Integral Low-Energy Electron
Mössbauer Spectroscopy (ILEEMS):
a useful variant for true surface studies
E. De Grave and R.E. Vandenberghe
Department of Subatomic and Radiation Physics
Ghent University
Proeftuinstraat 86, B-9000 Gent, Belgium
E-mail: eddy.degrave@ugent.be
Principle of Electron Mössbauer Spectroscopy
A Mössbauer event at 57Fe results only for 10% in re-emission of gamma rays
due to the large internal conversion giving rise to the emission of electrons and
secondary X-rays. Backscattering measurements of resonant 14.4 keV gamma
rays are therefore less attractive because of their low efficiency. The detection
of the secondary 6.3 keV X-rays can give better results, however, many
precautions must be taken in order to reduce the noise and the non-resonant
background. Hence, the most efficient part of the backscattering techniques
comprehends the detection of the resonant electrons, which due to their limited
escape depth yields information of the surface complementary to that of
transmission measurements. The resonant electrons can be classified into:
conversion electrons with an energy of 13.6 keV or higher (L- and Mconversion); those of 7.3 keV (K-conversion); KLL-Auger electrons with an
energy of 5.4 keV; LMM-Auger electrons of about 580 eV; and electrons from
further degradation processes (LMM, MMM, MVV, shake-off…) with very
low energy (< 15eV) (see next slide). According to the detected energy range
various electron-backscattering techniques have been proposed.
Decay process - 57Fe
non-resonant
photo electrons
conversion
Compton electrons electrons
57Co
57Fe
resonant
K 7.3 keV
L 13.6 keV
M 14.3 keV
136.3 keV
9%
91%
14.4 keV
0
57Fe
8.2%
ABSORBER
DCEMS
Auger electrons
KLL 5.4 keV
LMM ~0.6 keV
MMM < 15 eV
shake-off electr.
< 15 eV
14.4 keV -quantum
X-rays
SOURCE
e-
(I)CEMS
I
L
E
E
M
S
Techniques and detectors
1.
2.
CEMS or ICEMS technique
One of the most frequently used electron detector is a simple proportional
counter consisting of a small chamber with the sample mounted inside and a
ionizing gas flow of helium with a few % of methane or other mixtures.
Another detection method consists of a vacuum chamber with a built-in
channeltron.
In both cases primarely high-energy conversion electrons are detected and
the technique is called CEMS or ICEMS = (Integral) Conversion Electron
Mössbauer Spectroscopy.
Because of the high energy of the involved electrons the probing depth is
relatively high.
DCEMS technique
Another, more sophisticated method is based on a high-resolution electron
spectrometer enabling to make an energy selection and therefore indirectly
probing at different depths. This is the so-called DCEMS = Depth-selective
Conversion Electron Mössbauer Spectroscopy.
ILEEMS Technique
The ILEEMS technique (Integrated LowEnergy Electron Mössbauer Spectroscopy)
is also based on a channeltron detector.
elow V
high V
The principle of a channeltron consists of
accelerating the electrons in a curved (glass
or ceramic) tube creating secondary
electrons and evoking an avalanche effect as
a pulse signal.
signal
channeltron
detector input
An example of a channeltron is the so-called
spiraltron made of glass. A typical diameter
of the horn is 15 mm.
Efficiency
Efficiency of a channeltron detector
Low E
LMM
Auger
K Conv
1
10
102
103
104
Energy eV
The detection efficiency for low-energy electrons can be improved by
adding an energy of a few hunderds of eV, thus inducing the optimal
efficiency range of the detector. This can be realized by imposing a bias
voltage of about 200V to the channeltron input. This procedure does not
significantly affect the detection efficiency of the high-energy electrons.
In contrast, the efficiency of the detection of low-energy electrons
increases drastically because these electrons are focused towards the
horn inlet by the bias voltage.
low-E electrons
high-E electrons
Conclusion: by applying a proper bias voltage between sample and
channeltron detector input the intensity of the low-energy electrons can
be 10-15 times higher than that of the conversion and Auger electrons.
Low-energy electrons (~10eV) have an escape depth of only ~5 nm.
ILEEMS (Integrated Low-Energy Electron Mössbauer
Spectroscopy) is a practical and suitable technique for studies of the top
surface layers of solid materials.
Experimental set-up
Bias
+
to HV
signal
insulated feedthroughs
Al housing
~ 18 x 18 cm
channeltron
Be window
Transducer
sample
collimator
source
to pump
Experimental set-up
Inside view of the ILEEMS chamber
with collimator and chaneltron
Disassembled top cover with chaneltron and voltage divider
Experimental set-up
Cryostat insert with “cold-finger” sample
holder attached to flow cryostat
The ILEEMS set-up ready to measure
at low temperatures
Application of ILEEMS to Fe oxides
1) Ferrihydrite ~5Fe2O3.9H2O
Transmission (%)
100
Transmission spectra
97
94
Two natural ferrihydrite samples with
different crystallinity result in the
typical broadened doublets which can
be perfectly fitted with a distribution of
quadrupole splittings.
91
88
SOOS1 RT
85
Transmission (%)
100
97
94
There are no traces of otherphases.
91
88
85
LC31 RT
82
-2
-1
0
1
Velocity (mm/s)
2
3
1) Ferrihydrite (cont’d)
106
emission (%)
105
ILEEMS spectra
SOOS1 RT
104
The spectra of the two ferrihydrite
samples show additionally a sextet
of hematite (a-Fe2O3)
103
102
101
Conclusion:
The ferrihydrite particles (flakes) are
covered with a hematite layer.
100
emission (%)
105
LC31
104
103
102
101
100
-10 -8 -6 -4 -2
0
2
4
Velocity (mm/s)
6
8 10
Remark:
Freshly prepared ferrihydrite did not
show any hematite in the ILEEMS
spectrum meaning that hematite is
formed by aging.
1) Ferrihydrite (cont’d)
Quadrupole-splitting distributions
From transmission MS spectra
From ILEEMS spectrum
0.06
SOOS1- TMS RT
0.04
0.02
0.00
0.06
LC31- TMS RT
0.04
0.02
0.00
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
EQ (mm/s)
prob. (arb. units)
0.05
Fh SOOS1
0.04
0.03
0.02
0.01
0.00
0.2
0.6
1.0
1.4
1.8
2.2
EQ (mm/s)
The EQ distribution derived from the
ILEEMS shows 4 well-resolved peaks
4 distinct O6 coordinations with
progressively increasing distortion from
octahedral symmetry
2) Magnetite – Fe3O4 (bulk powder and thin film)
ILEEMS spectra
105
108
Fe3O4 bulk
104
106
103
104
102
102
101
100
100
-8
-6
-4
-2
0
2
4
Velocity (mm/s)
6
8
-8
-6
-4
-2
0
2
4
6
8
Velocity (mm/s)
The ILEEMS spectra of thin-film and powdered bulk magnetite show the
presence of hematite (pink coloured spectra). The latter was not observed in
the transmission spectrum of bulk magnetite. The amount of a-Fe2O3 in the
thin film (42%) is considerably higher than in the bulk ( 13%)
Emission (%)
Emission (%)
Fe3O4 thin film 100 nm
3) Morin transition in hematite - a-Fe2O3
[111]
S1 AF
WF
(111)
S2
TM
T
Pure, well-crystallized hematite
exibits at TM  265K a sharp
transition between an antiferromagntic (AF) arrangement of the
spins in the [111] direction and a
slightly canted, weakly ferromagnetic (WF) arrangement in the
(111) basal plane.
Mössbauer spectrocopy is an excellent tool to study the Morin transition
because there is a large difference in the quadrupole shift 2e between the
two magnetic phases (2eAF  0.4 mm/s - 2eWF  0.2 mm/s).
3) Morin transition in hematite - a-Fe2O3 (cont’d)
Small particle effects in hematite result in:
• decrease of TM
• broad transition region TM with coexistence of both AF and WF states
RA
superposition of two
Mössbauer subspectra for
which the relative area (RA) of
the WF spectrum increases
with increasing T at the
expense of the relative area of
the AF spectrum.
1.0
AF
WF
TM
0.0
0
TM
300 T
Six samples with different particle size have been measured with transmission
MS and ILEEMS. The spectra in the next slide are those of HLB2 (av. dim.
~25 nm); HL86(av. dim. ~40 nm) and HL65(av. dim. ~130 nm)
3) Morin transition in hematite - a-Fe2O3 (cont’d)
110
107
ILEEMS
spectra at
80 K
106
HL65
HL86
108
HLB2
Emission (%)
105
108
106
104
103
104
102
102
101
100
100
100
100
WF phase
AF phase
99
100
99
98
Absorption (%)
98
98
96
Transmission
MS spectra at
80 K
96
97
96
94
94
95
HLB2
92
HL65
HL86
92
94
93
90
90
-10
-5
0
5
Velocity (mm/s)
10
-10
-5
0
5
Velocity (mm/s)
10
-10
-5
0
5
10
Velocity (mm/s)
Conclusion: The Morin transition region shifts to lower temperatures at the
surface of small particles. This effect increases for the smaller particles.
Literature
CEMS
J. Fenger, Nucl. Instrum. Methods 69 (1969) 268
C.M. Yagnik, R.A. Mazak and R.L. Collins, Nucl. Instrum. Methods 114 (1974) 1
Y. Isozumi, D.-I. Lee and I. Kádár, Nucl. Instrum. Methods 120 (1974) 23
M.J. Tricker, A.G. Freeman, A.P. Winterbottom and J.M. Thomas, Nucl. Instrum.
Methods 135 (1976) 117
J.A. Sawicki, B.D. Sawicka and J. Stanek, Nucl. Instrum. Methods 138 (1976) 565
D.C. Cook and E. Agyekum, Nucl. Instrum. Methods Phys. Res. B12 (1985) 515
J.R. Gancedo, M. Garcia, J.F. Marco and J.A. Tabares, Hyperfine Interactions 111 (1998)
83
H. Nakagawa, Y. Ujihira and M. Inaba, Nucl. Instrum. Methods 196 (1982) 573
A.P. Kuprin and A.A. Novakova, Nucl. Instrum. Methods Phys. Res. B62 (1992) 493
DCEMS
J. Parellada, M.R. Polcari, K. Burin and G.M. Rothberg, Nucl. Instrum. Methods 179
(1981) 113
T.-S. Yang, B. Kolk, T. Kaxhnowski, J. Trooster and N. Benczer-Koller, Nucl. Instrum.
Methods 197 (1982) 545
T. Toriyama, K. Asano, K. Saneyoshi and K Hisatake, Nucl. Instrum. Methods Phys. Res.
B4 (1984) 170
Literature (cont’d)
P. Auric, A. Baudry, M. Bogé, J. Rocco and L. Trabut, Hyperfine Interactions 58 (1990)
2491
B. Stahl, G. Klingelhöfer, H. Jäger, H. Keller, Th. Reitz and E. Kankeleit, Hyperfine
Interactions (1990) 2547
S.C. Pancholi, H. de Waard, J.L.W. Petersen, A. van der Wijk and J. van Klinken, Nucl.
Instrum. Methods Phys. Res. 221 (1984) 577
D. Liljequist, T. Eckdahl and U. Bäverstam, Nucl. Instrum. Methods 155 (1978) 529
D. Liljequist and M. Ismail, Phys. Rev. B 31 (1985) 4131
D. Liljequist, M. Ismail, K. Saneyoshi, K. Debusmann, W. Keune, R.A. Brand and W.
Kiauka, Phys. Rev. B 31 (1985) 4137
ILEEMS
G. Klingelhöfer and W. Meisel, Hyperfine Interact. 57 (1990) 1911
G. Klingelhöfer and E. Kankeleit, Hyperfine Interact. 57 (1990) 1905
E. De GraveE, R.E. Vandenberghe and C. Dauwe, Hyperfine Interact. 161 (2005) 147