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Journal of Undergraduate Reports in Physics

Revisiting the Room-Temperature Metastable 2E Lifetime in Ruby for an Upper Division Phosphorescence Laboratory Experiment

Zachary D. Jones, J. M. Hinds, S. Woznichak, Anthony G. Calamai

Published Jan 1, 2020• 2 citations

Abstract

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.

DOI: 10.1063/10.0002044

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Physical Review•Aug 10, 1967•124 citations

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 Ruby Phosphorescence Laboratory: Measuring the

^2

E-Term Room-Temperature Lifetime of Cr

^{3+}

.

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.

Abstract A long‐time phosphorescence of ruby is found under intense optical excitation in the spectral region of the R‐lines. From the initial brightness decay, as a function of the intensity and wavelength of the exciting light, it is suggested that the phosphorescence has a recombination character. The long‐wavelength limit of excitation corresponds to about 2 eV. These results indicate a complex character for the phosphorescence excitation (two‐photon or two‐step). To differentiate between these two possibilities experiments are performed using two excitation pulses with various time delays between them. These experiments show that two‐step excitation of electrons into the conduction band of ruby takes place over 2E levels of the Cr ions. From the results, and absorption data of ruby and corundum, a band scheme is proposed which explains the long‐wavelength limit of phosphorescence excitation in terms of indirect transitions.

The first appreciable decrease in the lifetime of the

^{2}E

state of

{\mathrm{Cr}}^{3+}

in ruby and emerald as the temperature is raised is shown to be the result of populating the shorter-lived

^{4}T_{2}

level in thermal equilibrium with the metastable

^{2}E

. The direct transition from the

^{4}T_{2}

to the ground

^{4}A_{2}

state is shown to be mostly radiative. In both crystals, the emission peak of the $^{4}T_{2}\ensuremath{\leftrightarrow}^{4}A_{2}$ transition is shifted considerably toward longer wavelength compared to the absorption peak of that transition. The reduction in lifetime and quantum efficiency at very high temperatures is also considered.