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

Outline

4fN and 4fN-15d states.

Transitions between configurations.

Ab-inito calculations of excited-state geometry.

Spectroscopic probes of excited-state geometry.

FEL study of excitons in CaF2:Yb2+

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Reid's goal rescues Kiwis

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Lanthanide 2+/3+ ground state: 5s2 5p6 4fN 5d0

5d

4f

5s5p

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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

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Understanding the energy levels: 4fN

Coulomb Spin-orbit “Crystal-field”

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Understanding the energy levels: 4fN-15d

T2

Cubic: higher energy

ECubic:

lower energy

Crystal-field Coulomb, etc

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Absorption Emission

Stokesshift

Vibrational configurations4f

5d

Displacement [Note: may be expansion or contraction!]

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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

Conduction Band

Valence Band

4f

5d

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Ce3+ : 4f1 5d1

Pr3+ : 4f2 4f15d1

Nd3+ : 4f3 4f25d1

CaF2 (cubic sites)ET2

Energy

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Tm3+:LiYF4: 4f12→4f115d1

SFSA

GS

HS

LS

Low Spin High Spin

Second half of series

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Radiative Lifetimes: Tm3+:LiYF4

spin-allowed: 10s of ns(also non-radiative)

spin-forbidden: 10s of µs

NR

SFSA

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

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E

T2

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?

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6PJ

8S7/2

6IJ

6DJ

0

33000

49500

E (cm-1)

3/25/27/2

6GJ

First excitation energyis fixed: ~33000 cm-1

Second excitation isscanned in energy:~16000-30000 cm-1

Excitation range~49000-63000

Excited State Absorption (ESA) Gd3+ Paul Peijzel, Andries Meijerink

278 nm luminescence

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LaF3:Gd3+ ESA

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)

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Yb2+ Emission/Absorption not symmetricin some cases

Moine et al, J. Phys. France 50, 2105 (1989)

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4f14

4f135d

4f13+e

Moine et al, J. Phys. France 50, 2105 (1989)

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F-Ca2+

Yb3+

Yb2+

Ca2+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 4f14 bond length than lowest exciton state.

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4f135d

4f141

4f13+e

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2

10K

40K

(University of Utrecht)ΔR

40cm-1

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FELIXSynchonized UV laser + FEL

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UVIR

Emission

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UV

IR

Emission

50μsNote: Lifetime is 13ms!

10 Hz 6μs IRmacropulse

1kHz ps UV

4f135d

4f141

4f13+e

3

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2 40cm-1

Temperature Dependence

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As the temperature increases higher exciton states are populated so the FEL pulse has less effect. ΔR

4f135d

4f141

4f13+e

3

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2 40cm-1

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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

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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

Triplet

Cry

stal

Fie

ld

Sharp features?

Exc

hang

e

Sharp lines

The sharp lines can be explained by transitions within the 4f13 hole.

Not all transitions are allowed.

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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.

34ΔR from 4f14

“s”

“p”

“d”

ΔE

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.

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