Site-selective spectroscopy of hydrogenic sites in CaF2:Er3+ crystals

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Abstract
Laser selective excitation and optical absorption studies of hydrogenated CaF2:0.05% Er3+ crystals have revealed 16 Er3+ ion sites involving H− or D− ion charge compensation. The relative occurrence of these sites can be controlled over a wide range by varying the duration of the hydrogenation treatment. The hydrogenic nature of these sites is established by the observation of local mode absorption lines in the infrared, Er3+ ion electronic line isotope shifts and associated local mode vibronic lines involving the H− and D− ions. Local mode infrared absorption frequencies of four sites are reported for most rare‐earth ions. Five new Er3+−F− sites were identified in the laser selective excitation study. For 〈100〉 and 〈111〉 oriented crystals the fluorescence spectra of several of the sites exhibit well‐defined polarization which determines their Er3+ ion site symmetries. Symmetry confirmations are also established for the two previously reported single Er3+ ion sites involving fluoride ion compensation. Eleven of the hydrogenic sites reported here are classified into two families, each derived from a single Er3+ ion site of the parent crystal, while three others are attributed to sites of nearly cubic Er3+ ion symmetry. Detailed spectroscopic results for these sites are presented, together with a crystal‐field analysis for the C4v symmetry sites and some model assignments.