1. The document studies excitons and traps in rare-earth materials like Yb2+ doped CaF2 and SrF2 using a free-electron laser (FEL) combined with ultraviolet excitation. 2. Excitation creates an excitonic state where the excited electron becomes delocalized, leading to bond length changes and broad, red-shifted emission bands. 3. FEL excitation of the excitonic state is used to probe trap liberation, showing enhanced emission even after exciton decay, relevant to applications like phosphors.

1. Excitons and traps in rare-earth materials probed by a free-electron laser Michael F Reid Jon Wells, Pubudu Senanayake Alex Salkeld, Roger Reeves Giel Berden, FELIX Andries Meijerink, Utrecht Chang-Kui Duan, USTC, China NZIP, Wellington, October 18, 2011

2. Outline 4f N , 4f N-1 5d, and exciton states FELIX FEL Excited state absorption with UV + IR Yb 2+ in CaF 2 , SrF 2 Marsden Fund

3. Lanthanides (rare earth) materials Generally form 3+ or 2+ ions Valence electrons are 4f. Chemically very similar since 4f electrons are close to nucleus and shielded by 5s and 5p electrons. N = 1..14 means optical and magnetic properties can be tuned. Widely used in phosphors, amplifiers, lasers, etc...

5. Filling of orbitals s s p d f

6. Lanthanides: 4f N , 4f N-1 5d, Excitons 4f N No configuration shift Sharp lines Long lifetimes 4f N-1 5d Different bond length Broad absorption bands from 4f N Broad emission bands Short lifetimes Excitons Excited electron can become delocalized, giving an excitonic state Large bond-length change Very broad, red-shifted, emission bands Long lifetimes e -

7. 4f N : Sharp-line spectra

8. Vibrations

9. Bonds are like springs Equilibrium Change in electronic state can change spring constant New equilibrium

10. Quantized Vibration Version

11. Conduction Band, Free Electrons, Excitons Conduction Band Valence Band 4f 5d

12. Excitons: Can be “free”… Ours are “bound”

13. Excited-state geometry: BaF 2 :Ce 3+ Pascual, Schamps, Barandiaran, Seijo, PRB 74, 104105 (2006) BaF 2 :Ce 3+ cubic sites. Potential surfaces: 5d E is contracted 5d T 2 is expanded As bond length contracts 6s orbital becomes delocalized. E T 2 Energy Ce 3+ :CaF 2 4f 1  5d 1 Ce 3+ : 4f 1  5d 1

14. SrCl 2 :Yb 2+ : Sánchez-Sanz et al. J. Chem. Phys. 133, 114509 (2010) Yb 2+ Cl - /F - Sr 2+ /Ca 2+ Sr Cl 2 : Yb 2+ / Ca F 2 : Yb 2+

15. SrCl 2 :Yb 2+ 4f 14 4f 13 5d (mixed) 4f 13 6s Sánchez-Sanz et al. J. Chem. Phys. 133, 114509 2010 bond length 4f 13 5d (E) 30000 cm -1 Exciton state forms as excited electron becomes delocalized and bonds shorten

16. SrCl 2 :Yb 2+ Absorption “ Normal”Emission

17. CaF 2 :Yb 2+ Absorption “ anomalous” Emission 4f 14 4f 13 5d 4f 13 +e ? τ rad =15ms τ rad =260μs

18. FEL Excited State Absorption UV Laser Exciton emission IR FEL 4f 13 5d 4f 14 4f 13 +e 40cm -1 τ rad =15ms τ rad =260μs

19. FELIX Nieuwegein, Netherlands

20. FELIX Synchonized UV laser + FEL UV IR

22. UV + FEL + Emission Spectrometer UV IR Emission

23. 1kHz ps UV 10 Hz 6 μ s IR macropulse UV IR Emission 365 nm 12.1 µ m 825 cm -1 Lowest state τ rad =15ms !

24. 10K Time-resolved spectrum Shift similar to temperature Probably same emission

25. 12.1 µ m 825 cm -1 16 µ m 625 cm -1 Faster emission from higher exciton state More sites radiating

26. Scan IR 12.1 µ m 825 cm -1 16 µ m 625 cm -1

27. Integrate over time to obtain spectrum

28. Sharp lines The sharp lines can be explained by transitions within the 4f 13 hole. Not all transitions are allowed. Yb 3+ crystal field Exchange Splitting

29. Electron Trap Liberation? Long-time enhancement must be trap liberation Coulomb trap model

30. Localized Mobile

31. Applications of Traps X-ray storage phosphors Persistent Luminescence

32. SrF 2 :Yb 2+ Larger lattice, lower energy. SrF 2 :Yb 2+ CaF 2 :Yb 2+

33. CaF 2 :Yb 2+ SrF 2 :Yb 2+

34. Trap Liberation in SrF 2 :Yb 2+ Effective even after exciton decay UV IR Exciton ESA + Trap Liberation Only Trap Liberation 200μs 400μs 600μs 800μs

35. Conclusion FEL experiments give us a unique tool to investigate: Excitonic states Trap states More experiments and analysis FEL Synchrotron Local laser experiments Detailed modeling