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

Outline1. 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 Gd3+ ions: spectral properties, decay kinetics, thermal quenching; assignment to Gd3+ 5d-4f luminescence; vibronic structure; the strength of electron-phonon coupling.4. VUV luminescence from Lu3+ ions: spectral and timing properties; assignment to Lu3+ 5d-4f luminescence; vibronic structure; the strength of electron-phonon coupling; spin-forbidden and spin-allowed Lu3+ 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 4fn electronic configuration of trivalent rare earth ions (Dieke diagram)

Crystal field splitting for 4f electronic configurationBecause 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 4fn configuration are only weakly sensitive to the type of the crystal host. Splitting of energy levels of 4fn electronic configuration due to: I Coulomb interaction;II spin-orbit interaction; III crystal-field interaction

Crystal field splitting for 4fn-15d electronic configurationThe 5d electrons are not effectively shielded by other electrons, and the crystal field influence on the energy levels of 4fn-15d electronic configuration is strong. Accordingly, crystal field splitting of 5d levels is large and the energies of levels within 4fn-15d electronic configuration can strongly differ for different crystal hosts. Crystal-field splitting of 5d1 configuration for tetragonal Ce3+ center: I free ion, II Oh, III Oh + spin-orbit, IV 4V

4f and 5d energy levels of Ce3+ in tetragonal environmentSOSite symmetry S4

Energies of the lowest 4fn-15d levels for RE3+ ions doped into LiYF4 crystal

Schematic electron configurations for the ground state (GS) 4f8, the lowest energy high-spin (HS) 4f75d state and the lowest energy low-spin (LS) 4f75d state for Tb3+

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% ?8560.25/0.17 = 1.47We need phosphor with Q > 100%

Quantum splitting (quantum cutting) schemes

Visible quantum cutting by two-step energy transfer upon excitation in the 6GJ levels of Gd3+1 violet photon absorbed on Gd3+ 8S7/26GJ transitions, 2 red photons emitted onEu3+ 5D07F1 transitionsLiGdF4:Eu3+GdF3:Eu3+

Visible quantum cutting via down-conversion in LiGdF4:Er3+,Tb3+1 VUV photon absorbed on Er3+ 4f11 4f105d transition, 2 photons emitted on :1) Er3+ 4S3/24I15/2 transition; 2) Tb3+ 5D3,47FJ transitions;

Scintillators for medical applications (PET)

Principles of PETRing of PhotonDetectorsPatient 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 Ce3+Lifetime of the emitting state (scintillation decay time): em3 shorter-wavelength emission is needed for increasing time resolution of scintillation detector: Pr3+, Nd3+, activator ions with shorter-wavelength (UV/VUV) and faster 5d 4f transitions can be used instead of Ce3+.Requirements to new scintillators:

Experimental setup for VUV spectroscopy with synchrotron radiation

SUPERLUMI station at HASYLAB (DESY)G. Zimmerer, Radiation Measurements 42 (2007) 859 Primary monochromator3 secondary monochromatorsPosition-sensitive detectorsMechanical chopperIn-situ cleaving4 to 900 K

5d 4f luminescence from Gd3+

The scheme of radiative and nonradiative transitions in Gd3+Nonradiative relaxation (intersystem crossing)is heavily spin-forbidden

VUV emission spectra of GdF3, LiGdF4 and CaF2:Gd3+(0.1%)M. Kirm, J.C. Krupa, V.N. Makhov, M. True, S. Vielhauer, G. Zimmerer, Phys. Rev B 70, 241101(R) (2004) S>5S~1

Decay curves of VUV luminescence from Gd3+-containing samples

Temperature dependence of VUV luminescence from GdF3Mott law

Temperature dependence of decay kinetics for Gd3+ 4f65d 4f7 emission from CaF2:Gd3+(0.1%),Ce3+(0.05%) crystal in the range of 8 149 KMott law

Comparison of Gd3+ 5d 4f emission spectrum from LiGdF4 and Ce3+ 4f 5d excitation (absorption) spectrum from LiGdF4:Ce3+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, 075111 (2007)

Charge compensation of RE3+ ion in CaF2 by interstitial ions compensationC4VC3VC2VIf optically active RE3+ ions substitute for other (optically non-active) RE3+ ions of thesame charge state: Y3+, Sc3+, La3+, 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 isdifferent (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 Ce3+ (C4v) doped into CaF24f 2F5/2 5d5d 4f 2F5/25d 4f 2F7/2480 cm-1

High-resolution ( ~1 ) VUV emission spectrum under 124.7 nm excitation and excitation spectrum of Gd3+ 4f65d 4f7 emission at 129 nm from CaF2:Gd3+(0.1%),Ce3+(0.05%) crystalSpectral lines tentatively ascribed to ZPLs are marked by symbol , and to dominating vibronic lines by symbol ~1970 cm-1V.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 Gd3+ 4f65d configuration by KrF excimer laserD. Lo, V.N. Makhov, N.M. Khaidukov, J.C. Krupa, J. Luminescence 119-120, 28 (2006)

5d 4f luminescence from Lu3+

Lu3+ 4f135d 4f14 emission and 4f14 4f135d excitation spectra for several fluoride matricesM. Kirm, J.C. Krupa, V.N. Makhov, M. True, S. Vielhauer, G. Zimmerer, Phys. Rev B 70, 241101(R) (2004)

Lu3+ d-f emission and f-d excitation spectra from CaF2:Lu3+(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 observableZPL ?ZPLV.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 Lu3+ at higher temperatures due to thermal population of the higher-lying low-spin 5d stateSASFM. 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, 075111 (2007)

Normalized spectra of VUV emission due to Lu3+ 5d 4f transitions in LuF3 measured at different temperatures SASF

Normalized time-resolved spectra of VUV emission due to Tm3+ 5d 4f transitions in LiYF4:Tm3+

Temperature dependence of 5d 4f luminescence from Er3+ doped into LiYF4: time-resolved VUV emission spectraV.N. Makhov, N.M. Khaidukov, N.Yu. Kirikova, M. Kirm, J.C. Krupa, T.V. Ouvarova, G. Zimmerer, J. Lumin. 87-89, 1005 (2000)

Energy splitting between low-spin (LS) and high-spin (HS) 5d states of heavy RE3+ ions (from Tb3+ to Lu3+) in LiYF4Yb3+ Lu3+4f13 4f14LSLSHSHS1500800L. 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 LuF3, LiYF4:Tm3+ and LiYF4:Er3+ The curves are the best fits with the formula: I(T)/I(0) = (1+A exp(-a/kBT))-1 , a activation energy, A pre-exponent factor (fitting parameters), kBBoltzmann constant. a=0.04 eVa=0.50 eVV.N. Makhov, T. Adamberg, M. Kirm, S. Vielhauer, G. Stryganyuk, J. Lumin. 128, 725 (2008)

Different mechanisms of thermal quenchingfor RE3+ 5d 4f luminescenceMulti-phonon relaxationThermally activated ionization to conduction band Thermally activated intersystem crossing

Position of 4f and 5d energy levels of RE3+ and RE2+ ions in the band gap of the host crystal (CaF2)P. Dorenbos, J. Phys.: Condens. Matter 15, 8417 (2003)Valence bandConduction band, eV

Trends in 5d levels position with respect to conduction band for RE3+ ions in the second half of lanthanide seriesV.N. Makhov, M. Kirm, S. Vielhauer, G. Stryganyuk, G. Zimmerer, ECS Transactions 11, 1 (2008)

Concluding remarksHigh-resolution (~0.5 ) VUV emission and excitation spectra as well as decay kinetics of VUV luminescence obtained for LiGdF4, LiYF4:Gd3+(1.0, 10%), GdF3, YF3:Gd3+(1.0%), CaF2:Gd3+(0.1%), LiLuF4, LiYF4:Lu3+(0.5%, 1.0%, 5.0%), LuF3 and CaF2:Lu3+(0.04%), evidently show that this VUV luminescence originates from 4f65d 4f7 transitions in Gd3+ for Gd-containing materials and from 4f135d 4f14 transitions in Lu3+ 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 LiGdF4, LiYF4:Gd3+, CaF2:Gd3+, LiLuF4, LiYF4:Lu3+ and CaF2:Lu3+ indicate intermediate electron-lattice coupling (S ~1) between the 4fn-15d electronic configurations of the Gd3+ and Lu3+ ions and the lattice vibrations in these matrices, whereas the spectra of GdF3, YF3:Gd3+ and LuF3 have a smooth shape and large Stokes shift because of strong electron-lattice coupling (S > 5).The observation of Gd3+ 4f65d 4f7 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 Gd3+ compounds, whereas both spin-forbidden and spin-allowed d f luminescence has been detected from Lu3+ 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.

AcknowledgementsMany 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 LiYF4:Er3+ crystal due to spin-allowed (fast component) and spin-forbidden (slow component) 4f105d 4f11 transitions in Er3+

Decay kinetics for different emission bands corresponding to spin-allowed (S-A) and spin forbidden (S-F) 4f105d - 4f11 radiative transitions in Er3+doped into some fluoride crystals

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

Quantum cascades in Pr3+ doped materialsPhoton #1Photon #2

Table 1. Comparison between calculated and experimental values of the lowest 4f5d excitation energies (zero-phonon lines, cm-1) of Gd3+ ion in LiYF4:Gd3+, LiGdF4 and CaF2:Gd3+, EGd,Ce = 45800 cm-1 P. Dorenbos, J. Luminescence 91, 91, 155 (2000)

matrixLiYF4LiGdF4CaF2Ce3+ 4f-5d ZPL (exp)334503361531930Gd3+ 4f-5d ZPL (calc)792507941577730Gd3+ 4f-5d ZPL (exp)792507937777660

Table 2. Comparison between calculated and experimental values of the lowest S-A 4f5d excitation energies (zero-phonon lines, cm-1) of Lu3+ ion in LiYF4:Lu3+, LiLuF4 and CaF2:Lu3+ELu,Ce = 49170 cm-1

matrixLiYF4LiLuF4CaF2Ce3+ 4f-5d ZPL (exp)334503313031930Lu3+ 4f-5d ZPL (calc)826208230081100Lu3+ 4f-5d ZPL (exp)818778177782355