High-order multipole interaction in nanosecond Nd-Nd energy transfer

The nanosecond energy transfer kinetics of the (4)G(7/2)+K-2(13/2) multiplet of the Nd3+ ion in the LaF3 laser crystal was investigated. The energy transfer rates of electronic excitation from donors (excited Nd3+ ions) (D) to different coordination spheres of accepters (A) (unexcited Nd3+ ions) were determined considering no energy migration. The predominance of the dipole-quadrupole (dq) mechanism against dipole-dipole (dd) was found from the analysis of these rates for the donor-acceptor distances from 6.05 Angstrom and up to 17 Angstrom. For the distances in the range from 4.1 to 4.4 Angstrom the mechanism of donor-acceptor interaction differes from that for the longer donor-acceptor distances. It was shown that a high rate and predominance of the dipole-quadrupole interaction is correlated with the larger value of the square of the low rank reduced matrix element U-(2) in comparison with those of the high rank U-(4) and U-(6) for one of the transition involved in the process of energy transfer. We point out that the dipole-dipole nonradiative energy transfer from the F-4(3/2) metastable level of Nd3+ ion is usually realized because of the small values of (U((2)2)) in comparison with (U((4)2)) and (U((6)2)) for both transitions involved in the quenching process.