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VUV spectroscopy of BaF 2 :Er A.J. Wojtowicz Institute of Physics, N. Copernicus Univ. Toruń, POLAND FPS 2007 French-Polish Symposium on Spectroscopy of Modern Materials in Physics, Chemistry and Biology. July 15 – 18 2007, Clermont-Ferrand, France. INTRODUCTION

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VUV spectroscopy of BaF2:Er

A.J. Wojtowicz

Institute of Physics, N. Copernicus Univ. Toruń, POLAND

FPS 2007

French-Polish Symposium on SpectroscopyofModernMaterials in Physics, Chemistry and Biology.

July 15 – 18 2007, Clermont-Ferrand, France


INTRODUCTION

Why VUV spectroscopy? Materials for UV, VUV solid state lasers quantum cutting phosphorsscintillators

GOAL of this work:identify various VUV excited emissions andidentify different excitation mechanisms in BaF2:Er

A.J. Wojtowicz, FPS July 2007


Scintillators

Ce scintillators: LSO, LYSO, LuAP, LuYAP

Ce: 350 nm, 15-30 nsthen 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…

A.J. Wojtowicz, FPS July 2007


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

A.J. Wojtowicz, FPS July 2007


In general, for Ln 4fn4fn→ 5d4fn-1for Ce just one option:4f → 5d (no f electrons left behind)for Pr4f2↑↑(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

A.J. Wojtowicz, FPS July 2007


For n > 7 situation changes drastically. For Tb3+:4fn↑↑↑↑↑↑↑↓(2S+1=7, LS) → 5d↑4fn-1 ↑↑↑↑↑↑↑ (2S+1=9, HS) spin-forbidden, lower in energyand 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

A.J. Wojtowicz, FPS July 2007


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)

A.J. Wojtowicz, FPS July 2007


VIS emissions; lower Er3+ levels excited

A.J. Wojtowicz, FPS July 2007


UV emissions; higher Er3+ levels excited

A.J. Wojtowicz, FPS July 2007





STE, Er-BE and d-levels

A.J. Wojtowicz, FPS July 2007


STE emission under over the bandgap excitation

A.J. Wojtowicz, FPS July 2007


VUV emissions under d-level excitation

A.J. Wojtowicz, FPS July 2007


Excitation spectrum of VUV emissions

A.J. Wojtowicz, FPS July 2007


Excitation spectrum of Ce emission in BaF2

A.J. Wojtowicz, FPS July 2007


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

A.J. Wojtowicz, FPS July 2007


Excitation spectrum of VUV emissions

A.J. Wojtowicz, FPS July 2007



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 ?

A.J. Wojtowicz, FPS July 2007


SUMMARY

  • UNDER VUV EXCITATION THERE ARE 3 DISTINCT TYPES OF EMISSIONS:

  • VIS Er3+4f11 emission lines

  • UV and VUV 4f11, 5d-4f emission lines and bands

  • STE emisssion (no relation to Er)

A.J. Wojtowicz, FPS July 2007


SUMMARY

  • UNDER VUV EXCITATION THERE ARE 3 DISTINCT CHANNELS:

  • Below 124 nm, STE emission, no transfer to Er

  • 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)

  • In the range 130 to 160 nm, d-levels, direct? via ? (2F(2)5/2?)higher Er 4f11 energy levels (UV and VIS emissions)

A.J. Wojtowicz, FPS July 2007


SUMMARY

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)

A.J. Wojtowicz, FPS July 2007


ACKNOWLEDGMENT

ADVICE AND EXPERTISE in VUV experiments

Prof. 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, donated

by A. Lempicki, ALEM Assoc. Boston

A.J. Wojtowicz, FPS July 2007


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