Conversion electron Mossbauer spectroscopy - CEMS

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. 

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]. 

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. 

Finally, we should also mention the technique of Conversion Electron Mossbauer Spectroscopy (CEMS) [25], which uses the backscattered electrons from a surface produced by alternative decay processes of the excited nuclear state. This is particularly well suited to studying surfaces. 

As was mentioned in Section 3.1, conversion electron Mossbauer spectroscopy (CEMS) provides an advantageous means of studying surface structure as it affords much greater sensitivity than measurements in the transmission... 

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