1 Thermally induced 4f – 5d transitions in LuAlO 3 :Ce (LuAP) A.J. Wojtowicz, S. Janus Institute of Physics, N. Copernicus Univ. Toruń, POLAND IEEE 9th International Conference on Inorganic Scintillators and their Applications, Winston-Salem, NC USA June 4 – 8, 2007

A.J. Wojtowicz, SCINT 2007, June INTRODUCTION LuAP: 8.34 g/cm 3, photofraction 0.3, 365 nm emission, 17 ns decay time, LY over 2xBGO, yet problems GOAL of this work study, report and explain details of optical transitions generating scintillation light

A.J. Wojtowicz, SCINT 2007, June SAMPLES 2 pixels 2x2x10 mm, grown in 2004 at Institute of Electronic Materials Technology, Warsaw, Poland by prof. Lukasiewicz et al LuAlO 3 :0.07%at Ce, LuAlO 3 :0.15%at Ce Emission and excitation spectra: Superlumi station, I – beamline, Hasylab, Hamburg, Germany (prof. Zimmerer)

A.J. Wojtowicz, SCINT 2007, June LuAlO 3 :Ce, luminescence spectrum

A.J. Wojtowicz, SCINT 2007, June Luminescence spectra, summary: Two spin – orbit split bands (1600 cm -1 ) lowest d → 2 F 5/2 357 nm lowest d → 2 F 7/2 379 nm Crystal field structure not resolved more or less comparable intensities

A.J. Wojtowicz, SCINT 2007, June Corrected excitation spectra

A.J. Wojtowicz, SCINT 2007, June EXCITATION SPECTRA, summary:  Strong triple 2 F → T 2 band two strong (306, 295 nm), one weak transitions (276 nm) ± 1800 cm -1  Weaker double 2 F → E band (226, 214 nm) ± 1200 cm -1

A.J. Wojtowicz, SCINT 2007, June  Stokes shift~ 4700 cm -1  DOMINANT CUBIC FIELD (O h ); 10Dq~ cm -1 weak low symmetry field ~ 1500 cm -1  The intensity ratio of two d–bands 2 F → T 2 / 2 F → E 12 K ~ K ~ 3.8

A.J. Wojtowicz, SCINT 2007, June  Both T 2 and E d–bands strongly expand toward longer wavelengths with increasing T  Intensities of 5d E subbands; „230 nm” subband increases, „215 nm” subband decreases with T,  „230 nm” subband peak shifts toward lower λ

A.J. Wojtowicz, SCINT 2007, June E subbands positions vs T

A.J. Wojtowicz, SCINT 2007, June Corrected excitation spectra vs T

A.J. Wojtowicz, SCINT 2007, June Corrected excitation spectra vs T

A.J. Wojtowicz, SCINT 2007, June Corrected excitation spectra vs T

A.J. Wojtowicz, SCINT 2007, June THEORY - MODEL D.J. Robbins, YAG J. Electrochem. Soc Thermally activated transitions from the higher ground state level Different transition moments BAND SHIFTS p 1 < p 1 ’ p 1 < p 2 p 2 ’ < p 1 ’

A.J. Wojtowicz, SCINT 2007, June d E subbands intensities vs T

A.J. Wojtowicz, SCINT 2007, June „230 nm” intensity vs T, experiment and theory

A.J. Wojtowicz, SCINT 2007, June „215 nm” intensity vs T, experiment and theory

A.J. Wojtowicz, SCINT 2007, June ENERGY LEVEL DIAGRAM, 5d levels Sequence of levels must agree with the experiment Γ 7, Γ 8 doubled-valued representations of O h

A.J. Wojtowicz, SCINT 2007, June ENERGY LEVEL DIAGRAM, 4f levels Sequence of levels must agree with the experiment Γ 6, Γ 7, Γ 8 doubled-valued representations of O h

A.J. Wojtowicz, SCINT 2007, June Transition moment matrix elements (line strenghts) between Γ 6, Γ 7, Γ 8 states originating from 2 F 7/2 and Γ 7, Γ 8 states from 2 F 5/2 term of 4f configuration and Γ 8, Γ 7 states originating from T 2 and E terms of 5d electron configuration T. Hoshina, J. Phys. Soc. Jap., 1980

A.J. Wojtowicz, SCINT 2007, June dT 2 Γ 8 a5dT 2 Γ 8 b5dT 2 Γ 7 5dE Γ 8 a5dE Γ 8 b 4f 2 F 7/2 Γ 6 8,571325,71420,0025,714425,714 4f 2 F 7/2 Γ 8 b12,244812,244924,490218,3674 4f 2 F 7/2 Γ 8 a12,244912,24524,4918,3674 4f 2 F 7/2 Γ 7 4, ,3076,1224 4f 2 F 5/2 Γ 8 b6,802833,46962,18761,632641,6327 4f 2 F 5/2 Γ 8 a33,4696,80282,176841,63141,6326 4f 2 F 5/2 Γ 7 34,01434,01381,36058,1632

A.J. Wojtowicz, SCINT 2007, June d T 2 5d E5dE/5dT 2 4f 2 F 5/2 Γ 8 b42, ,26531,02 4f 2 F 5/2 Γ 8 a42,448643,2641,02 4f 2 F 5/2 Γ 7 69, ,32640,24 The lowest 4f level (absorption), options: 0.24 is the only option where T 2 takes larger share (4.17), consistent with the experiment (3.8–4.4).

A.J. Wojtowicz, SCINT 2007, June The sequence of 5d T 2 levels (absorption), relevant matrix elements: Only 4f Γ 7 ground state and 5d Γ 8 below Γ 7 are consistent with two strong and one weak transitions generating 4f 2 F 5/2 → 5d T 2 band 5dT 2 Γ 8 a5dT 2 Γ 8 b5dT 2 Γ 7 4f 2 F 5/2 Γ 8 b6,802833,469622, f 2 F 5/2 Γ 8 a33,4696,80282,1768 4f 2 F 5/2 Γ 7 34,01434,01381,36053

A.J. Wojtowicz, SCINT 2007, June The lowest 5d level (emission), options: Γ 8 a and Γ 8 b give reasonable values for the ratios of 2 F 7/2 and 2 F 5/2 bands – Γ 8 below Γ 7 4f 2 F 7/2 4f 2 F 5/2 TOTAL 5dT 2 Γ 8 a37,142674, ,4284 5dT 2 Γ 8 b54, , , dT 2 Γ 7 114,28725, , dE Γ 8 a68,571651, ,9988 5dE Γ 8 b68,571251, ,9997

A.J. Wojtowicz, SCINT 2007, June Thermally induced transitions; line strengths between 5dEΓ 8 states and two lowest 4f levels: 5dE Γ 8 a5dE Γ 8 b 4f 2 F 5/2 Γ 8 b1,632641,6327 4f 2 F 5/2 Γ 8 a41,63141,6326 4f 2 F 5/2 Γ 7 8,1632 p 1 /p 2 = 0.2 (experiment 0.15±0.02) p ’ 1 /p ’ 2 = 5 (experiment 7±5)

A.J. Wojtowicz, SCINT 2007, June SUMMARY  Dominant cubic and low symmetry crystal field components determine the ground and excited states of the Ce 3+ ion in LuAP, in order of increasing energy: 4f 2 F 5/2 : 2 F 5/2 Γ 7, 2 F 5/2 Γ 8 a, 2 F 5/2 Γ 8 b 5d 2 D: T 2 Γ 8 a, T 2 Γ 8 b, T 2 Γ 7, EΓ 8 a, EΓ 8 b

A.J. Wojtowicz, SCINT 2007, June  The transition moment matrix elements between these states are consistent with the observed emission and absorption intensities and their temperature dependences  In contrast to Ce–activated YAG and BaF 2, in LuAP, YAP and LuYAP sequence of ground and excited state energy levels may enhance self–absorption of Ce emission;  consequences for scintillation of large crystals; radiation trapping and energy migration