Rare earth ions, sensitized fluorescence

A study has been made of the relative efficiencies with which various transition metal chelates quench triplet benzophenone.194 The chelates vary widely in efficiency, and no generalizations can be drawn except that in some cases triplet energy transfer to a coupled metal-ligand triplet energy level probably accounts for at least part of the quenching. Rare earth ions can quench excited triplets by energy transfer, since, as discussed earlier, sensitized fluorescence of the metal ion results. 

Solutions of salts of Pr, Sm, Eu, Gd, Tb and I y exhibit characteristic fluorescence spectra when irradiated by ultraviolet light but the fluorescence is low in intensity and often quenched by other ions or compounds present in solution as noted by Aladjem (1970), p. 182. Consequently the direct determination of the rare earth ions in solutions based on their fluorescence spectra is not generally attempted. Very sensitive methods for the determination of several rare earths based on the fluorescence of these elements in solid matrices have been developed and are described in another section. 

The X-ray determination of REE in geological samples is normally complicated by the relatively low concentrations of the REE, their complex X-ray spectra, the high concentration of matrix elements and the lack of reference standards with certified values for REE. A rapid and sensitive ion exchange and X-ray fluorescence procedure for the determination of trace quantities of rare earths is described. The REE in two U.S.G.S. standards, two inhouse synthetic mixtures and three new Japanese standards have been determined and corrections for inter-rare earth element interferences are made. 

The optical pumping efficiency and output power of many rare earth lasers can be increased by codoping the medium with other ions which absorb pump radiation and effectively transfer the excitation to the upper laser level. This transfer may be either radiative or nonradiative. In general, sensitization schemes used for phosphors and other luminescence phenomena are also applicable to lasers (Van Uitert, 1966). Requirements for the sensitizer ion include (a) no ground- or excited-state absorption at the laser wavelength, (b) absorption bands which complement rather than compete with absorption bands of the laser ion, since the fluorescence conversion efficiency usually is less for the former, (c) one or more metastable energy levels above the upper laser level,... 

The addition of sensitizer ions improves optical pumping efficiency those actually used for glass lasers are noted in table 35.6. Many additional ions have been used to sensitize rare earth fluorescence, such as Cr - Nd, Cr - Yb, and Mo - Er (see Young, 1969, for references). Energy transfer and sensitization schemes for rare earths in glasses are discussed by Reisfeld (1973). 

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