Excitons and traps in rare-earth materials probed by a free-electron laser

Michael F ReidMichael F Reid

Jon Wells, Pubudu Senanayake Jon Wells, Pubudu Senanayake

Alex Salkeld, Roger ReevesAlex Salkeld, Roger Reeves

Giel Berden, FELIXGiel Berden, FELIX

Andries Meijerink, UtrechtAndries Meijerink, Utrecht

Chang-Kui Duan, USTC, ChinaChang-Kui Duan, USTC, China

NZIP, Wellington, October 18, 2011

2

Outline

4fN, 4fN-15d, and exciton states FELIX FEL Excited state absorption with UV + IR Yb2+ in CaF2, SrF2

Marsden Fund

3

Lanthanides (rare earth) materials

Generally form 3+ or 2+ ions

• Valence electrons are 4f.

• Chemically very similar since 4f electrons are close to nucleus and shielded by 5s and 5p electrons.

• N = 1..14 means optical and magnetic properties can be tuned.

• Widely used in phosphors, amplifiers, lasers, etc...

4

5

s

sp

df

Filling of orbitals

6

Lanthanides: 4fN, 4fN-15d, Excitons

4fN

No configuration shift Sharp lines Long lifetimes

4fN-15d Different bond length Broad absorption bands from 4fN

Broad emission bands Short lifetimes

Excitons

Excited electron can become delocalized, giving an excitonic state

Large bond-length change

Very broad, red-shifted, emission bands

Long lifetimes

e-

7

4fN: Sharp-line spectra

8

Vibrations

9

Bonds are like springs

Equilibrium

Change in electronic state can change spring constant

New equilibrium

10

Quantized Vibration Version

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Conduction Band, Free Electrons, Excitons

Conduction Band

Valence Band

4f

5d

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Excitons: Can be “free”… Ours are “bound”

Excited-state geometry: BaF2:Ce3+

Pascual, Schamps, Barandiaran, Seijo, PRB 74, 104105 (2006)BaF2:Ce3+ cubic sites.

Potential surfaces:

5d E is contracted

5d T2 is expanded

As bond length contracts 6s orbital becomes delocalized.

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E

T2

EnergyCe3+:CaF

2 4f1 5d1

Ce3+ : 4f1 5d1

SrCl2:Yb2+: Sánchez-Sanz et al. J. Chem. Phys. 133, 114509 (2010)

Yb2+

Cl-/F-

Sr2+/Ca2+

SrCl2:Yb2+ / CaF

2:Yb2+

SrCl2:Yb2+

4f14

4f135d (mixed)

4f136s

Sánchez-Sanz et al. J. Chem. Phys. 133, 114509 2010

bond length

4f135d (E)

30000 cm-1

Exciton state forms as excited electron

becomes delocalized and bonds shorten

SrCl2:Yb2+

Absorption“Normal”Emission

CaF2:Yb2+Absorption

“anomalous”Emission

4f14

4f135d

4f13+e

??

τrad

=15ms

τrad

=260μs

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FEL Excited State Absorption

UV Laser

Exciton emission

IR FEL

4f135d

4f14

4f13+e

40cm-1

τrad

=15ms

τrad

=260μs

FELIX Nieuwegein, Netherlands

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FELIXSynchonized UV laser + FEL

UV IR

UV + FEL + Emission Spectrometer

UVIR

Emission

1kHz ps UV 10 Hz 6μs IRmacropulse

UV

IR

Emission

365 nm

12.1 µm825 cm-1

Lowest state τ

rad=15ms !

10K

Time-resolved spectrum

Shift similar to temperatureProbably same emission

12.1 µm825 cm-1

16 µm625 cm-1

Faster emission from higher exciton state

More sites radiating

Scan IR 12.1 µm825 cm-1

16 µm625 cm-1

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Integrate over time to obtain spectrum

Sharp lines

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

Not all transitions are allowed.

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Yb3+ crystal field

Exchange Splitting

Electron Trap Liberation?

Long-time enhancement must be trap liberation

Coulomb trap model

Localized

Mobile

Applications of TrapsX-ray storage phosphors Persistent Luminescence

SrF2:Yb2+ Larger lattice, lower energy.

SrF2:Yb2+

CaF2:Yb2+

CaF2:Yb2+ SrF

2:Yb2+

Trap Liberation in SrF2:Yb2+

Effective even after exciton decay

UV IR

Exciton ESA+ Trap Liberation

Only Trap Liberation

200μs

400μs

600μs

800μs

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Conclusion

• FEL experiments give us a unique tool to investigate:• Excitonic states• Trap states

• More experiments and analysis• FEL• Synchrotron• Local laser experiments• Detailed modeling