Mossbauer transmission spectrum

Fig. A.l Contributions to a Mossbauer transmission spectrum. N, is the nonresonant background from scattered high-energy y-radiation and X-ray fluorescence in the source and the absorber...

Fig. 1.7 A Mossbauer transmission spectrum produced by Doppler scanning, and the factors influencing it.

Fig. 2.5 The Mossbauer transmission spectrum of a resonance line and the corresponding derivative spectrum.

Fig. 2.6 Schematic illustration of a Mossbauer transmission experiment in five steps. The Absorption bars indicate the strength of recoilless nuclear resonant absorption as determined by the overlap of emission and absorption lines when the emission line is shifted by Doppler modulation (velocities Uj,. .., 1)5). The transmission spectrum T v) is usually normalized to the transmission T oo) observed for v oo by dividing T(v)IT(oo). Experimental details are found in Chap. 3...

Fig. 3.16 Schematic drawing of the MIMOS II Mossbauer spectrometer. The position of the loudspeaker type velocity transducer to which both the reference and main Co/Rh sources are attached is shown. The room temperature transmission spectrum for a prototype internal reference standard shows the peaks corresponding to hematite (a-Fe203), a-Fe, and magnetite (Fe304). The internal reference standards for MIMOS II flight units are hematite, magnetite, and metallic iron. The backscatter spectrum for magnetite (from the external CCT (Compositional Calibration Target) on the rover) is also shown...

In addition to the four detectors used to detect backscattered radiation from the sample, there is a fifth detector to measure the transmission spectrum of the reference absorber (a- Fe, a- Fe203, Fc304 see Fig. 3.16). Sample and reference spectra are recorded simultaneously, and the known temperature dependence of the Mossbauer parameters of the reference absorber can be used to give a measurement of the average temperature inside the SH, providing a redundancy to measurements made with the internal temperature sensor (see Sect. 3.3.4). 

Figure 2 shows the conversion electron and y-ray transmission MOssbauer spectra of Eu2(C2O4)3 IOH2O after UV-irradiation with a low-pressure mercury lamp. Although the formation of Eu(II) was not detected in the y-ray transmission spectrum (Fig. 2b), a Eu(II) peak showed in the conversion electron spectrum (Fig. 2a), obviously indicating that Eu(III) was reduced to Eu(II) by UV-irradiation. This was also confirmed by ESR measurements. 

We applied this technique to the study of photolytic reactions in solid potassium tris(oxalato)ferrate(111) (12). Figure 4 compares the Mflssbauer spectra at 293 K of photoirradiated Kj[5 Fe(C204)3] 3H2O obtained by three types of MOssbauer measurements y-ray transmission spectrum, integral conversion electron spectrum(with the He-CH4 gas flow proportional counter), and depth-resolved conversion electron spectrum with the above spectrometer for 7.2-keV electrons. The spectra in Figs. 4a, b, and c charac-... 

This is the basic form of a Mossbauer spectrum a plot of transmission versus a series of Doppler velocities between source and absorber (i.e. versus the eflfective y-ray energy), the absorption line being Lorentzian in shape with a width at half-height corresponding to IF. The various factors influencing the transmission spectrum are illustrated schematically in Fig. 1.7, which is self-explanatory. 

In Mossbauer spectroscopy, the peak-shape of the transmission spectrum (measured with a thin absorber, see O Fig. 9.22) is described as the convolution of the Lorentzians characteristic of the source and the absorber. According to the addition theorem, the result of such a convolution will be another Lorentzian with a halfWidth equal to the sum of both halfwidths. In other words, in this case, the FWHM is twice of the natural linewidth F. 

FIGURE 14. In situ Mossbauer transmission spectra for 11-nm Fe film in borate buffer (pH 8.4) at -0.4 V (metallic Fe) (curve A) and after passivation at + 1.3 V versus a-Pd/H (curve B). Curves C and D were obtained ex situ after drying the film and in situ (+1.3 V) after reintroducing the passive film in the same electrolyte. Spectrum E was recorded at + 1.3 V after two reduction-passivation cycles. See text for additional details. 

The expression is known as the transmission integral in the actual formulation, which is valid for ideal thin sources without self-absorption and homogeneous absorbers assuming equal widths F for source and absorber [9]. The transmission integral describes the experimental Mossbauer spectrum as a convolution of the source emission Une N(E,o) and the absorber response exp —cr( )/abs M - The substitution of N E,d) and cr( ) from (2.19) and (2.20) yields in detail ... 

Since the actual motion of the Mossbauer drive, as for any frequency transmission system, can show phase shifts relative to the reference signal, the ideal folding point (FP) of the raw data in terms of channel numbers may be displaced from the center at channel number (N — l)/2 (= 255.5 in the example seen earlier). The folding routine must take this into account. Phase shift and FP depend on the settings of the feedback loop in the drive control unit. Therefore, any change of the spectrometer velocity tuning requires the recording of a new calibration spectrum. 

Fig. 3.19 Schematic illustration of the measurement geometry for Mossbauer spectrometers. In transmission geometry, the absorber (sample) is between the nuclear source of 14.4 keV y-rays (normally Co/Rh) and the detector. The peaks are negative features and the absorber should be thin with respect to absorption of the y-rays to minimize nonlinear effects. In emission (backscatter) Mossbauer spectroscopy, the radiation source and detector are on the same side of the sample. The peaks are positive features, corresponding to recoilless emission of 14.4 keV y-rays and conversion X-rays and electrons. For both measurement geometries Mossbauer spectra are counts per channel as a function of the Doppler velocity (normally in units of mm s relative to the mid-point of the spectrum of a-Fe in the case of Fe Mossbauer spectroscopy). MIMOS II operates in backscattering geometry circle), but the internal reference channel works in transmission mode...

Figure 3 shows the Mossbauer spectra for an alloy of 75% iron and 25% rhodium after two different heat treatments. Since absorption rather than transmission is plotted, these curves are right side up. The upper spectrum is taken from an alloy which was annealed in the low temperature field (cesium chloride structure), and there are two six-hne patterns... 

Doppler velocity may provide a Mossbauer spectrum with a large increase in the signal to noise ratio compared to that obtained in the transmission mode. The type of radiation used to generate the scattered Mossbauer spectrum depends on the internal conversion coefficient a a large value of a, which favors the emission of X rays by the Mdssbauer isotope, makes X-ray detection appropriate, while a small value of a favors y-ray detection. 

Figure 5.4 provides a schematic diagram of a Mossbauer experiment in transmission mode with a moving single-line source and the absorbing sample in fixed position. A Mossbauer spectrum is a plot of the y-ray intensity transmitted by the sample, against the velocity v of the source. The latter is related to the... 

Figure 4.55 shows a Mossbauer spectrum obtained with a spectrometer similar to that shown in Figure 4.54, that is, in transmission geometry. 

Fig. 1. Basic apparatus for measuring a Mossbauer spectrum in transmission mode.

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]. 

Mossbauer experiments were performed in transmission mode using a calcium stannate source. Spectra were deconvoluted using standard methods to separate contributions from Tin (II) and Tin (IV) peaks. The Mfissbauer spectrum of the cured rubber (see Figure 1) shows the Tin (II) and Tin (IV) oxidation states, with the Tin (IV) species representing approximately 67% of the total tin signal. Overall, the IS/QS values suggest that the Tin (IV) species formed in the rubber is most probably Tin (IV) oxide, Sn02. The presence of some residual unreacted Tin (II) catalyst within the cured rubber is clearly evident. 

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