Two-photon dye-laser excitation is used to observe numerous sharp $4f^7$ levels of ${\mathrm{Eu}}^{2+}$ in Ca${\mathrm{F}}_2$ and Sr${\mathrm{F}}_2$ which are overlapped by the $4f^{6}5d$ band, and thus inaccessible to single-photon spectroscopy. $4f^7$ excited states belonging to the ${}_{}{}^6{}_{}{}^{}I$ and ${}_{}{}^6{}_{}{}^{}D$ groups have been identified for the first time. Energy-level calculations are presented which confirm the $f^7$-level assignments and determine freeion and crystal-field parameters for both host lattices. Measurements of the relative intensities and polarization dependence of the $f^7\to f^7$ transitions confirm the existence of third- and fourth-order contributions to the two-photon intensities which have been postulated in recent analyses of the two-photon spectrum of the isoelectronic ${\mathrm{Gd}}^{3+}$ ion. In some cases (${}_{}{}^8{}_{}{}^{}S_{\frac{7}{2}}^{}{}_{}{}^{}\to {}_{}{}^6{}_{}{}^{}P_{\frac{7}{2},\frac{5}{2}}^{}{}_{}{}^{}$,${}_{}{}^6{}_{}{}^{}D_{\frac{7}{2}}^{}{}_{}{}^{}$), however, these higher-order contributions are weaker than expected. It is shown that by relaxing the closure approximation in summing over the intermediate states, the agreement with the data is improved.

• Received 22 July 1983

DOI:https://doi.org/10.1103/PhysRevB.28.4931