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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 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
3
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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
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.
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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