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physica status solidi (b)

On the effect of exchange interaction on the impurity ion optical spectrum

Anatoliy E. Nikiforov, V. I. Cherepanov

Published Mar 1, 1971• 10 citations
Publisher

Abstract

Abstract The method of effective Hamiltonian (EFH) is applied to the excited state (2E4A2) fine structure of Cr3+–Cr3+ fourth order pairs in ruby. Comparison with experimental data illustrates the importance of both the anisotropic and antisymmetric exchange interactions, and that part of isotropic exchange interaction, which is nondiagonal in the orbital variables. Various microscopical mechanisms responsible for terms in the effective Hamiltonian are discussed.

DOI
10.1002/pssb.2220440128

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The absorption cross sections of the ${\mathrm{Cr}}^{3+}$ ion in the metastable states $^{2}E (\overline{E} \mathrm{and} 2\overline{A})$ of lightly doped ruby (\ensuremath{\sim}0.01%) have been measured as a function of polarization at temperatures of 30, 105, and 296\ifmmode^\circ\else\textdegree\fi{}K in the region from 2300 to 7000 \AA{}. The lifetime of these states was measured by observing the decay rate of the excited-state absorption, and agreed with the value of the measured fluorescence decay rate at various temperatures. The population of the excited states was measured by the change of the area under the first two vibronic sidebands at low temperatures. At room temperature the population was measured by the change of transmitted intensity of a ruby laser.

Physical Review•Sep 10, 1968•22 citations
Abstract The method of effective Hamiltonian (EFH) is applied to the excited state (2E4A2) fine structure of Cr3+–Cr3+ fourth order pairs in ruby. Comparison with experimental data illustrates the importance of both the anisotropic and antisymmetric exchange interactions, and that part of isotropic exchange interaction, which is nondiagonal in the orbital variables. Various microscopical mechanisms responsible for terms in the effective Hamiltonian are discussed.
A rather simple method of determining the lifetimes of the excited states of exchange-coupled ion pairs in ruby is described. The technique utilizes a chopped pump light and phasesensitive detection. It has permitted a great increase in the resolution of optical transitions, and in particular, it allows for a virtual elimination of the vibronic sidebands of the RRR lines, in order that the true pair spectrum may stand out clearly. Furthermore, transitions originating from different excited levels may be definitely distinguished, thus aiding in the identification of particular transitions with a given pair type. The chronospectroscopic method is of potential value in the studies of energy-transfer mechanisms between single ions and pairs. It has also provided evidence for a fourth-neighbor absorption band at 6050 \AA{}.
The optical spectrum due to the exchange coupled Cr 3+ ions in ruby has been studied theoretically. The effect of the lowering of the Cr 3+ site symmetry due to the formation of a pair was taken into account. For the 2nd neighbor pair, a good fitting of the excited state energies between theory and experiment was obtained. The effect of the higher order exchange is found to be very important for this pair. It is also found that the deviation of the Cr 3+ site symmetry from C 3 is rather large for this pair. For the 4th neighbor pair, the level diagram of the excited state was obtained and the absorption intensities were calculated. Most of the observed lines in this pair are explained by this theory. The lines so called N 1 and N 2 are found to be the transitions due to the exchange induced electric dipole.
Abstract Atomistic rate equations are represented for the transfer of optical excitation energy between donor and acceptor ions in ionic crystals. The physical assumptions which are necessary to deduce the phenomenological collective rate equations from the atomistic ones are specified. Time resolved measurements of the energy transfer from isolated Cr3+ ions to Cr3+ pairs in ruby are discussed. It is shown that, contrary to the present ideas, the collective rate equations cannot be applied in this case.
The absorption spectrum of Cr3+ ions in the magnesium spinel MgAl2O4 was determined for crystals excited by strong optical pumping, and for unexcited crystals. The absorption cross section of the 4A2g level was found to be 2.5 10–20 cm2, which was an order of magnitude smaller than that for ruby. The spectrum obtained for the excited crystals consisted of five absorption bands with maxima at 250, 325, 390, 440, and 550 nm. The absorption cross section for the maximum of the first band was ~2 10–18 cm2. The absorption cross sections at the maxima of the other bands were (1 – 2.5) 10–20 cm2. An energy scheme for Cr3+ ions in MgAl2O4 was derived.
The absorption intensities, widths and wave-numbers of R 1 , R 2 and B 1 B 2 lines of Cr 3+ in ruby for the polarized light E ⊥ C 3 and E // C 3 are experimentally studied at 20°K and 4.2°K. Zeeman effect is also studied, using a magnetic field H 0 =24,000 φ, for both R and B lines. The Zeeman effect has not been observed yet for the B lines, while Lehmann has already observed the Zeeman effect for R lines. In our results of R lines, the quantitative aspects of Lehmann's experiment are improved. The comparison is made with the theoretical results given in Part A and it is shown that nice agreements can be obtained when suitable assignments of the spectra are adopted and when fairly large g -shifts of the excited states are introduced.
The absorption cross sections of the Cr3+{\mathrm{Cr}}^{3+}Cr3+ ion in the metastable states 2E(E‾and2A‾)^{2}E (\overline{E} \mathrm{and} 2\overline{A})2E(Eand2A) of lightly doped ruby (\ensuremath{\sim}0.01%) have been measured as a function of polarization at temperatures of 30, 105, and 296\ifmmode^\circ\else\textdegree\fi{}K in the region from 2300 to 7000 \AA{}. The lifetime of these states was measured by observing the decay rate of the excited-state absorption, and agreed with the value of the measured fluorescence decay rate at various temperatures. The population of the excited states was measured by the change of the area under the first two vibronic sidebands at low temperatures. At room temperature the population was measured by the change of transmitted intensity of a ruby laser.