1. 1 VUV spectroscopy of BaF 2 :Er A.J. Wojtowicz Institute of Physics, N. Copernicus Univ. Toruń, POLAND FPS 2007 French-Polish Symposium on Spectroscopy of Modern Materials in Physics, Chemistry and Biology. July 15 – 18 2007, Clermont-Ferrand, France

2. A.J. Wojtowicz, FPS July 20072 INTRODUCTION Why VUV spectroscopy? Materials for UV, VUV solid state lasers quantum cutting phosphors scintillators GOAL of this work: identify various VUV excited emissions and identify different excitation mechanisms in BaF 2 :Er

3. A.J. Wojtowicz, FPS July 20073 Scintillators Ce scintillators: LSO, LYSO, LuAP, LuYAP Ce: 350 nm, 15-30 ns then Pr i Nd, emitting at 250, 190 nm, should have 8-15 and 5-10 ns (more or less true) Heavy lantanides have even higher energy d-levels…

4. A.J. Wojtowicz, FPS July 20074 Could it be that 4d-5f emission from ions such as Tb, Dy, Ho, Er or Tm is efficient and even faster under excitation by ionizing radiation? Is the oscillator strength f up to the expectations? Is energy transfer from host to ion efficient? NO on both questions in the case of BaF 2 :Er

5. A.J. Wojtowicz, FPS July 20075 In general, for Ln 4f n 4f n → 5d4f n-1 for Ce just one option: 4f → 5d (no f electrons left behind) for Pr 4f 2 ↑↑(HS) → 5d↑4f↑(HS) or 5d↓4f↑(LS); the second option is higher in energy and forbidden; f almost the same for higher n it is getting worse; more allowed states lower f

6. A.J. Wojtowicz, FPS July 20076 For n > 7 situation changes drastically. For Tb 3+ : 4f n ↑↑↑↑↑↑↑↓(2S+1=7, LS) → 5d↑4f n-1 ↑↑↑↑↑↑↑ (2S+1=9, HS) spin-forbidden, lower in energy and 5d↓4f n-1 ↑↑↑↑↑↑↑, no spin flip (2S+1=7, LS) spin-allowed but higher in energy – Hund’s rule; the lowest excited state will be HS and emission transition will be spin-forbidden

7. A.J. Wojtowicz, FPS July 20077 SAMPLES and EXPERIMENTS BaF 2 :0.2%Er, grown by Bridgman, at Optovac, donated by A. Lempicki of ALEM, Boston Emission and excitation spectra: I–beamline, Superlumi, Hasylab, Hamburg, Germany (prof. G. Zimmerer) U9B beamline, National Synchrotron Light Source, Brookhaven, USA (dr J. Sutherland)

8. A.J. Wojtowicz, FPS July 20078 VIS emissions; lower Er 3+ levels excited

9. A.J. Wojtowicz, FPS July 20079 UV emissions; higher Er 3+ levels excited

10. A.J. Wojtowicz, FPS July 200710 Excitation spectra of VIS emissions; RT and 5 K

11. A.J. Wojtowicz, FPS July 200711 Excitation below 124 nm; STE, some VIS emission

12. A.J. Wojtowicz, FPS July 200712 VIS and UV emissions competition, Er-BE and d-levels

13. A.J. Wojtowicz, FPS July 200713 STE, Er-BE and d-levels

14. A.J. Wojtowicz, FPS July 200714 STE emission under over the bandgap excitation

15. A.J. Wojtowicz, FPS July 200715 VUV emissions under d-level excitation

16. A.J. Wojtowicz, FPS July 200716 Excitation spectrum of VUV emissions

17. A.J. Wojtowicz, FPS July 200717 Excitation spectrum of Ce emission in BaF 2

18. A.J. Wojtowicz, FPS July 200718 BaF 2 :Ce, crystal field splitting 10Dq, (3+2) structure, d-levels split into t and e In BaF 2 :Er four bands, no single electron crystal field structure (3+2) like in BaF 2 :Ce Er 3+ 4f 11 ground state 4 I 15/2 (LS, 2S+1=4) Excited state 5d(e)↓ 5 I 8 (4f 10 ) (HS, 2S+1=6) Excited state 5d(e)↑ 5 I 8 (4f 10 ) (LS, 2S+1=4) Excited state 5d(e)↑ 5 I 7 (4f 10 ) (LS, 2S+1=4) Excited state 5d(t)↓ 5 I 8 (4f 10 ) (HS, 2S+1=6) 10Dq ~ 12800 cm -1

19. A.J. Wojtowicz, FPS July 200719 Excitation spectrum of VUV emissions

20. A.J. Wojtowicz, FPS July 200720 Emission and absorption spectra; the lowest d-level

21. A.J. Wojtowicz, FPS July 200721 The shape, difference between emission and absorption Zero-phonon line, exp(-S) S, Huang Rhys factor S ~ 1-2 Surprisingly weak electron-phonon coupling, consequently relatively narrow bands in emission and absorption No good match to 2 F 5/2 or other high 4f 11 levels, transitions directly from d ?

22. A.J. Wojtowicz, FPS July 200722 UNDER VUV EXCITATION THERE ARE 3 DISTINCT TYPES OF EMISSIONS: 1)VIS Er 3+ 4f 11 emission lines 2)UV and VUV 4f 11, 5d-4f emission lines and bands 3)STE emisssion (no relation to Er) SUMMARY

23. A.J. Wojtowicz, FPS July 200723 UNDER VUV EXCITATION THERE ARE 3 DISTINCT CHANNELS: 1)Below 124 nm, STE emission, no transfer to Er 2)Above 124 nm, up to about 170 nm Er-BE (CT), large relaxation (F+H), subsequent transfer to lower Er 4f 11 energy levels (VIS emissions) 3)In the range 130 to 160 nm, d-levels, direct? via ? ( 2 F(2) 5/2 ?) higher Er 4f 11 energy levels (UV and VIS emissions) SUMMARY

24. A.J. Wojtowicz, FPS July 200724 No fast and efficient scintillator Although there is some competition from direct VIS, VUV and UV (and consequtive VIS) emissions prevail under selective VUV excitation into d-bands quantum cutting phosphors? VUV, UV lasers? Possible upconversion (reversed two photon cascade emission) SUMMARY

25. A.J. Wojtowicz, FPS July 200725 ADVICE AND EXPERTISE in VUV experiments Prof. G. Zimmerer, dr M. Kirm of Hasylab Drs. J. Sutherland, J. Trunk, U9B Brookhaven Help in experiments: W. Drozdowski, J. Glodo, M. Wisniewska, D.Wisniewski Crystals of BaF 2 :Er and others, donated by A. Lempicki, ALEM Assoc. Boston ACKNOWLEDGMENT