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Physical review. B, Condensed matter

Two-photon absorption spectroscopy of multisite Gd3+ in CaF2

C. D. Cordero-Montalvo

Published Apr 15, 1985• 10 citations
Publisher

Abstract

The energy levels of ${\mathrm{Gd}}^{3+}$ in the $^{6}\mathrm{P}$, $^{6}\mathrm{I}$, and $^{6}\mathrm{D}$ 4${f}^{7}$ configuration in cubic, tetragonal, and cluster sites in ${\mathrm{CaF}}_{2}$ have been excited by two-photon absorption. The relative two-photon intensities and polarization dependence show behavior which is similar to that previously observed for ${\mathrm{Gd}}^{3+}$:${\mathrm{LaF}}_{3}$ and ${\mathrm{Eu}}^{2+}$:${\mathrm{CaF}}_{2}$ and which requires higher-order perturbation theory in the virtual intermediate 4${f}^{6}$5d configuration.

DOI
10.1103/physrevb.31.5433

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The energy levels of Gd3+{\mathrm{Gd}}^{3+}Gd3+ in the 6P^{6}\mathrm{P}6P, 6I^{6}\mathrm{I}6I, and 6D^{6}\mathrm{D}6D 4f7{f}^{7}f7 configuration in cubic, tetragonal, and cluster sites in CaF2{\mathrm{CaF}}_{2}CaF2​ have been excited by two-photon absorption. The relative two-photon intensities and polarization dependence show behavior which is similar to that previously observed for Gd3+{\mathrm{Gd}}^{3+}Gd3+:LaF3{\mathrm{LaF}}_{3}LaF3​ and Eu2+{\mathrm{Eu}}^{2+}Eu2+:CaF2{\mathrm{CaF}}_{2}CaF2​ and which requires higher-order perturbation theory in the virtual intermediate 4f6{f}^{6}f65d configuration.
A comprehensive report and analysis of our observations of numerous parity-allowed direct two-photon transitions within the 4f74{f}^{7}4f7 configuration of the Gd3+{\mathrm{Gd}}^{3+}Gd3+ ion is presented. The experimental technique is ultraviolet fluorescence detection following optical excitation with a dye-laser beam. Previous results and analyses are extended in three ways. (1) The intensities and polarization dependence of individual Stark components, in addition to integrated multiplet intensities, are reported and analyzed for Gd3+{\mathrm{Gd}}^{3+}Gd3+:LaF3{\mathrm{F}}_{3}F3​. In those cases ($^{8}S_{\frac{7}{2}}\ensuremath{\rightarrow}^{6}P_{\frac{5}{2}},^{6}D_{\frac{9}{2},\frac{3}{2},\frac{5}{2}}$) where a second-order theory of two-photon absorption adequately explains the integrated intensity, it also (with the exception of 6D92^{6}D_{\frac{9}{2}}6D29​​) explains the Stark-component intensities. The remaining transitions are anomalously strong, and in some cases violate the angular momentum selection rules $\ensuremath{\Delta}L,\ensuremath{\Delta}J\ensuremath{\le}2$ and exhibit strong anisotropies, in contradiction to the second-order theory. It is shown that introduction of third- and fourth-order contributions involving spin-orbit and/or crystal-field interactions among levels of the 4f65d4{f}^{6}5d4f65d configuration, which serve as intermediate states, can account for both the integrated and Stark-component intensities of these transitions. (2) The strong anisotropy observed for the transitions $^{8}S_{\frac{7}{2}}\ensuremath{\rightarrow}^{6}P_{\frac{3}{2}},^{6}I_{J},^{6}D_{\frac{1}{2}}$ in Gd3+{\mathrm{Gd}}^{3+}Gd3+:LaF3{\mathrm{F}}_{3}F3​ is explained quantitatively as a polarization-dependent interference between contributions to the intensity which are comparable in magnitude. Interference is destructive for $\stackrel{\ensuremath{\rightarrow}}{E}\ensuremath{\parallel}\stackrel{^}{z}$ and constructive for $\stackrel{\ensuremath{\rightarrow}}{E}\ensuremath{\perp}\stackrel{^}{z}$, creating an order-of-magnitude contrast in the line strength for the two polarizations. (3) A two-photon excitation spectrum of Gd3+{\mathrm{Gd}}^{3+}Gd3+ in aqueous solution is reported. Linewidths comparable to those of room-temperature solids and Stark splittings which suggest a low-symmetry quasistatic environment are observed.
Two-photon absorption spectroscopy is used to investigate the 8 S 7/2 → 6 P J , 6 I 7/2 transitions of Gd 3+ ions in the cubic elpasolite Cs 2 NaGdCl 6 . The Gd 3+ ions occupy a centro-symmetric site (O h ); so the odd terms of the crystal field vanish and the pure electric dipole transitions should not be observed in one-photon experiments. As a consequence the electric dipole allowed two-photon transitions involving the 6 I 7/2 and 6 P 3/2 excited states are well observed in contrast with their weakness in the one-photon spectrum. Furthermore the magnetic dipole character of the transitions to the 6 P 5/2 and 6 P 7/2 levels is confirmed. The occurrence of the one-photon transition to 6 P 3/2 is tentatively explained by the J mixing involving the 6 P 7/2 excited state. A qualitative analysis of the relative intensities of the transitions to the 6 P J and 6 I 7/2 levels is presented.
Recent observations of two-photon absorption within the configuration 4f 7 of Gd 3+ in LaF 3 have indicated an anomalously high line strength for the transition 8 S 7/2 to 6 P 7/2 compared with 8 S 7/2 to 6 P 5/2 and 8 S 7/2 to 6 P 3/2 . It is shown that the standard expression for the two-photon intensity is unusually small for all three transitions and that a higher-order correction, involving matrix elements of the spin-orbit interaction within the intermediate configuration 4f 6 5d, predominates. The principal term is scalar, implying (in the absence of J mixing by the crystal field) isotropic contributions to the various components of the Delta J=0 transition. The line strengths that have been observed can be accounted for reasonably well without having to invoke variations in the radial integral (4f mod r mod 5d) or inadequacies in the closure approximation. Expressions for the transition matrix elements are given for the general configuration l N .
Intraconfiguration ${4f}^{7}\ensuremath{\rightarrow}{4f}^{7}$ transitions of Eu2+{\mathrm{Eu}}^{2+}Eu2+ in KMgF3{\mathrm{KMgF}}_{3}KMgF3​ have been investigated by two-photon excitation (TPE) spectroscopy at 10 K. Emission bands due to the ${}^{6}{\stackrel{\ensuremath{\rightarrow}}{{P}_{7/2}}}^{8}{S}_{7/2}$ and ${}^{6}{\stackrel{\ensuremath{\rightarrow}}{{I}_{7/2}}}^{8}{S}_{7/2}$ transitions are obtained by two-photon excitation into the 6IJ{}^{6}{I}_{J}6IJ​ (J=7/2,9/2,17/2,11/2,15/2,13/2)(J=7/2,9/2,17/2,11/2,15/2,13/2)(J=7/2,9/2,17/2,11/2,15/2,13/2) and 6DJ{}^{6}{D}_{J}6DJ​ (J=1/2,7/2,3/2,5/2)(J=1/2,7/2,3/2,5/2)(J=1/2,7/2,3/2,5/2) states. The TPE spectrum of the ${}^{6}{\stackrel{\ensuremath{\rightarrow}}{{I}_{7/2}}}^{8}{S}_{7/2}$ emission is observed to be different from that of the ${}^{6}{\stackrel{\ensuremath{\rightarrow}}{{P}_{7/2}}}^{8}{S}_{7/2}$ emission. This is explained by the relative positions of the lowest crystal-field component of the 6I7/2{}^{6}{I}_{7/2}6I7/2​ level and the lowest level of the 4f65d{4f}^{6}5d4f65d configuration for cubic-site and non-cubic-site Eu2+{\mathrm{Eu}}^{2+}Eu2+ ions. The observed TPE spectra are satisfactorily interpreted by a suggestion that the ${}^{6}{\stackrel{\ensuremath{\rightarrow}}{{I}_{7/2}}}^{8}{S}_{7/2}$ emission is produced by only cubic-site Eu2+,{\mathrm{Eu}}^{2+},Eu2+, while the ${}^{6}{\stackrel{\ensuremath{\rightarrow}}{{P}_{7/2}}}^{8}{S}_{7/2}$ emission is produced by both cubic-site Eu2+{\mathrm{Eu}}^{2+}Eu2+ and non-cubic-site Eu2+.{\mathrm{Eu}}^{2+}.Eu2+.