VUV spectroscopy of BaF 2 :Er A.J. Wojtowicz

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


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


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…


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


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


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


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)


VIS emissions; lower Er3+ levels excited


UV emissions; higher Er3+ levels excited


Excitation spectra of VIS emissions; RT and 5 K


Excitation below 124 nm; STE, some VIS emission


VIS and UV emissions competition, Er-BE and d-levels


STE, Er-BE and d-levels


STE emission under over the bandgap excitation


VUV emissions under d-level excitation


Excitation spectrum of VUV emissions


Excitation spectrum of Ce emission in BaF2


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


Excitation spectrum of VUV emissions


Emission and absorption spectra; the lowest d-level


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 ?


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


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


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


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

VUV spectroscopy of BaF 2 :Er A.J. Wojtowicz

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