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VUV spectroscopy of rare earth ions in solids: recent studies and possible applications

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Title: VUV spectroscopy of rare earth ions in solids: recent studies and possible applications


1
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
2
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 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.
3
General optical properties of trivalent rare
earth ions in solids
4
Rare earth elements
5
Energy level structure for 4fn electronic
configuration of trivalent rare earth ions (Dieke
diagram)
6
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 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
7
Crystal field splitting for 4fn-15d electronic
configuration
The 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
8
4f and 5d energy levels of Ce3 in tetragonal
environment
Site symmetry S4
?
SO
9
Energies of the lowest 4fn-15d levels for RE3
ions doped into LiYF4 crystal
10
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
11
Single configuration-coordinate diagram of the 4f
and 5d states and of 4f 4f and 4f 5d
transitions in rare earth ion
12
High-efficiency VUV-excited phosphors
13
Why we need VUV-phosphor efficiency gt 100 ?
85

6
0.25/0.17 1.47 We need phosphor with Q gt 100
14
Quantum splitting (quantum cutting) schemes
15
Visible quantum cutting by two-step energy
transfer upon excitation in the 6GJ levels of
Gd3
1 violet photon absorbed on Gd3 8S7/2?6GJ
transitions, 2 red photons emitted on Eu3
5D0?7F1 transitions
LiGdF4Eu3 GdF3Eu3
16
Visible quantum cutting via down-conversion in
LiGdF4Er3,Tb3
1 VUV photon absorbed on Er3 4f11 4f105d
transition, 2 photons emitted on 1) Er3
4S3/2?4I15/2 transition 2) Tb3 5D3,4?7FJ
transitions
17
Scintillators for medical applications (PET)
18
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
19
Scintillators for PET based on 5d 4f
transitions in Ce3
Requirements to new scintillators
Lifetime of the emitting state (scintillation
decay time) t ? ?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.
20
Experimental setup for VUV spectroscopy with
synchrotron radiation
21
SUPERLUMI station at HASYLAB (DESY)
Primary monochromator 3 secondary
monochromators Position-sensitive
detectors Mechanical chopper In-situ cleaving 4
to 900 K
G. Zimmerer, Radiation Measurements 42 (2007) 859
22
5d 4f luminescence from Gd3
23
The scheme of radiative and nonradiative
transitions in Gd3
Nonradiative relaxation (intersystem
crossing) is heavily spin-forbidden
24
VUV emission spectra of GdF3, LiGdF4 and
CaF2Gd3(0.1)
Sgt5
S1
M. Kirm, J.C. Krupa, V.N. Makhov, M. True, S.
Vielhauer, G. Zimmerer, Phys. Rev B 70, 241101(R)
(2004)
25
Decay curves of VUV luminescence from
Gd3-containing samples
26
Temperature dependence of VUV luminescence from
GdF3
Mott law
27
Temperature dependence of decay kinetics for Gd3
4f65d 4f7 emission from CaF2Gd3(0.1),Ce3(0.0
5) crystal in the range of 8 149 K
Mott law
28
Comparison of Gd3 5d 4f emission spectrum from
LiGdF4 and Ce3 4f 5d excitation (absorption)
spectrum from LiGdF4Ce3
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)
29
Charge compensation of RE3 ion in CaF2 by
interstitial ions
If optically active RE3 ions substitute for
other (optically non-active) RE3 ions of
the same 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
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.
C4V
C3V
compensation
C2V
30
Emission and absorption (excitation) spectra due
to 4f ? 5d transitions in Ce3 (C4v) doped into
CaF2
5d 4f 2F7/2
480 cm-1
5d 4f 2F5/2
4f 2F5/2 5d
31
High-resolution (?? 1 Å) VUV emission spectrum
under 124.7 nm excitation and excitation spectrum
of Gd3 4f65d 4f7 emission at 129 nm from
CaF2Gd3(0.1),Ce3(0.05) crystal
?1970 cm-1
370 cm-1
Spectral lines tentatively ascribed to ZPLs are
marked by symbol ? , and to dominating
vibronic lines by symbol ?
V.N. Makhov, S.Kh. Batygov, L.N. Dmitruk, M.
Kirm, G. Stryganyuk, and G. Zimmerer, phys. stat.
sol. (c) 4, 881 (2007)
32
Up-conversion excitation to Gd3 4f65d
configuration by KrF excimer laser
D. Lo, V.N. Makhov, N.M. Khaidukov, J.C. Krupa,
J. Luminescence 119-120, 28 (2006)
33
5d 4f luminescence from Lu3
34
Lu3 4f135d 4f14 emission and 4f14 4f135d
excitation spectra for several fluoride matrices
M. Kirm, J.C. Krupa, V.N. Makhov, M. True, S.
Vielhauer, G. Zimmerer, Phys. Rev B 70, 241101(R)
(2004)
35
Lu3 d-f emission and f-d excitation spectra from
CaF2Lu3(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)
36
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 state
SF
SA
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)
37
Normalized spectra of VUV emission due to Lu3 5d
4f transitions in LuF3 measured at different
temperatures
SF
SA
38
Normalized time-resolved spectra of VUV emission
due to Tm3 5d 4f transitions in LiYF4Tm3
39
Temperature dependence of 5d 4f luminescence
from Er3 doped into LiYF4 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. 87-89, 1005 (2000)
40
Energy splitting between low-spin (LS) and
high-spin (HS) 5d states of heavy RE3 ions (from
Tb3 to Lu3) in LiYF4
LS
LS
1500
800
HS
HS
Yb3 Lu3
4f13 4f14
L. van Pieterson, R.T. Wegh, A. Meijerink, M.F.
Reid, J. Chem. Phys. 115, 9382 (2001)
41
Temperature dependence of integrated intensity of
VUV luminescence from LuF3, LiYF4Tm3 and
LiYF4Er3
?a0.04 eV
?a0.50 eV
The curves are the best fits with the
formula I(T)/I(0) (1A exp(-?a/kBT))-1 , ?a
activation energy, A pre-exponent factor (fitting
parameters), kBBoltzmann constant.
V.N. Makhov, T. Adamberg, M. Kirm, S. Vielhauer,
G. Stryganyuk, J. Lumin. 128, 725 (2008)
42
Different mechanisms of thermal quenchingfor
RE3 5d 4f luminescence
Multi-phonon relaxation
Thermally activated ionization to
conduction band
Thermally activated intersystem crossing
43
Position of 4f and 5d energy levels of RE3 and
RE2 ions in the band gap of the host crystal
(CaF2)
Conduction band
, eV
Valence band
P. Dorenbos, J. Phys. Condens. Matter 15, 8417
(2003)
44
Trends in 5d levels position with respect to
conduction band for RE3 ions in the second half
of lanthanide series
V.N. Makhov, M. Kirm, S. Vielhauer, G.
Stryganyuk, G. Zimmerer, ECS Transactions 11, 1
(2008)
45
Concluding remarks
  • High-resolution (??0.5 Å) VUV emission and
    excitation spectra as well as decay kinetics of
    VUV luminescence obtained for LiGdF4,
    LiYF4Gd3(1.0, 10), GdF3, YF3Gd3(1.0),
    CaF2Gd3(0.1), LiLuF4, LiYF4Lu3(0.5, 1.0,
    5.0), LuF3 and CaF2Lu3(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, LiYF4Gd3, CaF2Gd3, LiLuF4,
    LiYF4Lu3 and CaF2Lu3 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, YF3Gd3
    and LuF3 have a smooth shape and large Stokes
    shift because of strong electron-lattice coupling
    (S gt 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.

46
  • 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.

47
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 !
48
Emission spectrum of LiYF4Er3 crystal due to
spin-allowed (fast component) and spin-forbidden
(slow component) 4f105d 4f11 transitions in
Er3
49
Decay kinetics for different emission bands
corresponding to spin-allowed (S-A) and spin
forbidden (S-F) 4f105d - 4f11 radiative
transitions in Er3doped into some fluoride
crystals
50
UV/ VUV excited phosphors in lighting devices
Schematic representation of one end of a
fluorescent tube, illustrating the process of the
generation of visible light.
Schematic representation of a single plasma
display cell, illustrating the process of light
generation.
 
51
Quantum cascades in Pr3 doped materials
Photon 1
Photon 2
52
Table 1. Comparison between calculated and
experimental values of the lowest 4f?5d
excitation energies (zero-phonon lines, cm-1) of
Gd3 ion in LiYF4Gd3, LiGdF4 and CaF2Gd3
matrix LiYF4 LiGdF4 CaF2
Ce3 4f-5d ZPL (exp) 33450 33615 31930
Gd3 4f-5d ZPL (calc) 79250 79415 77730
Gd3 4f-5d ZPL (exp) 79250 79377 77660
, ?EGd,Ce 45800 cm-1
P. Dorenbos, J. Luminescence 91, 91, 155 (2000)
53
Table 2. Comparison between calculated and
experimental values of the lowest S-A 4f?5d
excitation energies (zero-phonon lines, cm-1) of
Lu3 ion in LiYF4Lu3, LiLuF4 and CaF2Lu3
matrix LiYF4 LiLuF4 CaF2
Ce3 4f-5d ZPL (exp) 33450 33130 31930
Lu3 4f-5d ZPL (calc) 82620 82300 81100
Lu3 4f-5d ZPL (exp) 81877 81777 82355
?ELu,Ce 49170 cm-1
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