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 ReynoldsUniversity of CanterburyAndries Meijerink, Gabriele BellocchiUniversity of UtrechtGiel Berden, Britta Redlich, Lex van der MeerFELIX free electron laser facility, FOM Rijnhuizen, NieuwegeinChang-Kui DuanChongqing University of Post and Telecommunications
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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
5s 5p
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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
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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
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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
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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)
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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
3
54
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
54
2 40cm-1
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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
54
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.
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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?
Exch
ange
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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.
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“s”
“p”
“d”
ΔE
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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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