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Physical review. B, Solid state

Energy transport in ruby via microwave-optical experiments

J. C. Murphy, Leonard C. Aamodt, C. K. Jen

Published Mar 1, 1974• 10 citations
Publisher

Abstract

The process which results in enhanced pair fluorescence relative to single-ion fluorescence for ruby samples containing more than 0.2-wt% ${\mathrm{Cr}}^{3+}$ has been studied using microwave-optical double resonance. These experiments confirm that the enhancement is associated with energy transport among excited states of the system. However, the primary donor-level complex is not the normal ion excited state as postulated previously. Microwave-optical experiments indicate the existence of an additional excited level which is postulated to be the actual donor level. Evidence is presented which suggests that this additional level may be connected with exciton formation above a critical ${\mathrm{Cr}}^{3+}$ concentration.

DOI
10.1103/physrevb.9.2009

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The process which results in enhanced pair fluorescence relative to single-ion fluorescence for ruby samples containing more than 0.2-wt% Cr3+{\mathrm{Cr}}^{3+}Cr3+ has been studied using microwave-optical double resonance. These experiments confirm that the enhancement is associated with energy transport among excited states of the system. However, the primary donor-level complex is not the normal ion excited state as postulated previously. Microwave-optical experiments indicate the existence of an additional excited level which is postulated to be the actual donor level. Evidence is presented which suggests that this additional level may be connected with exciton formation above a critical Cr3+{\mathrm{Cr}}^{3+}Cr3+ concentration.
The transfer of energy from single chromium ions to closely coupled pairs of chromium ions in ruby has been investigated. Ruby samples whose concentrations ranged from 0.003 to 1.0% Cr2{\mathrm{Cr}}_{2}Cr2​O3{\mathrm{O}}_{3}O3​ were prepared and in each crystal the intensity of single-ion fluorescence and of pair fluorescence was accurately measured. Radiative-decay measurements were also made in an attempt to unravel the dynamics of the transfer process. The transfer from single ions to pairs is shown to occur nonradiatively, and its efficiency is enhanced by a fast energy transfer between individual single ions. The possibility is explored that an electric quadrupole-quadrupole interaction is responsible for this transfer process.
The transfer of energy from single chromium ions to closely coupled pairs of chromium ions in ruby has been investigated. Ruby samples whose concentrations ranged from 0.003 to 1.0% Cr2{\mathrm{Cr}}_{2}Cr2​O3{\mathrm{O}}_{3}O3​ were prepared and in each crystal the intensity of single-ion fluorescence and of pair fluorescence was accurately measured. Radiative-decay measurements were also made in an attempt to unravel the dynamics of the transfer process. The transfer from single ions to pairs is shown to occur nonradiatively, and its efficiency is enhanced by a fast energy transfer between individual single ions. The possibility is explored that an electric quadrupole-quadrupole interaction is responsible for this transfer process.
The critical concentration for single-ion-single-ion energy transfer in ruby is calculated using Anderson's method for transport between ions which are characterized by inhomogeneously broadened energy levels. The excitation on the chromium impurity is assumed to propagate by virtue of superexchange interactions. Using a continuum approximation, we find that the single-ion excitation is localized below concentrations of \ensuremath{\sim}0.3-0.4%, a range not inconsistent with that concentration at which a sudden change in the fluorescent lifetime of ruby occurs.
The transfer of energy from single chromium ions to closely coupled pairs of chromium ions in ruby has been investigated. Ruby samples whose concentrations ranged from 0.003 to 1.0 percent Cr2O3were prepared and in each crystal the intensity of single-ion fluorescence and of pair fluorescence was accurately measured. Radiative decay measurements were also made in an attempt to unravel the dynamics of the transfer process. The transfer from single ions to pairs is shown to occur nonradiatively, and its efficiency is enhanced by a fast energy transfer between individual single ions. The possibility is explored that an electric quadrupole-quadrupole interaction is responsible for this transfer process.
The fluorescence intensities and lifetimes of various no-phonon lines in ruby samples with 2.1 and 0.94% Cr3+{\mathrm{Cr}}^{3+}Cr3+ were studied from 13 to 700\ifmmode^\circ\else\textdegree\fi{}K. The temperature dependences of the intensities of numerous lines were used to determine the energy levels of two pair systems. The relative intensities of the single-ion (RRR) lines and the pair (NNN) lines are proportional to Boltzmann factors ${e}^{\frac{\ensuremath{-}\ensuremath{\Delta}E}{\mathrm{kT}}}$, where $\ensuremath{\Delta}E$ is the difference in energy of the metastable states, at high temperatures but not at low temperatures. The deviation from this law is greater and occurs at higher temperatures in the less concentrated sample. The fluorescence decays of the RRR and NNN lines are pure exponentials in the temperature region where the NNN lines follow a Boltzmann law; at low temperatures the NNN lines have an initial rise in fluorescence and then a decay. These results are interpreted in terms of coupled fluorescence systems consisting of single ions and pairs. A phonon-assisted cross-relaxation coupling process is suggested which allows for thermalization of the systems at high temperatures and direct pumping of the pair systems by the single-ion system at low temperatures. Energy transfer from single-ion to multi-ion systems was also observed.
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.