Optical detection of spin-lattice relaxation and hfs in the excited E(E2) State of V2+ and Mn4+ in Al2O3

The hyperfine structure (hfs) and spin-lattice relaxation in the excited E(E2) states of V2+ and Mn4+ in Al2O3 have been studied by optical-detection techniques. This is an extension of an earlier study of EPR (electron paramagnetic resonance) in the excited E(E2) state of Cr3+ in Al2O3. The experimental results for all three isoelectronic ions in the same host lattice allow us to make meaningful comparisons with theoretical ideas of hfs and spin-lattice relaxation. A well-resolved hfs is found for V2+ and Mn4+, which is in contrast to the absence of hfs in the case of the isoelectronic Cr53 ion studied earlier. The values of the hyperfine-splitting parameter for the three ions can be adequately explained by considering the combined effects of the corepolarization hyperfine field, orbital hyperfine field, and dipolar hyperfine field. The experimental data are fitted to a simple spin Hamiltonian for an effective spin S=12 with |g|=2.21980.001, and |A|=(46.31.5)×10-4 cm-1 for V2+, and |g|=3.09590.0006, and |A|=(1233)×10-4 cm-1 for Mn4+. Since g0 for both ions, A could not be determined. In the temperature range in which T1 could be measured (1.4-2.15°K for V2+, 6-9°K for Mn4+), the spin-lattice relaxation times T1 were found to follow an Orbach process: T1=cexp(kT), where is the 2A-E splitting of the E2 level. 12.3 and 80 cm-1 for V2+ and Mn4+, respectively, and the measured values of c were 5.2×10-8 and 1.6×10-10 sec, respectively. For Cr3+, where 29 cm-1, the value of c, obtained earlier, is 3.8×10-9 sec. The parameter c is related to the direct-process relaxation time for the spontaneous transition between the non-time-reversed states 2A and E, T2AE, in which a phonon of energy is emitted. For the case where kT, this direct-process relaxation time should vary inversely as (V(1))23, where V(1) is the orbit-lattice coupling parameter which can be determined from static-strain measurements, and the measured relaxation time is found to be so governed. In the vanadium experiment, the EPR signal was so weak that several new experimental techniques had to be used to extract the signal from the noise. In the case of manganese, a circular-polarization method to detect the excited-state EPR signal had to be developed. This technique of detecting excited-state EPR by monitoring circularly polarized light should be applicable to a wide variety of materials characterized by inhomogeneously broadened emission lines.