Mechanisms of Energy Transfer Involving Trivalent Tb and Sm

Abstract

The sodium (yttrium, rare‐earth) tungstates are particularly useful for the study of rare‐earth (RE) ion interactions that lead to the enhancement or quenching of emission. On the average a given RE ion interacts with only six others by nearest‐neighbor or next‐nearest‐neighbor exchange at maximum concentration. Therefore, quenching resulting from exchange coupling of ions of the fluorescent species of interest with such neighbors may be taken to vary with the fraction of dodecahedral sites occupied by RE ions (x) as [1—(1—x)⁶]. In general, interactions between cations that reduce intensity by a fraction that is less than [1—(1—x)⁶] probably involve exchange, while those that do so to a greater extent involve the cross relaxation of energy by multipolar transitions. Lifetime and intensity data obtained on sodium rare‐earth tungstates containing trivalent terbium (Tb) and trivalent samarium (Sm) are examined herein. Energy transfers between Sm ions by a dipole—quadrupole mechanism and from Tb to Sm—Sm or Sm—Tb couples by a similar multipolar process. Transfer to single Sm ions by exchange enhances emission from Sm. However, this process is only competitive at low Sm concentrations.