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

Outline  4f N and 4f N-1 5d states.  Transitions between configurations.  Ab-inito calculations of excited-state geometry.  Spectroscopic probes of excited-state geometry.  FEL study of excitons in CaF 2 :Yb 2+ 2

Reid's goal rescues Kiwis 3

4

5

6 Lanthanide 2+/3+ ground state: 5s 2 5p 6 4f N 5d 0 5d 4f 5s 5p

7 4f N and 4f N-1 5d  N can range from 0 to 14  Can tune the electronic structure  Small interaction with surrounding ions  Similar chemistry  Optical Applications:  4f N  Sharp lines  Long lifetimes  Similar patterns in all materials  So ideal for laser and phosphor applications  4f N-1 5d  Broad absorption bands from 4f N  Useful for absorbing energy  Short lifetimes useful in some applications, such as scintillators

8 Understanding the energy levels: 4f N CoulombSpin-orbit“Crystal-field” -

9 Understanding the energy levels: 4f N-1 5d T 2 Cubic: higher energy E Cubic: lower energy Crystal-fieldCoulomb, etc

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

11 Example: Energy levels in cubic systems such as CaF 2 Cubic environment splits E and T 2 orbitals Coulomb and spin-obit interactions adds extra structure Conduction band has an important influence on lifetimes Conduction Band Valence Band 4f 5d

12 Ce 3+ : 4f 1  5d 1 Pr 3+ : 4f 2  4f 1 5d 1 Nd 3+ : 4f 3  4f 2 5d 1 CaF 2 (cubic sites)‏ ET2T2 Energy

13 Tm 3+ :LiYF 4 : 4f 12 → 4f 11 5d 1 SF SA GS HS LS Low Spin High Spin Second half of series

14 Radiative Lifetimes: Tm 3+ :LiYF4 spin-allowed: 10s of ns (also non-radiative)‏ spin-forbidden: 10s of µs NR SF SA

Ab-initio calculations  Pascual, Schamps, Barandiaran, Seijo, PRB 74, (2006) BaF 2 :Ce 3+ cubic sites.  Potential surfaces:  5d E is contracted  5d T 2 is expanded  f-d transitions broadened 15 E T2T2

Yb 2+ :CsCaBr 3 Sánchez-Sanz, Seijo, and Barandiarán J. Phys. Chem. A 2009, 113, (2009)  Multi-electron system so more 4f 13 5d states than just the 5d(E) and 5d(T 2 ), 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? 16

17 6PJ6PJ 8 S 7/2 6IJ6IJ 6DJ6DJ E (cm -1 ) 3/2 5/2 7/2 6GJ6GJ First excitation energy is fixed: ~33000 cm -1 Second excitation is scanned in energy: ~ cm -1 Excitation range ~ Excited State Absorption (ESA) Gd 3+ Paul Peijzel, Andries Meijerink 278 nm luminescence

18 LaF 3 :Gd 3+ ESA

Exitons in CaF 2 :Yb 2+  When Yb 2+ or Eu 2+ is doped in some materials emission is too shifted and broadened to be from the 4f N-1 5d 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) 19

20 Yb 2+ Emission/Absorption not symmetric in some cases Moine et al, J. Phys. France 50, 2105 (1989)

21 4f 14 4f 13 5d 4f 13 +e Moine et al, J. Phys. France 50, 2105 (1989)

22 F-F- Ca 2+ Yb 3+ Yb 2+ Ca 2+ F-F- Exciton model Dorenbos J. Phys.: Condens. Matter 15, 2645 (2003) Moine et al, J. Phys. France 50, 2105 (1989)

Temperature Dependence: Excited state at 40cm -1 deduced by Moine et al from temperature studies must have bond length closer to 4f 14 bond length than lowest exciton state. 23 4f 13 5d 4f f 13 +e K 40K (University of Utrecht) ΔRΔR 40cm -1

24 FELIX Synchonized UV laser + FEL

25

26 UV IR Emission

27 UV IR Emission 50μs Note: Lifetime is 13ms! 10 Hz 6μs IR macropulse 1kHz ps UV 4f 13 5d 4f f 13 +e cm -1

Temperature Dependence 28 As the temperature increases higher exciton states are populated so the FEL pulse has less effect. ΔRΔR 4f 13 5d 4f f 13 +e cm -1

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

Water in low-energy spectrum 30

31

32 Modelling: Yb 3+ (4f 13 ) + “s” / “p”/”d” electron? Broad bands: Delocalized electron in different orbitals. Sharp lines: Re-arrangement of 4f 13 core. Lowest exciton state: 4f 13 +“s”: H = 4f spin-orbit + 4f crystal field + fs exchange Coulomb. Only extra parameter is G 3 (fs), giving triplet/singlet splitting. Singlet Triplet Crystal Field Sharp features? Exchange

Sharp lines  The sharp lines can be explained by transitions within the 4f 13 hole.  Not all transitions are allowed. 33

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. 34 ΔR from 4f 14 “s” “p” “d” ΔEΔE

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. 35

36