A fine spectral structure of pair and quartet centers of Nd ions in CaF2, crystals was investigated by the selective laser absorption saturation and time resolved pump/probe technique at 8-30 K. The spectral hole-antihole pairs were observed under laser excitation of the pair Nd ions. It was shown that the main absorption lines reflect a splitting of the lowest Stark level of the 4G(5/2) state caused by a strong coherent interaction between Nd ions in cluster with total splitting values 2.4 and 3.3 cm-1 for pair and quartet centers, respectively. Energy diagram, including singly and doubly excited states, was built for a strongly coherent interacting pair. The study of dynamics of the absorption saturation for different time delay between probe and pump pulses allowed us to estimate relaxation time (~8 ns) among ground and excited state sub-levels. (C) 2000 Elsevier Science B.V.
