Saturation effects in the excitation spectra of rare-earth ions

The concentration enhancement of the relative intensity of vibronic lines in the excitation spectra of rare-earth (RE) ions, and the line narrowing of the two-photon excitation transitions of RE ions relative to their one-photon equivalents have been investigated. These phenomena are not well understood and have been puzzling many researchers over the past decade. This paper gives direct proof that both effects are the result of a misinterpretation of the experimental results due to saturation of the absorption transitions of RE ions. It is shown that saturation effects can become significant at surprisingly low RE concentrations (viz. 0.5 mol%), even for powder samples.