UV i VUV spectroscopy of Ba F 2

UV i VUV spectroscopy of Ba. F 2: Ce crystals (Report from Hasylab experiments) Andrzej J. Wojtowicz IF UMK Optoelectronics Seminar, Oct. 26, 2009

Hasylab experiments, July 2009 and Aug. 2007 Experimental teams: 2009: 2007: Piotr Palczewski, Sebastian Janus Marek Różański, Kinga Jastak Marcin Witkowski Robert Theis Andrzej Wojtowicz

OUTLINE Experimental set-up Samples Some physics: localized and band states of charge carriers UV i VUV spectroscopy of Ba. F 2: Ce crystals: results and discussion Summary

Superlumi station, beamline I, Doris III ring, Hasylab, DESY, Hamburg, Germany Prof. G. Zimmerer i Dr Aleksei Kotlov Funding: UE (via Hasylab; large experimental facilities), IF UMK

Superlumi station, technical details Excitation: synchrotron, Mc. Pherson monochromator (primary) , grating (spherical mirror) 2 m, 1200 gr/mm, Al+Mg. F 2 (50 -330 nm), res. 0. 32 nm Luminescence: Home-made Pouey VUV monochromator, 0. 5 m, f/2. 8, res. 1. 1 nm, solar blind R 6836 Hamamatsu PMT (115 -300 nm) UV-VIS Monochromator, Acton Research 0. 3 m Czerny-Turner Spectra. Pro 300 i, f/4, Hamamatsu PMT R 6358 P (200 -800 nm). 3 gratings, 1200 gr/mm (2. 7 nm/mm, blazed at 300 nm), 300 gr/mm (10. 8 nm/mm, blazed at 300 i 500 nm)

OUTLINE Experimental set-up Samples Some physics: localized and band states of charge carriers UV i VUV spectroscopy of Ba. F 2: Ce crystals: results and discussion Summary

Samples cut from larger boules: Ba. F 2: 0. 05 m%Ce, 0. 2 m%Na Ba. F 2: 0. 015 m%Ce Grown at Optovac, MA, USA donated by Prof. A. Łempicki of Boston University Scintillator, phosphor, laser material Not the best in any category, convenient as a model material: efficient host-ion energy transfer emission at 290 -350 nm, decay time 25 -30 ns, complex spectroscopy

OUTLINE Experimental set-up Samples Some physics: localized and band states of charge carriers UV i VUV spectroscopy of Ba. F 2: Ce crystals: results and discussion Summary

Two kinds of electron states in solid state: 1. Delocalized band states; k-vector 2. Localized (trapped or self-trapped) states; configuration coordinate Q

Electron band states, linear model of the crystal E, electron energy E = E 0 – 2 Acoskb A – electron transfer rate constant k, wave vector

Localized states (carrier trapped at impurity/defect or self-trapped) Additional (linear) term in energy, Elin = -k. Q

Band states: electronic energy reduced, no lattice relaxation Localized states (self-trapped electrons or holes) lattice relaxation no electronic energy loss The question for a free charge carrier in any material is: localize or delocalize? Two extreme cases: Si and Ba. F 2

Si, broad cb i vb bands; a lot to gain by staying delocalized AND nothing to gain upon localization (no lattice relaxation; covalent bonds) Band (quantum mechanical) conduction: high mobility, low effective mass No self – trapping, no lattice relaxation gain upon trapping at impurity/defect

Ba. F 2: Ce, narrow cb i vb bands; little to gain by staying delocalized AND large lattice relaxation (ionic bonds) hole/electron self-trapping, Vk centers, F – H complexes Bandgap energy 10. 6 e. V (117 nm), band exciton (n = 1) 10 e. V (124 nm) Free excitons – band states (relaxation? ) Bound excitons – combined band localized states Electron – hole pairs – band states (relaxation? ) Pay attention to 117 i 124 nm

OUTLINE Experimental set-up Samples Some physics: localized and band states of charge carriers UV i VUV spectroscopy of Ba. F 2: Ce crystals: results and discussion Summary

RT, 200 nm ### D: UsersTorunBFCe. Na 09. dat, started at 22. 07. 2009, 02: 11. ### Scan of ARC (0/0 grating) monochromator from 450. 00 to 210. 00 nm, with 1. 00 nm steps. Using 4 channels, counting time 2. 00 s. ### Primary monochromator: 200. 000 nm (6. 199 e. V). T Sample: 299. 80 K, Doris current: 0. 00 m. A. ### BF 2 Ce(0. 05)Na(0. 2) ### Exc 200 nm ### Temp 302 K ### 300/300 ### slits 0. 4 mm

RT, 323 nm ### D: UsersTorunBFCe. Na 02. dat, started at 21. 07. 2009, 22: 54. ### Scan of primary monochromator from 310. 00 to 50. 00 nm, with 0. 20 nm steps. Using 6 channels, counting time 2. 00 s. ### Monochromator Positions: ARC: 322. 99 nm (300/300), VUV: -4. 87 nm. T Sample: 305. 20 K, Doris current: 0. 00 m. A. ### BF 2 Ce(0. 05)Na(0. 2) ### Emi 323 nm ### Temp 306 K ### 300/300 ### slits 0. 4 mm

Crystal field: octahedral cubic tetrahedral

RT, 137. 5 nm ### D: UsersTorunBFCe. Na 03. dat, started at 22. 07. 2009, 00: 09. ### Scan of ARC (300/300 grating) monochromator from 450. 00 to 270. 00 nm, with 1. 00 nm steps. Using 6 channels, counting time 2. 00 s. ### Primary monochromator: 137. 495 nm (9. 017 e. V). T Sample: 302. 30 K, Doris current: 0. 00 m. A. ### BF 2 Ce(0. 05)Na(0. 2) ### Ex 137. 5 nm ### Temp 302 K ### 300/300 ### slits 0. 4 mm

RT, 129 nm ### D: UsersTorunBFCe. Na 05. dat, started at 22. 07. 2009, 00: 43. ### Scan of ARC (300/300 grating) monochromator from 450. 00 to 200. 00 nm, with 1. 00 nm steps. Using 6 channels, counting time 2. 00 s. ### Primary monochromator: 128. 999 nm (9. 611 e. V). T Sample: 301. 50 K, Doris current: 0. 00 m. A. ### BF 2 Ce(0. 05)Na(0. 2) ### Exc 129 nm ### Temp 302 K ### 300/300 ### slits 0. 4 mm

RT, 112 nm ### D: UsersTorunBFCe. Na 07. dat, started at 22. 07. 2009, 01: 23. ### Scan of ARC (0/0 grating) monochromator from 450. 00 to 180. 00 nm, with 1. 00 nm steps. Using 4 channels, counting time 2. 00 s. ### Primary monochromator: 112. 296 nm (11. 041 e. V). T Sample: 300. 60 K, Doris current: 0. 00 m. A. ### BF 2 Ce(0. 05)Na(0. 2) ### Exc 112, 3 nm ### Temp 302 K ### 300/300 ### slits 0. 4 mm

RT, 137. 5, 112, 129 nm

RT exc. D: UsersTorunBFCe. Na 02. dat, started at 22. 07. 2009, 00: 43 emi. 323 nm, black, total. D: UsersTorunBFCe. Na 02. dat, started at 22. 07. 2009, 00: 43 emi. 323 nm, red, slow. w. D: UsersTorunBFCe. Na 10. dat, started at 22. 07. 2009, 02: 48, emi. 400 nm, blue, total

RT, 320 nm e-h pairs ### D: UsersTorunBFCe. A 01. dat, started at 16. 08. 2007, 14: 40. ### Scan of primary monochromator from 315. 00 to 50. 00 nm, with 1. 00 nm steps. Using 4 channels, counting time 2. 00 s. ### Monochromator Positions: ARC: 319. 99 nm (300/300), VUV: -5. 91 nm. T Sample: 298. 50 K, Doris current: 99. 74 m. A. ### Ba. F 2: Ce (0. 015%), black line ### D: UsersTorunBFNa. Ce 01. dat, started at 18. 08. 2007, 13: 59. ### Scan of primary monochromator from 300. 00 to 65. 00 nm, with 0. 50 nm steps. Using 4 channels, counting time 3. 00 s. ### Monochromator Positions: ARC: 323. 49 nm (1200/300), VUV: -5. 92 nm. T Sample: 297. 20 K, Doris current: 101. 90 m. A. ### Ba. F 2: Ce(0. 05): Na, red line

RT D: UsersTorunPBFCe. Na 01. dat, emi. 323 nm, exc. 200 nm

RT D: UsersTorunPBFCe. Na 01. dat, emi. 323 nm, exc. 200. 0 nm, black. D: UsersTorunPBFCe. Na 02. dat, emi. 323 nm, exc. 137. 5 nm, red. D: UsersTorunPBFCe. Na 03. dat, emi. 323 nm, exc. 129 nm, blue.

RT D: UsersTorunPBFCe. Na 07. dat, emi. 400 nm, exc. 129 nm, black. D: UsersTorunPBFCe. Na 03. dat, emi 323 nm, exc. 129 nm, blue. D: UsersTorunPBFCe. Na 05. dat, emi 323 nm, exc. 112 nm, red.

10 K Exc. 200 nm ### D: UsersTorunBFCe. Na 13. dat, started at 24. 07. 2009, 17: 32. ### Scan of ARC (300/300 grating) monochromator from 450. 00 to 210. 00 nm, with 0. 50 nm steps. Using 4 channels, counting time 2. 00 s. ### Primary monochromator: 199. 997 nm (6. 199 e. V). T Sample: 9. 78 K, Doris current: 0. 00 m. A. ### BF 2: Ce (0. 05)Na(0. 2) ### Exc 200 nm ### Temp 10 K ### 300/300 ### slits 0. 5 mm

10 K ### D: UsersTorunBFCe. Na 12. dat, started at 24. 07. 2009, 16: 07. ### Scan of primary monochromator from 315. 00 to 50. 00 nm, with 0. 20 nm steps. Using 6 channels, counting time 2. 00 s. ### Monochromator Positions: ARC: 322. 99 nm (300/300), VUV: -4. 87 nm. T Sample: 9. 25 K, Doris current: 0. 00 m. A. ### BF 2: Ce (0. 05)Na(0. 2) ### Emi 323 nm ### Temp 10 K ### 300/300 ### slits 1 mm

10 K Exc. 132 nm ### D: UsersTorunBFCe. Na 15. dat, started at 24. 07. 2009, 18: 36. ### Scan of ARC (0/0 grating) monochromator from 450. 00 to 210. 00 nm, with 0. 50 nm steps. Using 4 channels, counting time 2. 00 s. ### Primary monochromator: 132. 000 nm (9. 393 e. V). T Sample: 9. 78 K, Doris current: 0. 00 m. A. ### BF 2: Ce (0. 05)Na(0. 2) ### Exc 132 nm ### Temp 10 K ### 300/300 ### slits 0. 5 mm

10 K Exc. 114 nm ### D: UsersTorunBFCe. Na 17. dat, started at 24. 07. 2009, 19: 31. ### Scan of ARC (0/0 grating) monochromator from 450. 00 to 240. 00 nm, with 0. 50 nm steps. Using 4 channels, counting time 2. 00 s. ### Primary monochromator: 114. 000 nm (10. 876 e. V). T Sample: 10. 18 K, Doris current: 0. 00 m. A. ### BF 2: Ce (0. 05)Na(0. 2) ### Exc 114 nm ### Temp 10 K ### 300/300 ### slits 0. 5 mm

10 K PBFCe. Na 12. dat, emi. 321. 1 exc. 200 nm black PBFCe. Na 13. dat, emi. 321. 1 exc. 132 nm red PBFCe. Na 14. dat, emi. 321. 1 exc. 114 nm blue

10 K PBFCe. Na 12. dat, emi. 321. 1 exc. 200 nm

10 K PBFCe. Na 14. dat, emi. 321. 1 exc. 114 nm black PBFCe. Na 15. dat, emi. 321. 1 exc. 83. 2 nm red PBFCe. Na 16. dat, emi. 321. 1 exc. 76. 4 nm green PBFCe. Na 17. dat, emi. 321. 1 exc. 67 nm blue

10 K PBFCe. Na 16. dat, emi. 321. 1 exc. 76. 4 nm green

10 K 0. 015 m%Ce ### D: UsersTorunBFCe. A 09. dat, started at 18. 08. 2007, 18: 42, red line, emi 323. 5 nm. ### D: UsersTorunBFCe. A 08. dat, started at 18. 08. 2007, 18: 03, black line, exc. 199. 0 nm. Exc. 291 nm – 34, 400 cm-1 Emi. 303 nm – 33, 000 cm-1 Stokes shift – 1400 cm-1

Summary Excitation spectra point to: dominant cubic crystal field, 10 Dq about 15, 000 cm-1, low symmetry component about 2000 -3000 cm-1 Stokes shift about 1400 cm-1 Three host-ion energy transfer channels 1) „bound exciton” 2) „band exciton” 3) electron –hole pairs plus 4) Direct excitation (no host involved)

• direct excitation: fast Ce emission, no rise time, no slow components • „bound exciton” excitation: fast Ce emission, short rise time, no slow components • „exciton” excitation: Ce and host emission, some fast component, dominant slow components • „e-h pair” excitation: Ce and host emissions, fast and slow components, branching coefficient dependent on Ce concentration