principle of electron mössbauer spectroscopy
DESCRIPTION
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: [email protected]. - PowerPoint PPT PresentationTRANSCRIPT
“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
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 PhysicsGhent University
Proeftuinstraat 86, B-9000 Gent, BelgiumE-mail: [email protected]
“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
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 M-conversion); 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.
“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
Decay process - 57Fe
ILEEMS
non-resonant resonant
(I)CEMS
DCEMS
SOURCE
photo electronsCompton electrons
57Fe
ABSORBER
conversion electrons K 7.3 keV L 13.6 keV M 14.3 keV
e-
14.4 keV -quantum
X-rays
Auger electrons KLL 5.4 keV LMM ~0.6 keV MMM < 15 eV
shake-off electr. < 15 eV
0
57Co
57Fe 136.3 keV9%
91%
14.4 keV
8.2%
“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
1. CEMS or ICEMS techniqueOne 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.
2. 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.
Techniques and detectors
“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
ILEEMS Technique
The ILEEMS technique (Integrated Low-Energy Electron Mössbauer Spectroscopy) is also based on a channeltron detector.
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.
An example of a channeltron is the so-called spiraltron made of glass. A typical diameter of the horn is 15 mm.
low V high V
e-
signal
channeltron
detector input
“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
Efficiency of a channeltron detector
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.
LMM Auger
K Conv
Low E
Energy eV
Eff
icie
ncy
10 102 103 1041
“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
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.
“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
Experimental set-up
Transducer
to HV +Bias
sample
sourcecollimator
Be window
channeltron
signal
insulated feedthroughs
to pump
Al housing~ 18 x 18 cm
“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
Inside view of the ILEEMS chamber with collimator and chaneltron
Disassembled top cover with chanel-tron and voltage divider
Experimental set-up
“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
Cryostat insert with “cold-finger” sample holder attached to flow cryostat
The ILEEMS set-up ready to measure at low temperatures
Experimental set-up
“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
Application of ILEEMS to Fe oxides
85
88
91
94
97
100
Tran
smis
sio
n (%
)
SOOS1 RT
LC31 RT
-2 -1 0 1 2 3Velocity (mm/s)
82
85
88
91
94
97
100
Tran
smis
sio
n (%
)
Two natural ferrihydrite samples with different crystallinity result in the typical broadened doublets which can be perfectly fitted with a distribution of quadrupole splittings.
There are no traces of otherphases.
1) Ferrihydrite ~5Fe2O3.9H2O
Transmission spectra
“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
The spectra of the two ferrihydrite samples show additionally a sextet of hematite (-Fe2O3)
Conclusion: The ferrihydrite particles (flakes) are covered with a hematite layer.
Remark:Freshly prepared ferrihydrite did not show any hematite in the ILEEMS spectrum meaning that hematite is formed by aging.
1) Ferrihydrite (cont’d)
ILEEMS spectra
100
101
102
103
104
105
106
emis
sion
(%
)
SOOS1 RT
-10 -8 -6 -4 -2 0 2 4 6 8 10Velocity (mm/s)
100
101
102
103
104
105
emis
sion
(%
) LC31
“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
1) Ferrihydrite (cont’d)
0.00
0.02
0.04
0.06 SOOS1- TMS RT
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8EQ (mm/s)
0.00
0.02
0.04
0.06 LC31- TMS RT 0.2 0.6 1.0 1.4 1.8 2.2EQ (mm/s)
0.00
0.01
0.02
0.03
0.04
0.05
prob
. (a
rb.
units
) Fh SOOS1
Quadrupole-splitting distributions
From ILEEMS spectrumFrom transmission MS spectra
The EQ distribution derived from the ILEEMS shows 4 well-resolved peaks 4 distinct O6 coordinations with progressively increasing distortion from octahedral symmetry
“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
2) Magnetite – Fe3O4 (bulk powder and thin film)
-8 -6 -4 -2 0 2 4 6 8
Velocity (mm/s)
100
102
104
106
108
Em
ission (%)
Fe3O4 bulk Fe3O4 thin film 100 nm
-8 -6 -4 -2 0 2 4 6 8
Velocity (mm/s)
100
101
102
103
104
105
Em
issi
on (
%)
ILEEMS spectra
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 -Fe2O3 in the thin film (42%) is considerably higher than in the bulk ( 13%)
“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
3) Morin transition in hematite - -Fe2O3
(111)
AF WF
TTM
[111]
S1
S2
Pure, well-crystallized hematite exibits at TM 265K a sharp transition between an antiferro-magntic (AF) arrangement of the spins in the [111] direction and a slightly canted, weakly ferro-magnetic (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 between the two magnetic phases (2AF 0.4 mm/s - 2WF 0.2 mm/s).
“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
3) Morin transition in hematite - -Fe2O3 (cont’d)
RA1.0
0.03000 T
AF
TM
WF
TM
Small particle effects in hematite result in:• decrease of TM
• broad transition region TM with coexistence of both AF and WF states
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.
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)
“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
100
101
102
103
104
105
106
107
100
102
104
106
108
110
99
108
-10 -5 0 5 1090
92
94
96
98
100
-10 -5 0 5 10
93
94
95
96
97
98
99
100
-10 -5 0 5 1090
92
94
96
98
100
HLB2
Em
issi
on
(%
)
HL86
HL65
HLB2
Ab
sorp
tio
n (
%)
Velocity (mm/s)
HL86
Velocity (mm/s)
HL65
Velocity (mm/s)
3) Morin transition in hematite - -Fe2O3 (cont’d)
WF phase
AF phase
ILEEMS spectra at 80 K
Transmission MS spectra at 80 K
Conclusion: The Morin transition region shifts to lower temperatures at the surface of small particles. This effect increases for the smaller particles.
“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
Literature CEMSJ. Fenger, Nucl. Instrum. Methods 69 (1969) 268C.M. Yagnik, R.A. Mazak and R.L. Collins, Nucl. Instrum. Methods 114 (1974) 1Y. Isozumi, D.-I. Lee and I. Kádár, Nucl. Instrum. Methods 120 (1974) 23M.J. Tricker, A.G. Freeman, A.P. Winterbottom and J.M. Thomas, Nucl. Instrum. Methods 135 (1976) 117J.A. Sawicki, B.D. Sawicka and J. Stanek, Nucl. Instrum. Methods 138 (1976) 565D.C. Cook and E. Agyekum, Nucl. Instrum. Methods Phys. Res. B12 (1985) 515J.R. Gancedo, M. Garcia, J.F. Marco and J.A. Tabares, Hyperfine Interactions 111 (1998) 83H. Nakagawa, Y. Ujihira and M. Inaba, Nucl. Instrum. Methods 196 (1982) 573A.P. Kuprin and A.A. Novakova, Nucl. Instrum. Methods Phys. Res. B62 (1992) 493
DCEMSJ. Parellada, M.R. Polcari, K. Burin and G.M. Rothberg, Nucl. Instrum. Methods 179 (1981) 113T.-S. Yang, B. Kolk, T. Kaxhnowski, J. Trooster and N. Benczer-Koller, Nucl. Instrum. Methods 197 (1982) 545T. Toriyama, K. Asano, K. Saneyoshi and K Hisatake, Nucl. Instrum. Methods Phys. Res. B4 (1984) 170
“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
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) 529D. Liljequist and M. Ismail, Phys. Rev. B 31 (1985) 4131D. Liljequist, M. Ismail, K. Saneyoshi, K. Debusmann, W. Keune, R.A. Brand and W.
Kiauka, Phys. Rev. B 31 (1985) 4137
ILEEMSG. Klingelhöfer and W. Meisel, Hyperfine Interact. 57 (1990) 1911G. Klingelhöfer and E. Kankeleit, Hyperfine Interact. 57 (1990) 1905E. De GraveE, R.E. Vandenberghe and C. Dauwe, Hyperfine Interact. 161 (2005) 147