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Excited Kramers fine level splittings of Nd 3+ pair (M) and quartet (N) clusters in CaF 2 crystal have been found and investigated by using absorption spectroscopy and accumulated photon echo (APE) techniques. Absorption spectra on the 4 I 9/2 → 4 G 5/2 transition between Kramers states split into four lines with overall splitting ∼5 cm −1 ( T =9 K, NdF 3 concentration 0.1, 0.3 and 1 wt%). At the picosecond laser excitation of the M- and N-centers on the same transition the APE kinetics was observed to be strongly modulated with multiple coherent beatings on pico- and nano-second time scale. Fast Fourier Transform (FFT) of the APE kinetics decay shows that nearest-neighbour absorption lines splittings 0.9, 1.5 and 2.1 cm −1 (M-center) are clearly defined in FFT spectrum. Theoretical study revealed that in general case the electrostatic interionic coupling lifts completely the eightfold degeneracy of the excited level of a pair of similar ions. The selection rules for the forced electric-dipole, magnetic dipole and quadrupole intermultiplet transitions are stated. It is suggested that in the case of Nd 3+ –Nd 3+ pair in CaF 2 (M-center), the strong quadrupole–quadrupole coupling is responsible for the fine splitting of the excited 4 I 9/2 × 4 G 5/2 levels and leads to delocalization of the optical electrons within ion pair. |
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