Strong coherent interaction of Nd3+-Nd3+ pair ions in CaF2 crystal

Excited Kramers fine level splittings of Nd3+ pair (M) and quartet (N) clusters in CaF2 crystal have been found and investigated by using absorption spectroscopy and accumulated photon echo (APE) techniques. Absorption spectra on the I-4(9/2) --> (4)G(5/2) transition between Kramers states split into four lines with overall splitting similar to 5 cm(-1) (T = 9 K, NdF3 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 Nd3+-Nd3+ pair in CaF2 (M-center), the strong quadrupole-quadrupole coupling is responsible for the fine splitting of the excited I-4(9/2) X (4)G(5/2) levels and leads to delocalization of the optical electrons within ion pair. (C) 1999 Elsevier Science B.V. All rights reserved.