Mossbauer spectroscopy conversion electron

An additional technique, known as conversion electron Mossbauer Spectroscopy (GEMS), is applied to investigate the properties of physical surfaces (thin films) and the surface properties. To know about GEMS, we will have to understand what the conversion electron is  

Gonversion Electron Mossbauer Spectroscopy (GEMS) is an alternative to normal Mossbauer spectroscopy. The y-rays emitted by the source, enter the electron detector, through a thin A1 window as shown in Fig. 4.11. The sample is mounted on the detector. Resonance absorption of y-rays in the sample is followed by de-excitation of Mossbauer nuclei, as a result of which conversion and corresponding Auger electrons enter the detector volume, and trigger an electronic impulse. The recoilless absorption is observed as a peak in the spectrum. 

The Gonversion Electron Mossbauer Spectroscopy experiments are very useful, particularly in nondestructive testing and study of surfaces and thin layers because low-energy conversion electrons in matter have a limited range (typically 100 nm for the nucleus Fe), the method is especially useful for the study of thin layers. Apart from the transmission spectroscopy, the 

It is possible to sample different depth regions with electrons by selecting the electrons emerging from the surface with particular energies. For the most promising isotopes, Fe and Sn, the main t3q es of scattered photons or electrons, their intensities and their approximate ranges are given in Tables 4.1 and 4.2. 

The fairly wide use of the Fe, Sn and Eu source is due to the fact that these sources are long-lived and easily available. The spectra are normally recorded at room temperatures. If spectra have to be recorded at low temperatures (liquid Nitrogen 80 K or liquid helium 4.2 K) for the investigation of magnetic materials with low transition temperatures, it is common practice to keep the source at room temperatures and to cool the absorber only in a horizontal-beam transition cryostat. 

---

Typical Mossbauer spectra for the fresh, reduced, carblded and used Fe/ZSM-5 system are shown in a composite Fig. 5. Similar spectra were obtained for the Fe-Co/ZSM-5 system. The product distribution for the F-T reaction, using the Fe and Fe-Co systems, are shown in Table 1. The gasoline range hydrocarbon yield increased from 75 to 94%, when the Fe-Co clusters were used in place of Fe only. In a typical CEMS (Conversion Electron Mossbauer Spectroscopy) of the Fe-Co system, no spectrum for 57pg vas observed even after one week from this. It was concluded that in the Fe-Co clusters Co was predominantly in the "mantle" and Fe species were In their "core," in the parlance of metallurgy/geophysics. This model Is sometimes referred to as the cherry model. 

Conversion electron Mossbauer spectroscopy (CEMS) measurements with back scattering geometry have the merit that spectra can be obtained from a sample with much less isotope content compared with transmission measurements. Another merit is that a sample, deposited on a thick substrate, could be measured, and that because of the limited escape depth of the conversion electrons, depth-selective surface studies are possible. The CEMS technique was found to be best applicable to specimens of 10-100 pg Au cm, i.e., about two orders of magnitudes thinner than required for measurements in transmission mode [443]. This way (1) very thin films of gold alloys, as well as laser- and in beam-modified surfaces in the submicrometers range of depth [443], and (2) metallic gold precipitates in implanted MgO crystals [444] were investigated. 

It is noteworthy that a low-temperature apparatus has been described recently that allows y-ray and conversion electron Mossbauer spectroscopy measurements at the same time, in the temperature range from 10 K to room temperature (28). 

CEMS = conversion electron Mossbauer spectroscopy DFT = density functional theory EFG = electric field gradient EPR = electron paramagnetic resonance ESEEM = electron spin echo envelope modulation spectroscopy GTO = Gaussian-type orbitals hTH = human tyrosine hydroxylase MIMOS = miniaturized mossbauer spectrometer NFS = nuclear forward scattering NMR = nuclear magnetic resonance RFQ = rapid freeze quench SAM = S -adenosyl-L-methionine SCC = self-consistent charge STOs = slater-type orbitals TMP = tetramesitylporphyrin XAS = X-ray absorption spectroscopy. 

The Mossbauer-effect experiment can also be applied to the study of surfaces in the variation known as conversion electron Mossbauer spectroscopy (CEMS). Here, what is monitored as a function of incident y-ray energy is not absorption, but the emission of electrons through a process of internal conversion (i.e., as a byproduct of the absorption of Mossbauer y rays). Since the conversion electrons can only escape from the surface layers of the solid, data are selectively acquired for the surface region, arising from the Mossbauer effect in the (most commonly iron) atoms of the surface layers. The monitoring of emitted electrons results in a mirror image of the usual absorption spectrum. Transmission and CEM spectra of vivianite [Ee3(P04)2-8H20] are illustrated in Fig. 2.49 (after Tricker et al., 1979]. 

Jones, W., J. M. Thomas, R. K. Thorpe, and M. J. Tricker (1978). Conversion electron Mossbauer spectroscopy and the study of surfaee properties and reactions. Appl. Surf. Sci. 1, 388 07. 

Application of Depth-resolved Conversion Electron MOssbauer Spectroscopy to Photochemistry in the Solid Surface... 

Angle-resolved X-ray photoemission spectroscopy (3, 38, 39, 49, 50) Conversion electron Mossbauer spectroscopy (4, 52-55)... 

M.J. Tricker, J.M. Thomas, and A.P. Winterbottom. Conversion Electron Mossbauer Spectroscopy for the Study of Solid Surfaces. Surf. Sci. 45 601 (1974). 

Fe conversion electron Mossbauer spectroscopy (CEMS) is an interesting technique for studying reactions taking place on, or near, the surface of materials which contain iron in one form or another [9]. The decay processes following the resonant excitation of a Fe nucleus are shown in Fig. 8. The number of photons and electrons produced following the capture of 100 resonant 14.4 keV recoil-free y-ray photons is indicated in this figure. 

With the scattering technique, it is possible to detect y rays (back-scattering y-ray Mossbauer spectroscopy), X-rays (X-ray Mossbauer spectroscopy), or conversion electrons (conversion electron Mossbauer spectroscopy, CEMS), which are characterized hy different escape depths. Consequently, by the detection of different forms of back-scattered radiation, the surface of the samples can be investigated at different depths. An important advantage of the scattering Mossbauer technique is that it provides a nondestructive way of material testing. 

Chang and Wei (1990) used electrochemical and conversion-electron Mossbauer spectroscopy (CEMS) methods to study the corrosion behavior of electrodeposited Fe tZnj, wherej = 0.15-0.85,on 1010 steel immersed in a 0.1% NaCl solution at room temperature for 2 weeks. The corrosion rate measured by metal weight loss and electrochemical methods revealed that the Feo,25Zno,75 specimen was more corrosion resistant than the others. CEMS analysis showed that the corrosion product of the pure 1010 steel and the higher iron-containing Fe-Zn alloys on steel in 0.10% NaCl solution is (3-FeOOH. 

---