Excited-state structure and dynamics of high-energy states in lanthanide materials

1. Excited-state structure and dynamics of high-energy states in lanthanide materials Mike Reid, Jon-Paul Wells, Roger Reeves, Pubudu Senanayake, Adrian ReynoldsUniversity of Canterbury Andries Meijerink, Gabriele BellocchiUniversity of Utrecht Giel Berden, Britta Redlich, Lex van der MeerFELIX free electron laser facility, FOM Rijnhuizen, Nieuwegein Chang-Kui DuanChongqing University of Post and Telecommunications

2. Outline • 4fN and 4fN-15dstates. • Transitions between configurations. • Ab-inito calculations of excited-state geometry. • Spectroscopic probes of excited-state geometry. • FEL study of excitons in CaF2:Yb2+

3. Reid's goal rescues Kiwis

6. Lanthanide 2+/3+ ground state: 5s2 5p6 4fN 5d0 5d 4f 5s 5p

7. 4fN and 4fN-15d • N can range from 0 to 14 • Can tune the electronic structure • Small interaction with surrounding ions • Similar chemistry • Optical Applications: • 4fN • Sharp lines • Long lifetimes • Similar patterns in all materials • So ideal for laser and phosphor applications • 4fN-15d • Broad absorption bands from 4fN • Useful for absorbing energy • Short lifetimes useful in some applications, such as scintillators

8. - Understanding the energy levels:4fN Coulomb Spin-orbit “Crystal-field”

9. Understanding the energy levels:4fN-15d T2 Cubic: higher energy E Cubic: lower energy Crystal-field Coulomb, etc

10. Absorption Emission 5d Stokes shift Vibrational configurations 4f Displacement [Note: may be expansion or contraction!]

11. Conduction Band 5d 4f Valence Band Example: Energy levels in cubic systems such as CaF2 • Cubic environment splits E and T2 orbitals • Coulomb and spin-obit interactions adds extra structure • Conduction band has an important influence on lifetimes

12. CaF2 (cubic sites)‏ T2 E Ce3+ : 4f1 5d1 Energy Pr3+ : 4f2  4f15d1 Nd3+ : 4f3 4f25d1

13. Low Spin High Spin Tm3+:LiYF4:4f12→4f115d1 LS HS SA SF GS Second half of series

14. NR SA SF Radiative Lifetimes: Tm3+:LiYF4 spin-allowed: 10s of ns (also non-radiative)‏ spin-forbidden: 10s of µs

15. Ab-initio calculations • Pascual, Schamps, Barandiaran, Seijo, PRB 74, 104105 (2006)BaF2:Ce3+ cubic sites. • Potential surfaces: • 5d E is contracted • 5d T2 is expanded • f-d transitions broadened T2 E

16. Yb2+:CsCaBr3 Sánchez-Sanz, Seijo, and Barandiarán J. Phys. Chem. A 2009, 113, 12591 (2009) • Multi-electron system so more 4f135d states than just the 5d(E) and 5d(T2), with splitting due to Coulomb and spin-orbit interactions. • Transitions where the 5d state does not change should give sharp lines. • How to observe these transitions?

17. Excited State Absorption (ESA) Gd3+Paul Peijzel, Andries Meijerink E (cm-1) First excitation energy is fixed: ~33000 cm-1 Second excitation is scanned in energy: ~16000-30000 cm-1 Excitation range ~49000-63000 6GJ 49500 6DJ 6IJ 3/2 5/2 7/2 6PJ 33000 278 nm luminescence 8S7/2 0

18. LaF3:Gd3+ ESA

19. Exitons in CaF2:Yb2+ • When Yb2+ or Eu2+ is doped in some materials emission is too shifted and broadened to be from the 4fN-15d states. • Studied extensively by McClure, Pedrini, Moine, etc. • Moine et al, J. Phys. France 50, 2105 (1989) • Moine et al, J. Lum. 48/49, 501 (1991) • Summary: Dorenbos J. Phys.: Condens. Matter 15, 2645 (2003)

20. Yb2+ Emission/Absorption not symmetricin some cases • Moine et al, J. Phys. France 50, 2105 (1989)

21. 4f135d 4f13+e 4f14 • Moine et al, J. Phys. France 50, 2105 (1989)

22. Exciton model • Moine et al, J. Phys. France 50, 2105 (1989) Yb2+ Ca2+ Ca2+ F- F- Yb3+ • Dorenbos J. Phys.: Condens. Matter 15, 2645 (2003)

23. Temperature Dependence:Excited state at 40cm-1 deduced by Moine et al from temperature studies must have bond length closer to 4f14 bond length than lowest exciton state. 4f135d 5 4 40K 3 40cm-1 2 4f13+e 10K 1 4f14 ΔR • (University of Utrecht)

24. FELIXSynchonized UV laser + FEL

26. IR UV Emission

27. IR Emission 4f135d UV 5 4 40cm-1 3 1kHz ps UV 10 Hz 6μs IR macropulse 2 4f13+e 1 50μs 4f14 Note: Lifetime is 13ms!

28. Temperature Dependence 4f135d 5 4 3 40cm-1 2 4f13+e As the temperature increases higher exciton states are populated so the FEL pulse has less effect. 1 4f14 ΔR

29. Graph is ratio of visible emission with/without FEL. Three different wavelength ranges/windows/setups. Dips are water absorption of IR.

30. Water in low-energy spectrum

32. Modelling: Yb3+(4f13) + “s” / “p”/”d” electron? Broad bands: Delocalized electron in different orbitals. Sharp lines: Re-arrangement of 4f13 core. Lowest exciton state: 4f13+“s”: H = 4f spin-orbit + 4f crystal field + fs exchange Coulomb. Only extra parameter is G3(fs), giving triplet/singlet splitting. Singlet Sharp features? Exchange Triplet Crystal Field

33. Sharp lines • The sharp lines can be explained by transitions within the 4f13 hole. • Not all transitions are allowed.

34. Broad Band • Broad band must involve change in wavefunction of delocalized electron. • Change in bond length is proportional to band width. • Energy level at 40cm-1 has longer bond length than lowest exciton state (from temperature data). • Broad band in ESA at 600cm-1 must be another arrangement of delocalized electron with longer bond length. “d” ΔE “p” “s” ΔR from 4f14

35. Conclusions • ESA experiments can give much more detailed information about excited states. • Structure and dynamics of exciton states measured with FEL. • More experiments and modelling to come.