UV and VUV spectroscopy of rare earth activated wide bandgap materials A.J. Wojtowicz Institute 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 BaF 2 :Er, BaF 2 :Ce and (Ba,La)F 2 :Er; experimental results Model; configuration coordinate diagram SUMMARY

RE 3+ ions [Xe]4f n, [Xe]4f n-1 5d Intraconfigurational transitions 4f n → 4f n (sharp lines, parity forbidden slow emissions) Interconfigurational transitions 4f n → 4f n-1 5d broad bands, parity allowed, FAST?

Scintillators Ce scintillators: LSO, LYSO, LuAP, LuYAP Ce: 350 nm, 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 4f n 4f n → 5d4f n-1 for Ce just one option: 4f → 5d (no f electrons left behind) for Pr 4f 2 ↑↑(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 Tb 3+ (8 electrons, 7+1): 4f n ↑↑↑↑↑↑↑↓(2S+1=7, LS) → 5d↑4f n-1 ↑↑↑↑↑↑↑ (2S+1=9, HS) spin-forbidden, lower in energy and 5d↓4f n-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 Tb 3+ (8 electrons, 7+1): 4f n ↑↑↑↑↑↑↑↓(2S+1=7, LS) → 5d↑4f n-1 ↑↑↑↑↑↑↑ (2S+1=9, HS) spin-forbidden, lower in energy and 5d↓4f n-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 Tb 3+ (8 electrons, 7+1): 4f n ↑↑↑↑↑↑↑↓(2S+1=7, LS) → 5d↑4f n-1 ↑↑↑↑↑↑↑ (2S+1=9, HS) spin-forbidden, lower in energy and 5d↓4f n-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 Tb 3+ (8 electrons, 7+1): 4f n ↑↑↑↑↑↑↑↓(2S+1=7, LS) → 5d↑4f n-1 ↑↑↑↑↑↑↑ (2S+1=9, HS) spin-forbidden, lower in energy and 5d↓4f n-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 4f n-1 5d configuration: 4f electrons – weak crystal field, strong spin – orbit, we have multiplets: 2S+1 L J split by crystal field

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

5d-4f n-1 coupling (imposed on CF structure), PLUS f – d exchange splitting (LS and HS states)

So the state of the 4f n-1 5d configuration can be described as (e.g.): (HS) 4f10( 4 I 15/2 ) 5d(e) This is the lowest excited state of the 4f 10 5d configuration of Er 3+ ion in BaF 2

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 Ce 3+ excited states Er: [Xe]4f 11, [Xe]4f 10 5d configurations overlap; significant f-d coupling, f-d exchange (LS and HS d-levels)

high VUV sensitivity, good scintillator material Ce

Fast emission (30 ns), only the lowest d–level emits Ce

2 F 7/2, 2 F 5/ and 103,2 nm 2 G 9/2, 2 G 7/2, 2 F 5/ , and nm Er

no VUV sensitivity, poor scintillator material Er

FAST!!!! Er

IF fast relaxation then only the lowest level emits transition is spin forbidden hence SLOW We must have emission from (LS, J = 8) !!

Fast emission starts indeed from the LS band at 158 nm… Notice slow band at 170 nm! (HS emission)

Ce 3+ in BaF 2 d-f emission –fast (30 ns) Er 3+ in BaF 2 4f 10 5d → 4f 11 emission slow (HS) Er 3+ in (Ba,La)F 2 4f 10 5d → 4f 11 emission fast (45 ns at LHe) (?? LS ??)

BUT under 148 nm excitation (higher LS d-band) the emission from (Ba,La)F 2 :Er is different (sharp lines no bands) and slow…

UV spectra…

and VUV/UV spectra…

sharp slow lines excited at 148 nm start from the 2 G 7/2 …

Three emissions from (Ba,La)F 2 : 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) nm: fast d (LS) and slow 4f 11 2 G 7/2 263 nm: slow 4f 11 2 G 7/2

MODEL Configuration Coordinate Diagram Assumptions: All states of the same electronic configuration have the same equilibrium position BUT The equilibrium positions for states of the 4f 11 and 4f 10 5d configurations are different The energies taken from experiment

MODEL, BaF 2

MODEL, (Ba,La)F 2

Summary VUV response critical for scintillator (phosphor) materials (Ce good, Er bad) Relatively slow nonradiative relaxation in (La,Ba)F 2 :Er between the lowest LS and HS 4f 10 5d levels Fast and efficient 4f 10 5d → 4f 11 emissions from the (LS, J = 8) level bypassing (HS, J = 8) 4f 10 5d level 2 G 7/2 emission under the (LS, J = 7) level excitation at 10 K; indirect identification of the 2 G 7/2 level 2 G 7/2 - THE HIGHEST KNOWN EMITTING 4f 11 -level of Er 3+ ion in solid state material ( cm -1 )

ACKNOWLEDGMENTS Collaborators: experiment S. Janus (PhD student) R. Theis (PhD student) K. Jastak (student) Calculations of 4f n 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 4f 11 levels

SAMPLES and EXPERIMENTS (Ba,La)F 2 :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