Homogeneously broadened far-infrared absorption line due to the E → 2A transition of excited chromium ions in ruby

By use of far-infrared laser spectroscopy, we have found that for a high-quality ruby crystal at low temperature, the absorption line due to the electronic transition between the excited states $\text{E}\text{(}{}^2\text{E)}$ and $\text{2}\text{A}\text{(}{}^2\text{E) of Cr}{}^{\mathrm{3+}}$ ions is homogeneously broadened. From the linewidth (0.011 cm^-1) the lifetime of the 2ā level against emission of 29 cm^-1 phonons is obtained.