Abstract
Low-temperature, high-resolution measurements of the far-infrared absorption spectrum of CaF2:Er3+ have revealed two sharp lines at 21.10 and 32.76+or-0.05 cm-1. These lines correspond to transitions between the lowest 4I15/2 Stark levels of the Er3+ ion in a tetragonal crystal field. The temperature dependence of the lines is also presented and successfully analysed in terms of thermal population effects.