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Research 8: M Reid
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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
11
Conduction Band, Free Electrons, Excitons
Conduction Band
Valence Band
4f
5d
12
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.
13
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
18
FEL Excited State Absorption
UV Laser
Exciton emission
IR FEL
4f135d
4f14
4f13+e
40cm-1
τrad
=15ms
τrad
=260μs
FELIX Nieuwegein, Netherlands
20
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
27
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.
28
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
35
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
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