Photon cascade, radiative

A related phenomenon is the conversion of single visible photons with the result the quantum efficiency can be higher than 100%. If, for example, 0.1% of Pr + is incorporated in YF3 and excited with the mercury spectral line at 185 nm, the electron from 4f5d states decay non-radiatively to the Sq (4p) State. This system is able to generate two visible photons by So- f6> followed by non-radiative decay to the closely adjacent Po and by transition another photon is emitted by transitions to one of the six /-levels of or F. A condition for this cascade process is that the nephelauxetic effect for inter-shell transitions is sufficiently weakly pronounced for the lowest 4/5d state to be above Sq (Reisfeld and Jbrgensen 1977). 

Two visible photons per UV photon can be obtained by a cascade process with Pr +. In lattices generating a low crystal field, the lowest 5d states lie above the highest level of the configuration 4f, So (Figure 9), which decays radiatively by the transition So le, followed by a nonradiative transition... 

As pointed out by C.H.S.H., pairs of photons emitted in suitable atomic radiative cascades are good candidate for a sensitive test. Consider for instance a (J = 0)->(J = = 0)... 

Fig. 6. Radiative cascade emitting pairs of photons correlated in polarization, that only even isotopes can be used ).

---