Orbach Relaxation and Hyperfine Structure in the ExcitedE¯(E2)State ofCr3+inAl2O3

Abstract

Several methods of optical detection of electron paramagnetic resonance (epr) of excited states in crystals are outlined. The application of these methods to observation of epr in the excited metastable $\overline{E}(^{2}E)$ state of ${\mathrm{Cr}}^{3+}$ in ${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$ is described. The most recent technique made use of a high-resolution optical spectrometer to monitor the change of intensity of one of the Zeeman components of the ${R}_{1}$ fluorescent light [$\overline{E}(^{2}E)\ensuremath{\rightarrow}^{4}A_{2}$] as $\overline{E}$ was saturated by the microwaves when the magnetic field was swept through resonance. A sample doped with Cr enriched to 92% ${\mathrm{Cr}}^{53}$ was used to study the hfs in the $\overline{E}$ state. An upper limit of 25 kG is set on the hyperfine field per unit spin ($\frac{A}{{g}_{I}{\ensuremath{\beta}}_{N}}$). This small value compared to that of 200 kG for the ground-state results from a near cancellation of the negative core polarization field by an equally large but positive orbital field. The spin-lattice relaxation time ${T}_{1}$ in the $\overline{E}$ level was determined by observing the recovery of the light signal after the microwave saturating pulse was switched off. A continuous averaging technique using a multichannel analyzer was employed to improve the signal-to-noise ratio. ${T}_{1}$ is found to very accurately follow an Orbach process with ${T}_{1}=3.8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}{e}^{\frac{\ensuremath{\Delta}}{\mathrm{kT}}}$ sec where $\ensuremath{\Delta}=28.8\ifmmode\pm\else\textpm\fi{}1.0$ ${\mathrm{cm}}^{\ensuremath{-}1}$, i.e., the separation between $2\overline{A}$ and $\overline{E}$. In the range of temperature where the Orbach process was observed (4.2\ifmmode^\circ\else\textdegree\fi{}K \ensuremath{\rightarrow} 2.8\ifmmode^\circ\else\textdegree\fi{}K), ${T}_{1}$ was found to be essentially independent of concentration (0.05 and 0.005%) and frequency (24 and 48 kMc/sec), as anticipated. The coefficient 3.8\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}9}$ sec is significant in that it measures the maximum transition time from $2\overline{A}$ to $\overline{E}$, ${T}_{2\overline{A}\ensuremath{\rightarrow}\overline{E}}$, with spontaneous emission of a 29-${\mathrm{cm}}^{\ensuremath{-}1}$ phonon. This time plays an important role in considering the production of 29-${\mathrm{cm}}^{\ensuremath{-}1}$ acoustic phonons by the pumping light. Initial evidence is cited for the generation by the pumping light of a narrow-band hot-phonon spike at 29 ${\mathrm{cm}}^{\ensuremath{-}1}$ as seen by the spin-lattice relaxation in the $\overline{E}$ level.