STUDIES ON OPTICAL PROPERTIES OF Na_5Eu(WO_4)_4 CRYSTAL
Guo Changxin Pan Jun(Department of Physics, University of Science and Technology of China, Hefei 230026)
The absorption spectrum and the energy levels of Eu3+ of Na5Eu(WO4)4 crystal have been determined. The absorption edge lies at 290nm (4.3eV).The branch ratios of fluorescence lines of 5D0 to 7F1,2,3,4 are 0.041, 0.832, 0.006 and 0.121, respectively.The strongest fluorescence line(5D0 to 7F2(E)) is 617.2nm, the branch ratio of which is 0.637 and halfwidth is about 1.5nm. And the fluorescence of 5D1 and 5D2 to 7Fi is so weak at room temperature that they have not been observed in our experiment.According to Raman and infrared spectrum analyses, the energies of anti-symmetrical stretching vibration of WO42- in Na5Eu(WO4)4 crystal are very high.It will cause a strong non-radiation transition from 5D2 to 5D1 and then to 5D0. The lifetime experiment will support this further more.By using a YAG:Nd double frequency pulso laser (530nm, 6ns) to excite the Eu3+ ions from 7F0. to 5D1,the strong fluorescence was detected, which only belongs to 5D0→7Fj(j=1,2,3,4), but not to 5D1→7Fj. It means that an energy transfer from 5D1 to 5D0 is occured.The growing time of 5D0 to 7F2 (617.2nm) is 20ns. It indicates that the life time of 5D1 is about 20μs, which, is much less than about 1ms, the normal life time of 5D1 energy level (such as Eu3+ doped in Y2O3, Y2O2S and so on). It is shown that the nonradiation transition probability from 5D1 to 5D0 is very high. From this experiment, the decay time of 5D0 to 7F2 was determined to be 1.03ms. This decay time is in the same order of above materials doped with Eu3+ as an impurity. It is even shown that the concentration of Eu3+ is very high, but this material has no noticeable concentration quenching effect.Several parameters of Na5Eu(WO4)4 are determined. For example, transition probability of 5D0→7F2 is 724s-1, quantum efficiency of 5D0→ 7F2 is 0.90 and effective emission cross section of the 617.2nm main peak is l.5×10-20cm2, which is an order of magnitude larger than that in K5Eu(MoO4)4.