Phosphors with Two-Photon Emission

A much higher energy efficiency can be obtained if the absorbed ultraviolet photon would split into two visible photons. Such a process would, in principle, have a quantum efficiency of 2(X)%. 

Two-photon emission has been reported for (4/ ) in YF s (32,33] with a quantum efficiency of about 145%. The energy level scheme is given in Pig. 6.23. The Pr ion decays in two steps as indicated. The So level is at 46500 cm, and the H4 So transition parity forbidden. Excitation in a lamp has to occur in the 4/ configuration of Pr which is above the So level. Therefore the low-pre.ssure mercury dischaige cannot be applied and another solution has to be found. 

Although two-photon emission would greatly enhance the efficiency of luminescent lamps, this concept has, up till now, found no practical realisation for lack of suitable materials and ultraviolet sources. 

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