A.J. Wojtowicz, FPS July 2007 1
VUV spectroscopy of BaF2:Er
A.J. WojtowiczInstitute of Physics, N. Copernicus Univ. Toruń, POLAND
FPS 2007French-Polish Symposium on Spectroscopy
of Modern Materials in Physics, Chemistry and Biology.
July 15 – 18 2007, Clermont-Ferrand, France
A.J. Wojtowicz, FPS July 2007 2
INTRODUCTION
Why VUV spectroscopy?
Materials for UV, VUV solid state lasers
quantum cutting phosphors
scintillators
GOAL of this work:
identify various VUV excited emissions and
identify different excitation mechanisms in
BaF2:Er
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Scintillators
Ce scintillators: LSO, LYSO, LuAP, LuYAP
fn 2n31
1105.1
2
224
R
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 lantanides have even higher energy d-levels…
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Could it be that 4d-5f 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?
NO on both questions in the case of BaF2:Er
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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;
more allowed states lower f
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For n > 7 situation changes drastically.
For Tb3+:
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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SAMPLES and EXPERIMENTS
BaF2:0.2%Er, grown by Bridgman, at Optovac,
donated by A. Lempicki of ALEM, Boston
Emission and excitation spectra:
I–beamline, Superlumi, Hasylab, Hamburg,
Germany (prof. G. Zimmerer)
U9B beamline, National Synchrotron Light Source,
Brookhaven, USA (dr J. Sutherland)
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VIS emissions; lower Er3+ levels excited
A.J. Wojtowicz, FPS July 2007 9
UV emissions; higher Er3+ levels excited
A.J. Wojtowicz, FPS July 2007 10
Excitation spectra of VIS emissions; RT and 5 K
A.J. Wojtowicz, FPS July 2007 11
Excitation below 124 nm; STE, some VIS emission
A.J. Wojtowicz, FPS July 2007 12
VIS and UV emissions competition, Er-BE and d-levels
A.J. Wojtowicz, FPS July 2007 13
STE, Er-BE and d-levels
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STE emission under over the bandgap excitation
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VUV emissions under d-level excitation
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Excitation spectrum of VUV emissions
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Excitation spectrum of Ce emission in BaF2
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BaF2:Ce, crystal field splitting 10Dq, (3+2) structure, d-levels split into t and e
In BaF2:Er four bands, no single electron crystal field structure (3+2) like in BaF2:Ce
Er3+4f11 ground state 4I15/2 (LS, 2S+1=4)
Excited state 5d(e)↓ 5I8(4f10) (HS, 2S+1=6)
Excited state 5d(e)↑ 5I8(4f10) (LS, 2S+1=4)
Excited state 5d(e)↑ 5I7(4f10) (LS, 2S+1=4)
Excited state 5d(t)↓ 5I8(4f10) (HS, 2S+1=6)
10Dq ~ 12800 cm-1
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Excitation spectrum of VUV emissions
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Emission and absorption spectra; the lowest d-level
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The shape, difference between emission and absorption
Zero-phonon line, exp(-S)S, Huang Rhys factor
S ~ 1-2
Surprisingly weak electron-phonon coupling, consequently relatively narrow bands in emission and
absorption
No good match to 2F5/2 or other high 4f11 levels,transitions directly from d ?
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UNDER VUV EXCITATION THERE ARE 3 DISTINCT TYPES OF EMISSIONS:
1) VIS Er3+4f11 emission lines
2) UV and VUV 4f11, 5d-4f emission lines and bands
3) STE emisssion (no relation to Er)
SUMMARY
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UNDER VUV EXCITATION THERE ARE 3 DISTINCT CHANNELS:
1) Below 124 nm, STE emission, no transfer to Er
2) Above 124 nm, up to about 170 nm Er-BE (CT), large relaxation (F+H), subsequent transfer to lower
Er 4f11 energy levels (VIS emissions)
3) In the range 130 to 160 nm, d-levels, direct? via ? (2F(2)5/2?)
higher Er 4f11 energy levels (UV and VIS emissions)
SUMMARY
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No fast and efficient scintillator
Although there is some competition from direct VIS, VUV and UV (and consequtive VIS) emissions prevail
under selective VUV excitation into d-bands quantum cutting phosphors?
VUV, UV lasers?
Possible upconversion(reversed two photon cascade emission)
SUMMARY
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ADVICE AND EXPERTISE in VUV experimentsProf. G. Zimmerer, dr M. Kirm of Hasylab
Drs. J. Sutherland, J. Trunk, U9B Brookhaven
Help in experiments: W. Drozdowski, J. Glodo, M. Wisniewska, D.Wisniewski
Crystals of BaF2:Er and others, donatedby A. Lempicki, ALEM Assoc. Boston
ACKNOWLEDGMENT
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