The Ruby Phosphorescence Laboratory: Measuring the $^2$E-Term Room-Temperature Lifetime of Cr$^{3+}$.

Many existing advanced laboratory experiences associated with the metastable $^2$E term of Cr$^{3+}$ 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 $^2$E term.These projects typically use commercially available ruby spheres for which the manufacturer(s) only state an $\sim $ 2-percent chromium concentration. The uncertainty in Cr$^{3+}$ concentration represents one source of systematic error for this laboratory experience. In our local work developing a cost-effective laboratory experience in atomic phosphoresce, we noted a lack of consistency in the literature for the lifetime of the Cr$^{3+}$ $^2$E term. 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 $^2$E term is 3.3 $\pm $ 0.1 ms; which, unlike some more recent reports (e.g. [1]), compares favorably with that of Nelson and Sturge [2]. [1] Espositi, C.D. and Bizzocchi, L., J. Chem. Ed., V84, 1316, (2007). [2] Nelson, D.F. and Sturge, M.D., Phys. Rev., V137 #4A, A1117, (1965).