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The stepwise conversion of infrared (=0.97 p) radiation to near infrared {0. 81 p) and visible (0.475 p) light in YF3 sensitized with Yb ' and activated with Tm ' has been studied in detail. The lifetimes and fluorescence intensities of the important manifolds (Yb I"&~2, Tm 3H4, Tm E4, and Tm G4), as we11 as the Yb-to-Tm transfer probability coefficients for the three steps of the process, have been measured as functions of concentration. A saturation effect previously observed in the Tm emission versus infrared excitation intensity has been shown to result from the rate of depopulation of Tm H4 by the second Yb-to-Tm transfer exceeding the Tm H4 decay rate. This was used to determine the second transfer probability coefficient.The first transfer probability coefficient was found to be 1.2&&10 ' cm sec independent of Tm concentration and for Yb concentration of the order of 10 at. /o and greater. The second transfer probability coefficient was found to be approximately 10 4 cm sec for Yb concentrations of 25 at. % and greater. The third transfer probability coefficient was found to be 2. 7x10 cm sec . The dependence on Yb concentration of the relative efficiency of conversion to 0.475p light agrees well with the values calculated from a rate-equation model using the measured lifetimes and transfer probability coefficients.

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Efficiencies of ∼1%, 0.1%, and 0.01% have been achieved, respectively, in the red, green, and blue with simple infrared-to-visible conversion sources. Saturation effects observed in the blue-emitting phosphor Y0.65Yb0.35Tm0.001F3 have been explained. The first and second cross-transfer coefficients for this Yb, Tm system have been determined to be 1.3×10−17 and 8.3×10−15 cm3 sec−1.

Excitation can move from Tb to Eu at 4.2°K by processes involving exchange even though the electronic transitions concerned are not matched in energy. At 295°K, thermal effects cause an overlap permitting dipole-dipole transfer to occur. Transfer to Eu may be enhanced by excitation migration from more remote Tb ions to those closer to Eu at 295°K. This migration is not effective at 4.2°K, and hence appears to require thermal activation.