uv and vuv spectroscopy of rare earth activated wide bandgap materials a.j. wojtowicz institute of...
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![Page 1: UV and VUV spectroscopy of rare earth activated wide bandgap materials A.J. Wojtowicz Institute of Physics, N. Copernicus Univ. Toruń, POLAND II International](https://reader036.vdocuments.mx/reader036/viewer/2022070409/56649e9e5503460f94b9f3ab/html5/thumbnails/1.jpg)
UV and VUV spectroscopy of rare earth activated wide bandgap
materials
A.J. WojtowiczInstitute of Physics, N. Copernicus Univ. Toruń, POLAND
II International Workshop on Advanced Spectroscopy and Optical Materials
IWASOM ’08, July 13-17, Gdańsk
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OUTLINE
Introduction to Rare Earth ions in solid state materials; significance of UV
and VUV spectral ranges
VUV/ UV luminescence and luminescence excitation spectroscopy of BaF2:Er, BaF2:Ce
and (Ba,La)F2:Er; experimental results
Model; configuration coordinate diagram
SUMMARY
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RE3+ ions
[Xe]4fn, [Xe]4fn-15d
Intraconfigurational transitions 4fn → 4fn (sharp lines, parity forbidden slow emissions)
Interconfigurational transitions 4fn → 4fn-15d broad bands, parity allowed,
FAST?
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Scintillators
Ce scintillators: LSO, LYSO, LuAP, LuYAP
fn 2n31
1105.1
2
2 2
4R
Ce: 350 nm, 15-30 ns
then Pr i Nd, emitting at 250, 190 nm,
should have 8-15 and 5-10 ns (more or less true)
Heavy lanthanides d-levels are even higher…
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Could it be that 5d-4f emission from ions
such as Tb, Dy, Ho, Er or Tm is efficient and
even faster under excitation by ionizing
radiation?
Is the oscillator strength f up to the
expectations?
Is energy transfer from host to ion efficient?
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In general, for Ln 4fn
4fn → 5d4fn-1
for Ce just one option:
4f → 5d (no f electrons left behind)
for Pr
4f2↑↑(HS) → 5d↑4f↑(HS) or 5d↓4f↑(LS);
the second option is higher in energy and
forbidden; f almost the same
for higher n it is getting worse;
f spreads over more final states; lower f
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For n > 7 situation changes drastically.
For Tb3+ (8 electrons, 7+1):
4fn ↑↑↑↑↑↑↑↓(2S+1=7, LS) → 5d↑4fn-1 ↑↑↑↑↑↑↑
(2S+1=9, HS) spin-forbidden, lower in energy
and
5d↓4fn-1 ↑↑↑↑↑↑↑, no spin flip (2S+1=7, LS)
spin-allowed but higher in energy – Hund’s
rule;
the lowest excited state will be HS and
emission transition will be spin-forbidden
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For n > 7 situation changes drastically.
For Tb3+ (8 electrons, 7+1):
4fn ↑↑↑↑↑↑↑↓(2S+1=7, LS) → 5d↑4fn-1 ↑↑↑↑↑↑↑
(2S+1=9, HS) spin-forbidden, lower in energy
and
5d↓4fn-1 ↑↑↑↑↑↑↑, no spin flip (2S+1=7, LS)
spin-allowed but higher in energy – Hund’s
rule;
the lowest excited state will be HS and
emission transition will be spin-forbidden
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For n > 7 situation changes drastically.
For Tb3+ (8 electrons, 7+1):
4fn ↑↑↑↑↑↑↑↓(2S+1=7, LS) → 5d↑4fn-1 ↑↑↑↑↑↑↑
(2S+1=9, HS) spin-forbidden, lower in energy
and
5d↓4fn-1 ↑↑↑↑↑↑↑, no spin flip (2S+1=7, LS)
spin-allowed but higher in energy – Hund’s
rule;
the lowest excited state will be HS and
emission transition will be spin-forbidden
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For n > 7 situation changes drastically.
For Tb3+ (8 electrons, 7+1):
4fn ↑↑↑↑↑↑↑↓(2S+1=7, LS) → 5d↑4fn-1 ↑↑↑↑↑↑↑
(2S+1=9, HS) spin-forbidden, lower in energy
and
5d↓4fn-1 ↑↑↑↑↑↑↑, no spin flip (2S+1=7, LS)
spin-allowed but higher in energy – Hund’s
rule;
the lowest excited state will be HS and
emission transition will be spin-forbidden (?)
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Understanding the states of
the 4fn-15d configuration:
4f electrons – weak crystal field, strong spin – orbit, we have
multiplets:
2S+1LJ
split by crystal field
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5d electrons – strong crystal field, weak s – o
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5d electrons – strong crystal field, weak s – o
5d-4fn-1 coupling (imposed on CF structure),
PLUS
f – d exchange splitting (LS and HS states)
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So the state of the 4fn-15d configurationcan be described as (e.g.):
(HS) 4f10(4I15/2) 5d(e)
This is the lowest excited state of the 4f105dconfiguration of Er3+ ion in
BaF2
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In this presentation we concentrate on:
Ce: [Xe]4f, [Xe]5d, 4f and 5d configurations well
separated, no f-d coupling, no f-d exchange,
CF states provide good description of Ce3+ excited states
Er:
[Xe]4f11, [Xe]4f105d configurations overlap; significant f-d coupling, f-d exchange
(LS and HS d-levels)
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100 150 200 250 300
inte
ns
ity
, arb
. un
its
wavelength, nm
fast (0-40 ns) slow (150-190 ns)
BaF2:Ce, excitation spectrum
10 K, emi 323.5 nm
bandgap
d(t)
d(e)10Dq 16800cm-1
excitons?
e-h pairslow sym. CF
2-3 kcm-1
high VUV sensitivity, good scintillator material
Ce
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Fast emission (30 ns), only the lowest d–level emits
270 300 330 360
inte
ns
ity
, arb
. un
its
wavelength, nm
excitation emission
Excitation and emission spectraBaF2:Ce (0.015%), 10 K
Ce
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200 400 600 800 1000
0,05
0,10
0,15o
pti
ca
l de
ns
ity
wavelength, nm
Absorption, RTexp. and theoryBa,La)F2:Er (0.2%)
exp. SeJacalc. DaPi
2F7/2, 2F5/2 108.5 and 103,2 nm
2G9/2, 2G7/2, 2F5/2 142.8, 151.6 and 158.6 nm
Er
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90 120 150 180
2G9/2
2G7/2 2F5/2
LS, d(e)-4f10 5I6
LS, d(e)-4f10 5I7
LS, d(e)-4f10 5I8
inte
ns
ity
, arb
. un
its
wavelength, nm
excitation spectrum, 10 K(Ba,La)F2:Er (0.2%)
emission at 550 nm
calc. DaPi
x10
HS, d(e)-4f10 5I8
no VUV sensitivity, poor scintillator material
Er
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200 250 300 350 400 450 500 550
inte
ns
ity
, arb
. un
its
wavelength, nm
fast (0-40ns) slow (150-190ns)
Emission, 10 Kexc. 157 nm(Ba,La)F2:Er (0.2%)
FAST!!!!
Er
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IFfast relaxation
thenonly the lowest
level emits
transition is spin forbidden
henceSLOW
We must have emission from (LS, J = 8) !!
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Fast emission starts indeed from the LS band at 158 nm…
Notice slowband at 170 nm!(HS emission)
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Ce3+ in BaF2
d-f emission –fast (30 ns)
Er3+ in BaF2
4f105d → 4f11 emission slow (HS)
Er3+ in (Ba,La)F2
4f105d → 4f11 emission fast (45 ns at LHe)
(?? LS ??)
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BUT
under 148 nm excitation (higher LS d-band)
the emission from (Ba,La)F2:Er is different
(sharp lines no bands) and slow…
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200 250 300 350 400 450 500 550
inte
ns
ity
, arb
. un
its
wavelength, nm
fast (0-40ns) slow (150-190ns)
Emission, 10 Kexc. 148 nm(Ba,La)F2:Er (0.2%)
UV spectra…
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and VUV/UV spectra…
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sharp slow lines excited at 148 nm start from the 2G7/2…
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Three emissions from (Ba,La)F2:
FAST (d-f) (LS, J = 8)
SLOW (d-f) (HS, J = 8)
SLOW (f-f) (LS, J = 7)
CAN we confirm this by wavelength selective excitation?
YES!!
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170 nm:slow d (HS)
234.9 nm: fast d (LS) and slow 4f11 2G7/2
263 nm: slow 4f11 2G7/2
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MODELConfiguration Coordinate Diagram
Assumptions:
All states of the same electronic configuration have the same
equilibrium position
BUT
The equilibrium positions for states of the 4f11 and 4f105d configurations are different
The energies taken from experiment
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MODEL, BaF2
-50 0 50 100 150 200 250
60 000
65 000
70 000
4f
5d(LS)
5d(LS)
5d(HS)
En
erg
y, c
m-1
configuration coordinate, arb. units
HS (4f10)5I8 5d BF
LS (4f10)5I8 5d(e) BF
LS (4f10) 5I7 5d(e) BF
4f11(2F5/2)
4f11(2G7/2)
4f
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MODEL, (Ba,La)F2
-50 0 50 100 150 200 250
60 000
65 000
70 000
4f
5d(LS)
5d(LS)
5d(HS)
En
erg
y, c
m-1
configuration coordinate, arb. units
HS (4f10)5I8 5d BLF
LS (4f10)5I8 5d(e) BLF
LS (4f10) 5I7 5d(e) BLF
4f11(2F5/2)
4f11(2G7/2)
4f
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Summary
VUV response critical for scintillator (phosphor)materials (Ce good, Er bad)
Relatively slow nonradiative relaxation in (La,Ba)F2:Er between the lowest LS and HS 4f105d levels
Fast and efficient 4f105d → 4f11 emissions from the (LS, J = 8) level bypassing (HS, J = 8) 4f105d level
2G7/2 emission under the (LS, J = 7) level excitation at 10 K;
indirect identification of the 2G7/2 level
2G7/2 - THE HIGHEST KNOWN EMITTING 4f11-level of Er3+ ion in solid state material (66 100 cm-1)
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ACKNOWLEDGMENTS
Collaborators:
experimentS. Janus (PhD student)R. Theis (PhD student)
K. Jastak (student)
Calculations of 4fn energy levels D. Piatkowski (PhD student)
Prof. M.F. Reid (University of Canterbury,
Christchurch, New Zealand)is gratefully acknowledged for providing
f-shell empirical programs to calculate 4f11 levels
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SAMPLES and EXPERIMENTS
(Ba,La)F2:Er crystals grown at
Optovac, MA, USA,
donated by
Prof. A. Lempicki of Boston University
VUV and UV emission/excitation spectra,
and time profiles measured at
Superlumi station of I–beamline, DORIS III
Hasylab, Hamburg, Germany
Prof. G. Zimmerer and Dr G. Stryganyuk