Excitons and traps in  rare-earth materials probed by a  free-electron laser  Michael F Reid Jon Wells, Pubudu Senanayake ...
Outline <ul><li>4f N , 4f N-1 5d, and exciton states </li></ul><ul><li>FELIX FEL </li></ul><ul><li>Excited state absorptio...
Lanthanides (rare earth) materials  <ul><li>Generally form 3+ or 2+ ions  </li></ul><ul><ul><ul><li>Valence electrons are ...
Filling of orbitals s s p d f
Lanthanides:  4f N ,  4f N-1 5d,  Excitons   <ul><li>4f N </li></ul><ul><ul><li>No configuration shift </li></ul></ul><ul>...
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+ <ul><li>Pascual, Schamps, Barandiaran, Seijo, PRB 74, 104105 (2006) BaF 2 :Ce 3+  cub...
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...
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 (...
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 <ul><li>The sharp lines can be explained by transitions within the 4f 13  hole. </li></ul><ul><li>Not all tran...
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 Liberatio...
Conclusion  <ul><li>FEL experiments give us a unique tool to investigate: </li></ul><ul><ul><li>Excitonic states </li></ul...
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15.30 o11 m reid

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Research 8: M Reid

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  • E state has shorter bond length than ground state. T2 longer. Can tell from pressure experiments, etc.
  • UV laser at left goes through wall. Felix is synchronized with the UV laser UV 1kHz, Felix 10Hz macropulses.
  • 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.
  • Cubic, so MD allowed (or vibronic). The calculation is oscillator strength. The normalization of the excitation spectrum has not been worked out yet.
  • 15.30 o11 m reid

    1. 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. 2. Outline <ul><li>4f N , 4f N-1 5d, and exciton states </li></ul><ul><li>FELIX FEL </li></ul><ul><li>Excited state absorption with UV + IR </li></ul><ul><li>Yb 2+ in CaF 2 , SrF 2 </li></ul>Marsden Fund
    3. 3. Lanthanides (rare earth) materials <ul><li>Generally form 3+ or 2+ ions </li></ul><ul><ul><ul><li>Valence electrons are 4f. </li></ul></ul></ul><ul><ul><ul><li>Chemically very similar since 4f electrons are close to nucleus and shielded by 5s and 5p electrons. </li></ul></ul></ul><ul><ul><ul><li>N = 1..14 means optical and magnetic properties can be tuned. </li></ul></ul></ul><ul><ul><ul><li>Widely used in phosphors, amplifiers, lasers, etc... </li></ul></ul></ul>
    4. 4.
    5. 5. Filling of orbitals s s p d f
    6. 6. Lanthanides: 4f N , 4f N-1 5d, Excitons <ul><li>4f N </li></ul><ul><ul><li>No configuration shift </li></ul></ul><ul><ul><li>Sharp lines </li></ul></ul><ul><ul><li>Long lifetimes </li></ul></ul><ul><li>4f N-1 5d </li></ul><ul><ul><li>Different bond length </li></ul></ul><ul><ul><li>Broad absorption bands from 4f N </li></ul></ul><ul><ul><li>Broad emission bands </li></ul></ul><ul><ul><li>Short lifetimes </li></ul></ul><ul><li>Excitons </li></ul><ul><ul><li>Excited electron can become delocalized, giving an excitonic state </li></ul></ul><ul><ul><li>Large bond-length change </li></ul></ul><ul><ul><li>Very broad, red-shifted, emission bands </li></ul></ul><ul><ul><li>Long lifetimes </li></ul></ul>e -
    7. 7. 4f N : Sharp-line spectra
    8. 8. Vibrations
    9. 9. Bonds are like springs Equilibrium Change in electronic state can change spring constant New equilibrium
    10. 10. Quantized Vibration Version
    11. 11. Conduction Band, Free Electrons, Excitons Conduction Band Valence Band 4f 5d
    12. 12. Excitons: Can be “free”… Ours are “bound”
    13. 13. Excited-state geometry: BaF 2 :Ce 3+ <ul><li>Pascual, Schamps, Barandiaran, Seijo, PRB 74, 104105 (2006) BaF 2 :Ce 3+ cubic sites. </li></ul><ul><li>Potential surfaces: </li></ul><ul><ul><li>5d E is contracted </li></ul></ul><ul><ul><li>5d T 2 is expanded </li></ul></ul><ul><ul><li>As bond length contracts 6s orbital becomes delocalized. </li></ul></ul>E T 2 Energy Ce 3+ :CaF 2 4f 1  5d 1 Ce 3+ : 4f 1  5d 1
    14. 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. 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. 16. SrCl 2 :Yb 2+ Absorption “ Normal”Emission
    17. 17. CaF 2 :Yb 2+ Absorption “ anomalous” Emission 4f 14 4f 13 5d 4f 13 +e ? τ rad =15ms τ rad =260μs
    18. 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. 19. FELIX Nieuwegein, Netherlands
    20. 20. FELIX Synchonized UV laser + FEL UV IR
    21. 22. UV + FEL + Emission Spectrometer UV IR Emission
    22. 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 !
    23. 24. 10K Time-resolved spectrum Shift similar to temperature Probably same emission
    24. 25. 12.1 µ m 825 cm -1 16 µ m 625 cm -1 Faster emission from higher exciton state More sites radiating
    25. 26. Scan IR 12.1 µ m 825 cm -1 16 µ m 625 cm -1
    26. 27. Integrate over time to obtain spectrum
    27. 28. Sharp lines <ul><li>The sharp lines can be explained by transitions within the 4f 13 hole. </li></ul><ul><li>Not all transitions are allowed. </li></ul>Yb 3+ crystal field Exchange Splitting
    28. 29. Electron Trap Liberation? Long-time enhancement must be trap liberation Coulomb trap model
    29. 30. Localized Mobile
    30. 31. Applications of Traps X-ray storage phosphors Persistent Luminescence
    31. 32. SrF 2 :Yb 2+ Larger lattice, lower energy. SrF 2 :Yb 2+ CaF 2 :Yb 2+
    32. 33. CaF 2 :Yb 2+ SrF 2 :Yb 2+
    33. 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
    34. 35. Conclusion <ul><li>FEL experiments give us a unique tool to investigate: </li></ul><ul><ul><li>Excitonic states </li></ul></ul><ul><ul><li>Trap states </li></ul></ul><ul><li>More experiments and analysis </li></ul><ul><ul><li>FEL </li></ul></ul><ul><ul><li>Synchrotron </li></ul></ul><ul><ul><li>Local laser experiments </li></ul></ul><ul><ul><li>Detailed modeling </li></ul></ul>

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