observed a decay of about 20% within 2 h, indicating slow spontaneous hole-filling. A hole area recovery of ∼70 and ∼64% was observed after a temperature cycle from 2.5 to 70 and 90 K for the 0.4 atom % Mn3+ and 0.4 atom % Mn4+, Mg2+ samples, respectively.