Classification of Transitions in Upconversion Luminescence of Lanthanides by Two-Dimensional Correlation Analysis

Upconversion luminescence bands from Yb³⁺/Er³⁺ codoped into a matrix such as NaGdF₄ can show a very complex structure on account of multiple intra-f shell transitions occurring in the presence of random crystal fields. We demonstrate that two-dimensional correlation analysis, applied to such time-integrated luminescence spectra measured as a function of excitation power, allows us to gain substantial information about the states involved in transitions, without any additional theoretical input. The detailed correlation analysis allows us not only to identify the location of various transitions but further to club them into groups on the basis of their quantum mechanical origin, and finally subclassify the transitions with each group depending on whether they have a common initial or final state.