VUV spectroscopy of rare earth ions in solids: recent studies and possible applications V.N. Makhov P.N. Lebedev Physical Institute, Russian Academy of Sciences, Moscow, Russia Institute of Physics, University of Tartu, Tartu, Estonia

Outline 1. General optical properties of trivalent rare earth ions in solids: intraconfigurational 4f  4f and interconfigurational 5d  4f transitions; spin-allowed and spin- forbidden 5d  4f transitions. 2.Prospects for applications of rare earth containing materials: quantum cutting (multi- photon) phosphors for high-efficiency Hg-free fluorescent lamps and plasma display panels; new fast and efficient scintillators for medical imaging (PET). 3. VUV luminescence from Gd 3+ ions: spectral properties, decay kinetics, thermal quenching; assignment to Gd 3+ 5d-4f luminescence; vibronic structure; the strength of electron-phonon coupling. 4. VUV luminescence from Lu 3+ ions: spectral and timing properties; assignment to Lu 3+ 5d-4f luminescence; vibronic structure; the strength of electron-phonon coupling; spin-forbidden and spin-allowed Lu 3+ 5d-4f luminescence: interplay with temperature; thermal quenching. 5. Concluding remarks.

General optical properties of trivalent rare earth ions in solids

Rare earth elements

Energy level structure for 4f n electronic configuration of trivalent rare earth ions (Dieke diagram)

Crystal field splitting for 4f electronic configuration Because of the shielding effect of the outer 5s and 5p shell electrons, the crystal- field interaction with inner 4f electrons is weak and can be treated as a perturbation (Stark effect) of the free-ions states. Accordingly, the energies of the corresponding levels of 4f n configuration are only weakly sensitive to the type of the crystal host. Splitting of energy levels of 4f n electronic configuration due to: I – Coulomb interaction; II – spin-orbit interaction; III – crystal-field interaction

Crystal field splitting for 4f n-1 5d electronic configuration The 5d electrons are not effectively shielded by other electrons, and the crystal field influence on the energy levels of 4f n-1 5d electronic configuration is strong. Accordingly, crystal field splitting of 5d levels is large and the energies of levels within 4f n-1 5d electronic configuration can strongly differ for different crystal hosts. Crystal-field splitting of 5d 1 configuration for tetragonal Ce 3+ center: I – free ion, II – O h, III – O h + spin-orbit, IV – С 4V

4f and 5d energy levels of Ce 3+ in tetragonal environment  SO Site symmetry S 4

Energies of the lowest 4f n-1 5d levels for RE 3+ ions doped into LiYF 4 crystal

Schematic electron configurations for the ground state (GS) 4f 8, the lowest energy high-spin (HS) 4f 7 5d state and the lowest energy low-spin (LS) 4f 7 5d state for Tb 3+

Single configuration-coordinate diagram of the 4f and 5d states and of 4f – 4f and 4f – 5d transitions in rare earth ion

High-efficiency VUV-excited phosphors

Why we need VUV-phosphor efficiency > 100% ? /0.17 = 1.47 We need phosphor with Q > 100%

Quantum splitting (quantum cutting) schemes

Visible quantum cutting by two-step energy transfer upon excitation in the 6 G J levels of Gd 3+ 1 violet photon absorbed on Gd 3+ 8 S 7/2 → 6 G J transitions, 2 red photons emitted on Eu 3+ 5 D 0 → 7 F 1 transitions LiGdF 4 :Eu 3+ GdF 3 :Eu 3+

Visible quantum cutting via down-conversion in LiGdF 4 :Er 3+,Tb 3+ 1 VUV photon absorbed on Er 3+ 4f 11 – 4f 10 5d transition, 2 photons emitted on : 1) Er 3+ 4 S 3/2 → 4 I 15/2 transition; 2) Tb 3+ 5 D 3,4 → 7 F J transitions;

Scintillators for medical applications (PET)

Principles of PET Ring of Photon Detectors Patient injected with drug having  + emitting isotope. Drug localizes in patient. Isotope decays, emitting  +.  + annihilates with e – from tissue, forming back-to- back 511 keV photon pair. 511 keV photon pairs detected via time coincidence. Positron lies on line defined by detector pair (a chord). Produces planar image of a “slice” through patient

Scintillators for PET based on 5d – 4f transitions in Ce 3+ Lifetime of the emitting state (scintillation decay time): τ  λ em 3  shorter- wavelength emission is needed for increasing time resolution of scintillation detector: Pr 3+, Nd 3+, … activator ions with shorter-wavelength (UV/VUV) and faster 5d – 4f transitions can be used instead of Ce 3+. Requirements to new scintillators: Density (g/cm 3 ) Atten. length (mm) at 511 keV Phot. eff. % Light yield (phot/MeV) Dec. time (ns) Wavel. max. (nm) Bi 4 Ge 3 O 12 (BGO) Lu 2 SiO 5 :Ce (LSO) LuAlO 3 :Ce (LuAP) Lu 2 Si2O 7 :Ce (LPS) Lu 2 S 3 :Ce Gd 2 SiO 5 :Ce (GSO) LaCl 3 :Ce

Experimental setup for VUV spectroscopy with synchrotron radiation

SUPERLUMI station at HASYLAB (DESY) G. Zimmerer, Radiation Measurements 42 (2007) 859 Primary monochromator 3 secondary monochromators Position-sensitive detectors Mechanical chopper In-situ cleaving 4 to 900 K

5d – 4f luminescence from Gd 3+

The scheme of radiative and nonradiative transitions in Gd 3+ Nonradiative relaxation (intersystem crossing) is heavily spin-forbidden

VUV emission spectra of GdF 3, LiGdF 4 and CaF 2 :Gd 3+ (0.1%) M. Kirm, J.C. Krupa, V.N. Makhov, M. True, S. Vielhauer, G. Zimmerer, Phys. Rev B 70, (R) (2004) S>5S~1

Decay curves of VUV luminescence from Gd 3+ -containing samples

Temperature dependence of VUV luminescence from GdF 3 Mott law

Temperature dependence of decay kinetics for Gd 3+ 4f 6 5d – 4f 7 emission from CaF 2 :Gd 3+ (0.1%),Ce 3+ (0.05%) crystal in the range of 8 – 149 K Mott law

Comparison of Gd 3+ 5d – 4f emission spectrum from LiGdF 4 and Ce 3+ 4f – 5d excitation (absorption) spectrum from LiGdF 4 :Ce 3+ M. Kirm, G. Stryganyuk, S. Vielhauer, G. Zimmerer, V.N. Makhov, B.Z. Malkin, O.V. Solovyev, R.Yu. Abdulsabirov, S.L. Korableva, Phys. Rev. B 75, (2007)

Charge compensation of RE 3+ ion in CaF 2 by interstitial ions compensation C 4V C 3V C 2V If optically active RE 3+ ions substitute for other (optically non-active) RE 3+ ions of the same charge state: Y 3+, Sc 3+, La 3+, the site symmetry for optical centers will be the same as for the ions in the host crystal. If the charge state of the cation in the host crystal is different (e.g. +2) the charge compensation is necessary, which is reached usually by neighboring interstitial ions which reduce the local symmetry of optical center.

Emission and absorption (excitation) spectra due to 4f  5d transitions in Ce 3+ (C 4v ) doped into CaF 2 4f 2 F 5/2 – 5d 5d – 4f 2 F 5/2 5d – 4f 2 F 7/2 480 cm -1

High-resolution (  ~1 Å) VUV emission spectrum under nm excitation and excitation spectrum of Gd 3+ 4f 6 5d – 4f 7 emission at 129 nm from CaF 2 :Gd 3+ (0.1%),Ce 3+ (0.05%) crystal Spectral lines tentatively ascribed to ZPLs are marked by symbol “  ”, and to dominating vibronic lines by symbol “  “  ~1970 cm -1 V.N. Makhov, S.Kh. Batygov, L.N. Dmitruk, M. Kirm, G. Stryganyuk, and G. Zimmerer, phys. stat. sol. (c) 4, 881 (2007) 370 cm -1

Up-conversion excitation to Gd 3+ 4f 6 5d configuration by KrF excimer laser D. Lo, V.N. Makhov, N.M. Khaidukov, J.C. Krupa, J. Luminescence , 28 (2006)

5d – 4f luminescence from Lu 3+

Lu 3+ 4f 13 5d – 4f 14 emission and 4f 14 – 4f 13 5d excitation spectra for several fluoride matrices M. Kirm, J.C. Krupa, V.N. Makhov, M. True, S. Vielhauer, G. Zimmerer, Phys. Rev B 70, (R) (2004)

Lu 3+ d-f emission and f-d excitation spectra from CaF 2 :Lu 3+ (0.04%) Pure electronic spin-forbidden transitions (in emission): No zero-phonon line in spin- forbidden transitions because of extremely low probability for pure electronic transitions: only vibronic lines are observable ZPL ? ZPL V.N. Makhov, S.Kh. Batygov, L.N. Dmitruk, M. Kirm, S. Vielhauer, and G. Stryganyuk, Physics of the Solid State 50, 1565 (2008)

Appearance of emission band due to spin-allowed 5d – 4f transitions in Lu 3+ at higher temperatures due to thermal population of the higher-lying low-spin 5d state SA SF M. Kirm, G. Stryganyuk, S. Vielhauer, G. Zimmerer, V.N. Makhov, B.Z. Malkin, O.V. Solovyev, R.Yu. Abdulsabirov, S.L. Korableva, Phys. Rev. B 75, (2007)

Normalized spectra of VUV emission due to Lu 3+ 5d – 4f transitions in LuF 3 measured at different temperatures SA SF

Normalized time-resolved spectra of VUV emission due to Tm 3+ 5d – 4f transitions in LiYF 4 :Tm 3+

Temperature dependence of 5d – 4f luminescence from Er 3+ doped into LiYF 4 : time-resolved VUV emission spectra V.N. Makhov, N.M. Khaidukov, N.Yu. Kirikova, M. Kirm, J.C. Krupa, T.V. Ouvarova, G. Zimmerer, J. Lumin , 1005 (2000)

Energy splitting between low-spin (LS) and high-spin (HS) 5d states of heavy RE 3+ ions (from Tb 3+ to Lu 3+ ) in LiYF 4 Yb 3+ Lu 3+ 4f 13 4f 14 LS HS L. van Pieterson, R.T. Wegh, A. Meijerink, M.F. Reid, J. Chem. Phys. 115, 9382 (2001)

Temperature dependence of integrated intensity of VUV luminescence from LuF 3, LiYF 4 :Tm 3+ and LiYF 4 :Er 3+ The curves are the best fits with the formula: I(T)/I(0) = (1+A exp(-  a /k B T)) -1,  a activation energy, A pre-exponent factor (fitting parameters), k B Boltzmann constant.  a =0.04 eV  a =0.50 eV V.N. Makhov, T. Adamberg, M. Kirm, S. Vielhauer, G. Stryganyuk, J. Lumin. 128, 725 (2008)

Different mechanisms of thermal quenching for RE 3+ 5d – 4f luminescence Multi-phonon relaxationThermally activated ionization to conduction band Thermally activated intersystem crossing

Position of 4f and 5d energy levels of RE 3+ and RE 2+ ions in the band gap of the host crystal (CaF 2 ) P. Dorenbos, J. Phys.: Condens. Matter 15, 8417 (2003) Valence band Conduction band, eV

Trends in 5d levels position with respect to conduction band for RE 3+ ions in the second half of lanthanide series V.N. Makhov, M. Kirm, S. Vielhauer, G. Stryganyuk, G. Zimmerer, ECS Transactions 11, 1 (2008)

Concluding remarks  High-resolution (  ~0.5 Å) VUV emission and excitation spectra as well as decay kinetics of VUV luminescence obtained for LiGdF 4, LiYF 4 :Gd 3+ (1.0, 10%), GdF 3, YF 3 :Gd 3+ (1.0%), CaF 2 :Gd 3+ (0.1%), LiLuF 4, LiYF 4 :Lu 3+ (0.5%, 1.0%, 5.0%), LuF 3 and CaF 2 :Lu 3+ (0.04%), evidently show that this VUV luminescence originates from 4f 6 5d – 4f 7 transitions in Gd 3+ for Gd-containing materials and from 4f 13 5d – 4f 14 transitions in Lu 3+ for Lu-containing crystals.  The fine structure due to zero-phonon and vibronic lines along with wide side bands observed in VUV emission and excitation spectra of LiGdF 4, LiYF 4 :Gd 3+, CaF 2 :Gd 3+, LiLuF 4, LiYF 4 :Lu 3+ and CaF 2 :Lu 3+ indicate intermediate electron-lattice coupling (S ~1) between the 4f n-1 5d electronic configurations of the Gd 3+ and Lu 3+ ions and the lattice vibrations in these matrices, whereas the spectra of GdF 3, YF 3 :Gd 3+ and LuF 3 have a smooth shape and large Stokes shift because of strong electron-lattice coupling (S > 5).  The observation of Gd 3+ 4f 6 5d – 4f 7 luminescence requires an assumption that a dense 4f-level system behind the 5d-excitations not necessarily quenches 5d- emission. The influence of spin selection rules on energy relaxation should be taken into account.  Interplay with temperature of spin-allowed and spin-forbidden d-f luminescence from rare earth ions in the second half of lanthanide series agrees with the common trend in decreasing energy splitting between the lowest high-spin and low-spin 5d levels towards heavier rare earth ions.

 Thermal quenching of d-f luminescence agrees with the common trend in decreasing energy gap between the lowest 5d level and the bottom of the conduction band of the host crystal towards heavier rare earth ions.  Only fast spin-allowed d – f luminescence is observed from Gd 3+ compounds, whereas both spin-forbidden and spin-allowed d – f luminescence has been detected from Lu 3+ compounds, the latter being observed only at high enough temperatures.  Many new observations were obtained during past years concerning fundamental optical properties in VUV of RE ions in solids. However, possible practical application of RE containing materials with optical activity in VUV is still under discussion.

Acknowledgements Many thanks to all co-workers from P.N. Lebedev Physical Institute and various Institutions from Russia and other countries for fruitful collaboration when performing joint experiments with the use of synchrotron radiation. Thank you for your attention !

Emission spectrum of LiYF 4 :Er 3+ crystal due to spin-allowed (fast component) and spin-forbidden (slow component) 4f 10 5d – 4f 11 transitions in Er 3+

Decay kinetics for different emission bands corresponding to spin-allowed (S-A) and spin forbidden (S-F) 4f 10 5d - 4f 11 radiative transitions in Er 3+ doped into some fluoride crystals

UV/ VUV excited phosphors in lighting devices Schematic representation of a single plasma display cell, illustrating the process of light generation. Schematic representation of one end of a fluorescent tube, illustrating the process of the generation of visible light.

Quantum cascades in Pr 3+ doped materials Photon #1 Photon #2

Table 1. Comparison between calculated and experimental values of the lowest 4f  5d excitation energies (zero-phonon lines, cm -1 ) of Gd 3+ ion in LiYF 4 :Gd 3+, LiGdF 4 and CaF 2 :Gd 3+ matrix LiYF 4 LiGdF 4 CaF 2 Ce 3+ 4f-5d ZPL (exp) Gd 3+ 4f-5d ZPL (calc) Gd 3+ 4f-5d ZPL (exp) ,  E Gd,Ce = cm -1 P. Dorenbos, J. Luminescence 91, 91, 155 (2000)

Table 2. Comparison between calculated and experimental values of the lowest S-A 4f  5d excitation energies (zero-phonon lines, cm -1 ) of Lu 3+ ion in LiYF 4 :Lu 3+, LiLuF 4 and CaF 2 :Lu 3+ matrix LiYF 4 LiLuF 4 CaF 2 Ce 3+ 4f-5d ZPL (exp) Lu 3+ 4f-5d ZPL (calc) Lu 3+ 4f-5d ZPL (exp)  E Lu,Ce = cm -1