The absorption cross sections of the
Cr3+ ion in the metastable states
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.
We have measured the radiative efficiency (ratio of
R-line photons to the total number of photons emitted) and absorption cross sections for the
R lines of dilute ruby, as a function of polarization and temperature between 20 and 373\ifmmode^\circ\else\textdegree\fi{}K. We have also measured the fluorescent lifetime from 20 to 373\ifmmode^\circ\else\textdegree\fi{}K in optically thin crystals, and from 20 to 273\ifmmode^\circ\else\textdegree\fi{}K in optically thick crystals. The difference between the reciprocal lifetimes in thin and thick crystals is directly comparable (via the Einstein relations) with the integrated absorption in the
R lines. This comparison shows that for the
R lines at 20 and 77\ifmmode^\circ\else\textdegree\fi{}K detailed balance holds to well within the experimental error of 5%, a precision never to our knowledge previously approached in solids. An upper limit of 0.002 ${\mathrm{cm}}^{\ensuremath{-}1}$ was placed on any Stokes shift of the
R1 line, confirming that it is a no-phonon line. Comparison of $\ensuremath{\sigma}$ and $\ensuremath{\alpha}$ spectra confirms that the
R lines and their vibronic satellites are predominately electric-dipole in character. The radiative efficiency of the
R lines is strongly polarization dependent, a result which differs from that of previous workers. Its temperature dependence agrees well with that calculated from the observed vibronic spectrum at 77\ifmmode^\circ\else\textdegree\fi{}K. The integrated absorption in the
R lines increases by about 20% between 20 and 373\ifmmode^\circ\else\textdegree\fi{}K. The absorption in the vibronic satellites on the high-frequency side of the
R lines appears to be weaker than the corresponding emission on the low-frequency side. The temperature variation of the lifetime agrees well with that calculated from the absorption and radiative efficiency, but about 10% of the decay at 77\ifmmode^\circ\else\textdegree\fi{}K and 30% at 373\ifmmode^\circ\else\textdegree\fi{}K is either nonradiative or by emission at wavelengths outside the range of observation. The implications of our results for laser work are discussed.
Several experiments on optically excited ruby are described. First, absorption transitions initiated from the metastable states,t\min{2}\max{3}2Eandt\min{2}\max{3}2T1, are investigated in the photon energy range from 5500 to 45 000 cm-1. The absorption spectrum of these excited states is determined in both\pi-and σ-polarizations and assignments for the observed absorption bands are achieved. Obtained results are compared with other experiments reported in the literature. Next, emission properties of pink ruby are studied in the region near theRlines. The emission spectrum is determined point by point by means of intense flash excitation. The observed weak bands are ascribed to theR'and phonon assistedRtransitions. Laser actions in these bands are discussed and experimental results on phonon terminated laser amplification are presented. An experiment using a giant pulse as an excitation source reveals that the transition time fromt\min{2}\max{3}2Etot\min{2}\max{3}2T1is shorter than a fraction of 1μs at room temperature.
The fluorescent decay of the 2E level of Cr3+ over the range of 77 to 300 K has been studied in experimental arrangements providing strong trapping of the O-phonon lines. At 300 K, rubies as large as 2.5-cm diameter by 33-cm length were used and were packed in highly reflective BaSO4 powder to provide adequate trapping of the O-phonon fluorescence. The measurements resulted in the determination explicitly of the lifetime vibronic transitions and the O-phonon decay rate. These studies of lifetime enhancement have potential application in the design of efficient solid-state lasers.
Many existing advanced laboratory experiences associated with the metastable 2E term of Cr3+ in ruby, which gives rise to the R-lines at 692.7 and 694.3 nm, focus on a room-temperature measurement of the radiative lifetime of the 2E term. In our local work developing a laboratory experience in atomic phosphorescence, we noted a lack of consistency in the literature for the lifetime of the Cr3+ 2E term. These projects typically use commercially available ruby spheres for which the manufacturer(s) only state an ∼2% Cr concentration. The uncertainty in chromium concentration represents one major source of systematic error for this laboratory experience, as does the neglect of reabsorption of R-line photons within the respective sample. We present our results and corrections for systematic issues that make this project a more rewarding experience for students. Our result for the room temperature radiative-lifetime for the 2E term is 3.3 ± 0.1 ms; which, unlike some more recent reports1, compares favorably with that of Nelson and Sturge3.
The population ratio $\frac{{N}_{\ensuremath{-}}}{{N}_{+}}$ of the Zeeman states of
E(2E) has been measured over five decades of metastable-state concentrations ${N}_{\ensuremath{-}}$ in 2500-ppm ruby at 28 kG and 1.5 K, i.e., in the absence of thermalization between ${\overline{E}}_{\ifmmode\pm\else\textpm\fi{}}$. Upon raising the pumping, $\frac{{N}_{\ensuremath{-}}}{{N}_{+}}$ falls below the value corresponding to optical feeding only due to optically induced two-phonon processes via
2A(2E). Rate equations account for the findings under the assumption of marginal (ballistic regime) or moderate (spatially diffusive regime) trapping of these 29-${\mathrm{cm}}^{\ensuremath{-}1}$ phonons. At high pumping, $\frac{{N}_{\ensuremath{-}}}{{N}_{+}}$ further drops to unity, reflecting disturbance of the Boltzmann equilibrium within the
4A2 ground state.