Crystal field and energy levels of centres in

The polarized optical absorption and fluorescence of in crystals have been exhaustively investigated at low temperature in order to obtain and characterize the crystal field energy levels for every multiplet of the system. Up to 51 energy levels have been determined for this ion, and labelled by irreducible representations, E or A, assuming local symmetry. These energy levels have been used to estimate the crystal field ( parameters) acting on the ion in this host crystal. As the optical spectra present additional structure owing to the presence of four centres, laser site selective spectroscopy has been used to determine the Stark sublevels of the ground state for each particular centre. Thus, the slight variations of the parameters from one centre to another have been investigated and interpreted in terms of different -off centre positions for ions.