The hyperfine interaction in the ${}_{}{}^7{}_{}{}^{}\mathrm{F}_0^{}{}_{}{}^{}$ and ${}_{}{}^5{}_{}{}^{}\mathrm{D}_0^{}{}_{}{}^{}$ states in oxygen-compensated ${\mathrm{Eu}}^{3+}$ centers of $C_{3v}$ symmetry in ${\mathrm{CaF}}_2$ and ${\mathrm{CdF}}_2$ is reported. Optically detected nuclear-magnetic-resonance measurements determine (i) the hyperfine structure in the two states and (ii) their nuclear Zeeman splittings. In addition, optical hole burning is used to measure (iii) the quadratic electronic Zeeman effect on the ${}_{}{}^7{}_{}{}^{}\mathrm{F}_0^{}{}_{}{}^{}$→${\mathrm{}}^5$${\mathrm{D}}_0$ transition. These three measurements associated with both ${}_{}{}^{151}{}_{}{}^{}\mathrm{Eu}$ and ${}_{}{}^{153}{}_{}{}^{}\mathrm{Eu}$ isotopes are employed to determine the magnitudes and signs of the pseudoquadrupole interactions, of the quadrupole interactions, and of the effective nuclear magnetic moments. The quadrupole interaction parameters are found to be negative in the excited state but positive in the ground state. This has been confirmed using a double-radio-frequency experimental technique.

In the ${}_{}{}^5{}_{}{}^{}\mathrm{D}_0^{}{}_{}{}^{}$ excited state the quadrupole interaction energy is dominated by the lattice contribution and is equal to -22.8 (-58.4) MHz in ${\mathrm{CaF}}_2$ and -13.2 (33.7) MHz in ${\mathrm{CdF}}_2$ for ${}_{}{}^{151}{}_{}{}^{}\mathrm{Eu}$ ${(\mathrm{}}^{153}$Eu). In the ${}_{}{}^7{}_{}{}^{}\mathrm{F}_0^{}{}_{}{}^{}$ state there is additional contribution due to the polarization of the 4$f^6$ shell equal to +30.6 (78.2) MHz in ${\mathrm{CaF}}_2$ and +20.7 (+53.8) MHz in ${\mathrm{CdF}}_2$ for ${}_{}{}^{151}{}_{}{}^{}\mathrm{Eu}$ ${(}^{153}$Eu). The largest pseudoquadrupole contribution is for the ${}_{}{}^7{}_{}{}^{}\mathrm{F}_0^{}{}_{}{}^{}$ state and equals 0.45 (0.33 MHz) for the case of ${}_{}{}^{151}{}_{}{}^{}\mathrm{Eu}$ in ${\mathrm{CaF}}_2$ (${\mathrm{CdF}}_2$). The ground-state nuclear magnetic moments are only slightly affected by the 4$f^6$ screening effects for the magnetic field direction parallel to the c axis of the centers. However, there is significant quenching of the nuclear magnetic moment for fields transverse to the axis: ${\alpha }_x$=1.85 for ${\mathrm{CaF}}_2$ and 1.44 for ${\mathrm{CdF}}_2$.

This is the first time that a quenching greater than unity has been established. Matrix elements of N¯ and L¯+2S¯ between the ground state and the first excited states are also determined and found to deviate as much as 60% from the value they take in the free ion.

• Received 25 July 1986

DOI:https://doi.org/10.1103/PhysRevB.34.7554