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

Hasylab experiments, July 2009 and Aug Experimental teams: 2009: 2007: Piotr Palczewski, Sebastian Janus Marek Różański, Kinga Jastak Marcin WitkowskiRobert TheisAndrzej Wojtowicz

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

OUTLINE Experimental set-up Samples Some physics: localized and band states of charge carriers UV i VUV spectroscopy of BaF 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, McPherson monochromator (primary), grating (spherical mirror) 2 m, 1200 gr/mm, Al+MgF 2 ( nm), res nm Luminescence: Home-made Pouey VUV monochromator, 0.5 m, f/2.8, res. 1.1 nm, solar blind R6836 Hamamatsu PMT ( nm) UV-VIS Monochromator, Acton Research 0.3 m Czerny-Turner SpectraPro300i, f/4, Hamamatsu PMT R6358P ( 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 BaF 2 :Ce crystals: results and discussion Summary

Samples cut from larger boules: BaF 2 :0.05m%Ce, 0.2m%Na BaF 2 :0.015m%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 nm, decay time ns, complex spectroscopy

OUTLINE Experimental set-up Samples Some physics: localized and band states of charge carriers UV i VUV spectroscopy of BaF 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

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

Localized states (carrier trapped at impurity/defect or self-trapped) Additional (linear) term in energy, E lin = -kQ

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 BaF 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

BaF 2 :Ce, narrow cb i vb bands; little to gain by staying delocalized AND large lattice relaxation (ionic bonds) hole/electron self-trapping, V k centers, F – H complexes Bandgap energy 10.6 eV (117 nm), band exciton (n = 1) 10eV (124 nm) Free excitons – band states (relaxation?) Bound excitons – combined band and 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 BaF 2 :Ce crystals: results and discussion Summary

### D:\Users\Torun\BFCeNa09.dat, started at , 02:11. ### Scan of ARC (0/0 grating) monochromator from to nm, with 1.00 nm steps. Using 4 channels, counting time 2.00 s. ### Primary monochromator: nm (6.199 eV). T Sample: K, Doris current: 0.00 mA. ### ### BF2Ce(0.05)Na(0.2) ### Exc 200 nm ### Temp 302 K ### 300/300 ### slits 0.4 mm RT, 200 nm

### D:\Users\Torun\BFCeNa02.dat, started at , 22:54. ### Scan of primary monochromator from to nm, with 0.20 nm steps. Using 6 channels, counting time 2.00 s. ### Monochromator Positions: ARC: nm (300/300), VUV: nm. T Sample: K, Doris current: 0.00 mA. ### ### BF2Ce(0.05)Na(0.2) ### Emi 323 nm ### Temp 306 K ### 300/300 ### slits 0.4 mm RT, 323 nm

Crystal field: octahedralcubictetrahedral

### D:\Users\Torun\BFCeNa03.dat, started at , 00:09. ### Scan of ARC (300/300 grating) monochromator from to nm, with 1.00 nm steps. Using 6 channels, counting time 2.00 s. ### Primary monochromator: nm (9.017 eV). T Sample: K, Doris current: 0.00 mA. ### ### BF2Ce(0.05)Na(0.2) ### Ex nm ### Temp 302 K ### 300/300 ### slits 0.4 mm RT, nm

### D:\Users\Torun\BFCeNa05.dat, started at , 00:43. ### Scan of ARC (300/300 grating) monochromator from to nm, with 1.00 nm steps. Using 6 channels, counting time 2.00 s. ### Primary monochromator: nm (9.611 eV). T Sample: K, Doris current: 0.00 mA. ### ### BF2Ce(0.05)Na(0.2) ### Exc 129 nm ### Temp 302 K ### 300/300 ### slits 0.4 mm RT, 129 nm

### D:\Users\Torun\BFCeNa07.dat, started at , 01:23. ### Scan of ARC (0/0 grating) monochromator from to nm, with 1.00 nm steps. Using 4 channels, counting time 2.00 s. ### Primary monochromator: nm ( eV). T Sample: K, Doris current: 0.00 mA. ### ### BF2Ce(0.05)Na(0.2) ### Exc 112,3 nm ### Temp 302 K ### 300/300 ### slits 0.4 mm RT, 112 nm

RT, 137.5, 112, 129 nm

D:\Users\Torun\BFCeNa02.dat, started at , 00:43 emi. 323 nm, black, total. D:\Users\Torun\BFCeNa02.dat, started at , 00:43 emi. 323 nm, red, slow. w. D:\Users\Torun\BFCeNa10.dat, started at , 02:48, emi. 400 nm, blue, total RT exc.

### D:\Users\Torun\BFCeA01.dat, started at , 14:40. ### Scan of primary monochromator from to nm, with 1.00 nm steps. Using 4 channels, counting time 2.00 s. ### Monochromator Positions: ARC: nm (300/300), VUV: nm. T Sample: K, Doris current: mA. ### BaF2:Ce (0.015%), black line ### D:\Users\Torun\BFNaCe01.dat, started at , 13:59. ### Scan of primary monochromator from to nm, with 0.50 nm steps. Using 4 channels, counting time 3.00 s. ### Monochromator Positions: ARC: nm (1200/300), VUV: nm. T Sample: K, Doris current: mA. ### BaF2:Ce(0.05):Na, red line RT, 320 nm e-h pairs

### D:\Users\Torun\BFCeA01.dat, started at , 14:40. ### Scan of primary monochromator from to nm, with 1.00 nm steps. Using 4 channels, counting time 2.00 s. ### Monochromator Positions: ARC: nm (300/300), VUV: nm. T Sample: K, Doris current: mA. ### BaF2:Ce (0.015%), black line ### D:\Users\Torun\BFNaCe01.dat, started at , 13:59. ### Scan of primary monochromator from to nm, with 0.50 nm steps. Using 4 channels, counting time 3.00 s. ### Monochromator Positions: ARC: nm (1200/300), VUV: nm. T Sample: K, Doris current: mA. ### BaF2:Ce(0.05):Na, red line RT, 320 nm e-h pairs

D:\Users\Torun\PBFCeNa01.dat, emi. 323 nm, exc. 200 nm RT

D:\Users\Torun\PBFCeNa01.dat, emi. 323 nm, exc nm, black. D:\Users\Torun\PBFCeNa02.dat, emi. 323 nm, exc nm, red. D:\Users\Torun\PBFCeNa03.dat, emi. 323 nm, exc. 129 nm, blue. RT

D:\Users\Torun\PBFCeNa01.dat, emi. 323 nm, exc nm, black. D:\Users\Torun\PBFCeNa02.dat, emi. 323 nm, exc nm, red. D:\Users\Torun\PBFCeNa03.dat, emi. 323 nm, exc. 129 nm, blue. RT

D:\Users\Torun\PBFCeNa01.dat, emi. 323 nm, exc nm, black. D:\Users\Torun\PBFCeNa02.dat, emi. 323 nm, exc nm, red. D:\Users\Torun\PBFCeNa03.dat, emi. 323 nm, exc. 129 nm, blue. RT

D:\Users\Torun\PBFCeNa07.dat, emi. 400 nm, exc. 129 nm, black. D:\Users\Torun\PBFCeNa03.dat, emi 323 nm, exc. 129 nm, blue. D:\Users\Torun\PBFCeNa05.dat, emi 323 nm, exc. 112 nm, red. RT

### D:\Users\Torun\BFCeNa13.dat, started at , 17:32. ### Scan of ARC (300/300 grating) monochromator from to nm, with 0.50 nm steps. Using 4 channels, counting time 2.00 s. ### Primary monochromator: nm (6.199 eV). T Sample: 9.78 K, Doris current: 0.00 mA. ### ### BF2:Ce (0.05)Na(0.2) ### Exc 200 nm ### Temp 10 K ### 300/300 ### slits 0.5 mm 10K Exc. 200 nm

### D:\Users\Torun\BFCeNa12.dat, started at , 16:07. ### Scan of primary monochromator from to nm, with 0.20 nm steps. Using 6 channels, counting time 2.00 s. ### Monochromator Positions: ARC: nm (300/300), VUV: nm. T Sample: 9.25 K, Doris current: 0.00 mA. ### ### BF2:Ce (0.05)Na(0.2) ### Emi 323 nm ### Temp 10 K ### 300/300 ### slits 1 mm 10K

### D:\Users\Torun\BFCeNa15.dat, started at , 18:36. ### Scan of ARC (0/0 grating) monochromator from to nm, with 0.50 nm steps. Using 4 channels, counting time 2.00 s. ### Primary monochromator: nm (9.393 eV). T Sample: 9.78 K, Doris current: 0.00 mA. ### ### BF2:Ce (0.05)Na(0.2) ### Exc 132 nm ### Temp 10 K ### 300/300 ### slits 0.5 mm 10K Exc. 132 nm

### D:\Users\Torun\BFCeNa17.dat, started at , 19:31. ### Scan of ARC (0/0 grating) monochromator from to nm, with 0.50 nm steps. Using 4 channels, counting time 2.00 s. ### Primary monochromator: nm ( eV). T Sample: K, Doris current: 0.00 mA. ### ### BF2:Ce (0.05)Na(0.2) ### Exc 114 nm ### Temp 10 K ### 300/300 ### slits 0.5 mm 10K Exc. 114 nm

PBFCeNa12.dat, emi exc. 200 nm black PBFCeNa13.dat, emi exc. 132 nm red PBFCeNa14.dat, emi exc. 114 nm blue 10K

PBFCeNa12.dat, emi exc. 200 nm black PBFCeNa13.dat, emi exc. 132 nm red PBFCeNa14.dat, emi exc. 114 nm blue 10K

PBFCeNa12.dat, emi exc. 200 nm 10K

PBFCeNa14.dat, emi exc. 114 nm black PBFCeNa15.dat, emi exc nm red PBFCeNa16.dat, emi exc nm green PBFCeNa17.dat, emi exc. 67 nm blue 10K

PBFCeNa16.dat, emi exc nm green 10K

### D:\Users\Torun\BFCeA09.dat, started at , 18:42, red line, emi nm. ### D:\Users\Torun\BFCeA08.dat, started at , 18:03, black line, exc nm. Exc. 291 nm – 34,400 cm-1 Emi. 303 nm – 33,000 cm-1 Stokes shift – 1400 cm-1 10K 0.015m%Ce

Summary Excitation spectra point to: dominant cubic crystal field, 10Dq about 15,000 cm -1, low symmetry component about 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)

Summary Excitation spectra point to: dominant cubic crystal field, 10Dq about 15,000 cm -1, low symmetry component about 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)

Summary Excitation spectra point to: dominant cubic crystal field, 10Dq about 15,000 cm -1, low symmetry component about 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)

Summary Excitation spectra point to: dominant cubic crystal field, 10Dq about 15,000 cm -1, low symmetry component about 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

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

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

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