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

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

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?

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…

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?

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

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

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

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

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 (?)

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

5d electrons – strong crystal field, weak s – o

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)

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

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)

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

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

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

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

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

IFfast relaxation

thenonly the lowest

level emits

transition is spin forbidden

henceSLOW

We must have emission from (LS, J = 8) !!

Fast emission starts indeed from the LS band at 158 nm…

Notice slowband at 170 nm!(HS emission)

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 ??)

BUT

under 148 nm excitation (higher LS d-band)

the emission from (Ba,La)F2:Er is different

(sharp lines no bands) and slow…

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…

and VUV/UV spectra…

sharp slow lines excited at 148 nm start from the 2G7/2…

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!!

170 nm:slow d (HS)

234.9 nm: fast d (LS) and slow 4f11 2G7/2

263 nm: slow 4f11 2G7/2

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

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

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

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)

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

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