Knowledge Bank

Studies of the electronic $4f\to 4f$ transition intensities for $Eu3+$ in crystals of $Eu(DBM)3H2O\ast$, $Eu(ODA)3\cdot 2NaClO4\cdot 6H2O\ast$, and $Eu(AP)6\cdot I3\ast$ reveal the generality of the hypersensitivity phenomenon. Regardless of the metal ion site symmetry, the $^\{5\}D\_\{0,2\}\backslash leftrightarrow\; \{^\{7\}F\_\{0,2\}(\backslash Delta\; J\; =\; \backslash pm\; 2)$ multiplet-multiplet transition are always very hypersensitive as evidenced by their highly variable relative intensity, in contrast to certain other electric dipole transitions. While the absolute electric dipole intensity of hypersensitive transitions is directly moderated by the nature of the crystal field (i.e., centrosymmetric versus noncentrosymmotric), hypersensitivity itself is determined largely by the character of the free-ion wave functions of the initial and final transition states. In this sense, hypersensitivity is an intrinsic phenomenon of the $Eu3+$ ion, while electric dipole intensity is an extrinsic property of the coordination environment. Observations concerning the electronic spectra of the above crystals in relation to the $Eu3+$ ion point-group symmetry and the chemical properties of the ligands will be discussed in order to demonstrate this point.