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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
Outline ,[object Object],[object Object],[object Object],[object Object],Marsden Fund
Lanthanides (rare earth) materials  ,[object Object],[object Object],[object Object],[object Object],[object Object]
Filling of orbitals s s p d f
Lanthanides:  4f N ,  4f N-1 5d,  Excitons   ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],e -
4f N :  Sharp-line spectra
Vibrations
Bonds are like springs Equilibrium Change in electronic state  can change spring constant New equilibrium
Quantized Vibration Version
Conduction Band, Free Electrons, Excitons Conduction Band Valence Band 4f 5d
Excitons: Can be “free”… Ours are “bound”
Excited-state geometry: BaF 2 :Ce 3+ ,[object Object],[object Object],[object Object],[object Object],[object Object],E T 2 Energy Ce 3+ :CaF 2  4f 1     5d 1 Ce 3+  : 4f 1     5d 1
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+
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
SrCl 2 :Yb 2+ Absorption “ Normal”Emission
CaF 2 :Yb 2+ Absorption “ anomalous” Emission 4f 14 4f 13 5d 4f 13 +e ? τ rad =15ms τ rad =260μs
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
FELIX  Nieuwegein, Netherlands
FELIX Synchonized UV laser + FEL UV IR
 
UV +  FEL  +  Emission  Spectrometer UV IR Emission
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 !
10K Time-resolved spectrum  Shift similar to temperature Probably same emission
12.1  µ m 825 cm -1 16  µ m 625 cm -1 Faster emission from higher exciton state More sites radiating
Scan IR  12.1  µ m 825 cm -1 16  µ m 625 cm -1
Integrate over time to obtain spectrum
Sharp lines ,[object Object],[object Object],Yb 3+  crystal field Exchange Splitting
Electron Trap Liberation?  Long-time enhancement must be trap liberation  Coulomb trap model
Localized Mobile
Applications of Traps X-ray storage phosphors  Persistent Luminescence
SrF 2 :Yb 2+   Larger lattice, lower energy.  SrF 2 :Yb 2+ CaF 2 :Yb 2+
CaF 2 :Yb 2+ SrF 2 :Yb 2+
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
Conclusion  ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

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15.30 o11 m reid

  • 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.
  • 3.
  • 4.
  • 6.
  • 7. 4f N : Sharp-line spectra
  • 9. Bonds are like springs Equilibrium Change in electronic state can change spring constant New equilibrium
  • 11. Conduction Band, Free Electrons, Excitons Conduction Band Valence Band 4f 5d
  • 12. Excitons: Can be “free”… Ours are “bound”
  • 13.
  • 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
  • 21.  
  • 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.
  • 29. Electron Trap Liberation? Long-time enhancement must be trap liberation Coulomb trap model
  • 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.

Notas do Editor

  1. E state has shorter bond length than ground state. T2 longer. Can tell from pressure experiments, etc.
  2. UV laser at left goes through wall. Felix is synchronized with the UV laser UV 1kHz, Felix 10Hz macropulses.
  3. With ratio we get spectrum. High-energy from january with caf2 windows. I cut it off at 10microns, where it starts to dip. Medium with PbS windows Low with polypropolene windows and different FEL. Medium energy range is probably saturated.
  4. Cubic, so MD allowed (or vibronic). The calculation is oscillator strength. The normalization of the excitation spectrum has not been worked out yet.