Principle of Electron Mössbauer Spectroscopy

1. Integral Low-Energy Electron Mössbauer Spectroscopy (ILEEMS):a useful variant for true surface studiesE. De Grave and R.E. Vandenberghe Department of Subatomic and Radiation PhysicsGhent UniversityProeftuinstraat 86, B-9000 Gent, BelgiumE-mail: eddy.degrave@ugent.be

2. 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.

3. photo electrons Compton electrons conversion electrons K 7.3 keV L 13.6 keV M 14.3 keV (I)CEMS DCEMS 57Co 57Fe Auger electrons KLL 5.4 keV LMM ~0.6 keV MMM < 15 eV 57Fe e- 136.3 keV I L E E M S 9% 91% 14.4 keV 8.2% shake-off electr. < 15 eV 0 X-rays 14.4 keV -quantum ABSORBER SOURCE Decay process - 57Fe non-resonant resonant

4. Techniques and detectors • 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. 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.

5. e- signal low V high V channeltron detectorinput 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.

6. Efficiency LMM Auger Low E K Conv Energy eV 1 10 102 103 104 Efficiency of a channeltron detector The detectionefficiency 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.

7. low-E electrons high-E 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. 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 ofonly ~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.

8. Bias to HV signal + insulated feedthroughs Al housing ~ 18 x 18 cm channeltron Be window Transducer sample collimator source to pump Experimental set-up

9. Experimental set-up Disassembled top cover with chanel-tron and voltage divider Inside view of the ILEEMS chamber with collimator and chaneltron

10. Experimental set-up Cryostat insert with “cold-finger” sample holder attached to flow cryostat The ILEEMS set-up ready to measure at low temperatures

11. Application of ILEEMS to Fe oxides 1) Ferrihydrite ~5Fe2O3.9H2O Transmission spectra 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.

12. 1) Ferrihydrite (cont’d) ILEEMS spectra The spectra of the two ferrihydrite samples show additionally a sextet of hematite (a-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.

13. The EQ distribution derived from the ILEEMS shows 4 well-resolved peaks 4 distinct O6 coordinations with progressively increasing distortion from octahedral symmetry 1) Ferrihydrite (cont’d) Quadrupole-splitting distributions From transmission MS spectra From ILEEMS spectrum

14. 2) Magnetite – Fe3O4 (bulk powder and thin film) 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 a-Fe2O3 in the thin film (42%) is considerably higher than in the bulk ( 13%)

15. 3) Morin transition in hematite - a-Fe2O3 [111] 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. AF WF S1 (111) S2 T TM 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).

16. 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. WF RA 1.0 TM 0.0 0 300 T TM 3) Morin transition in hematite - a-Fe2O3 (cont’d) • Small particle effects in hematite result in: • decrease of TM • broad transition region DTM with coexistence of both AF and WF states AF 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)

17. ILEEMS spectra at 80 K WF phase AF phase Transmission MS spectra at 80 K 3) Morin transition in hematite - a-Fe2O3 (cont’d) Conclusion: The Morin transition region shifts to lower temperatures at the surface of small particles. This effect increases for the smaller particles.

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