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

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

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

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

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+


Reid s goal rescues kiwis

Reid's goal rescues Kiwis


Lanthanide 2 3 ground state 5s 2 5p 6 4f n 5d 0

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

5d

4f

5s

5p


4f n and 4f n 1 5d

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


Understanding the energy levels 4f n

-

Understanding the energy levels:4fN

Coulomb

Spin-orbit

“Crystal-field”


Understanding the energy levels 4f n 1 5d

Understanding the energy levels:4fN-15d

T2

Cubic:

higher energy

E

Cubic:

lower energy

Crystal-field

Coulomb, etc


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

Absorption

Emission

5d

Stokes

shift

Vibrational

configurations

4f

Displacement

[Note: may be expansion or contraction!]


Example energy levels in cubic systems such as caf 2

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


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

CaF2 (cubic sites)‏

T2

E

Ce3+ : 4f1 5d1

Energy

Pr3+ : 4f2  4f15d1

Nd3+ : 4f3 4f25d1


Tm 3 liyf 4 4f 12 4f 11 5d 1

Low Spin

High Spin

Tm3+:LiYF4:4f12→4f115d1

LS

HS

SA

SF

GS

Second half of series


Radiative lifetimes tm 3 liyf4

NR

SA

SF

Radiative Lifetimes: Tm3+:LiYF4

spin-allowed: 10s of ns

(also non-radiative)‏

spin-forbidden: 10s of µs


Ab initio calculations

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


Yb 2 cscabr 3 s nchez sanz seijo and barandiar n j phys chem a 2009 113 12591 2009

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?


Excited state absorption esa gd 3 paul peijzel andries meijerink

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


Laf 3 gd 3 esa

LaF3:Gd3+ ESA


Exitons in caf 2 yb 2

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)


Yb 2 emission absorption not symmetric in some cases

Yb2+ Emission/Absorption not symmetricin some cases

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


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

4f135d

4f13+e

4f14

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


Exciton model

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)


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

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)


Felix synchonized uv laser fel

FELIXSynchonized UV laser + FEL


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

IR

UV

Emission


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

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!


Temperature dependence

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


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

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

Water in low-energy spectrum


Modelling yb 3 4f 13 s p d electron

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


Sharp lines

Sharp lines

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

  • Not all transitions are allowed.


Broad band

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


Conclusions

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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