There has been considerable interest in the absorption spectra of the rare earth ethylsulphates for many years. The interest has been centered on checking the validity of the crystal field approximation for these salts of the rare earth metal ions which are at sites where the point group symmetry is C3h"
Bartel, Lewis C. and Spedding, F. H., "Nature of the energy levels in hydrated thulium ethylsulphate" (1964). Ames Laboratory Technical Reports. 80.