Analysis of the8S7/2to6P7/2two-photon absorption transitions of Gd3+in Gd(OH)3in an applied magnetic field

The authors report a detailed analysis of the intensity and polarisation dependence of the 8 S 7/2 to 6 P 7/2 two-photon transition Stark components of Gd 3+ in the concentrated magnetic insulator Gd(OH) 3 under an applied magnetic field (up to 3.5 T) at very low temperature (2 K). The experimental technique consisted of detecting the ultraviolet 6 P 7/2 to 8 S 7/2 one-photon fluorescent emission resulting from two-photon excitation in the 6 P 7/2 states, using a single pulsed dye-laser beam. The standard second-order theory of Axe is shown to fail in explaining the relative intensities of the Stark components. This is especially true in the case of the Delta M=0 transitions for which strong disagreements are observed. It is shown that the introduction of the third-order contribution involving the spin-orbit interaction within the 4f 6 5d states allows the experimental spectra to be reproduced in a correct way. Finally the effects of ion-ion interactions on the two-photon line profiles are discussed.