The Mossbauer Transmission Spectrum

In the following, we consider the shape and the width of the Mdssbauer velocity spectrum in more detail. We assume that the source is moving with velocity u, and the emission line is an unsplit Lorentzian according to (2.2) with natural width E. If we denote the total number of y-quanta emitted by the source per time unit toward the detector by Nq, the number N E)AE of recoU-free emitted y-rays with energy y in the range to -f dE is given by ([1] in Chap. 1) 

In a Mdssbauer transmission experiment, the absorber containing the stable Mdssbauer isotope is placed between the source and the detector (cf. Fig. 2.6). For the absorber, we assume the same mean energy q between nuclear excited and ground states as for the source, but with an additional intrinsic shift A due to chemical influence. The absorption Une, or resonant absorption cross-section cr( ), has the same Lorentzian shape as the emission line and if we assume also the same half width , cr( ) can be expressed as ([1] in Chap. 1) 

The absorption line is normalized to the maximum cross section (Tq at resonance, E = Eo+ AE, which depends on the y-energy Ey, the spins 4 and 4 of the 

We are interested in the transmission of y-quanta through the absorber as a function of the Doppler velocity. The radiation is attenuated by resonant absorption, in as much as emission and absorption lines are overlapping, but also by mass absorption due to photo effect and Compton scattering. Therefore, the number Tt E2)AE of recoilless y-quanta with energies EXo E + AE traversing the absorber is given by 

The total number of recoU-free photons arriving at the detector per time unit is then obtained by integration over energy 

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Fig. 2.5 The Mossbauer transmission spectrum of a resonance line and the corresponding derivative 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. 2.6 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...

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. 3.16 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 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... 

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]. 

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. 

Mossbauer spectroscopy is based on the phenomenon of recoil-free resonant absorption of y rays by atomic nuclei, and the spectrum reflects the perturbation induced in the nuclear levels due to the interaction of the Mossbauer atom with its chemical environment. The Mossbauer elements, iron and tin, can be used conveniently as in situ probes in environmental and geochemical applications. Since Mossbauer spectroscopy has now become relatively familiar to chemists, I present here only a brief description of typical experimental techniques used in Mossbauer measurements. There are two types of Mossbauer measurements transmission method and scattering method. 

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. 

We used Co(Rh) as the Mossbauer source for transmission mode spectroscopy. Pieces of the material were powdered and sandwiched between plastic disks. The Mossbauer spectrum for pristine BaFe2As2 shows a single line at room temperature and a magnetically split sextet at low temperatures, as expected (Fig. 26.2). On the other hand, Co-and Ni-doped materials exhibit single lines at 78 K and ambient temperature. The observed Mossbauer parameters for BaFe2As2 and Ni-doped material, obtained by the least-squares fitting procedure, are in concordance with those... 

The Mossbauer spectrum, a plot of the relative transmission as a function of Doppler velocity, shows maximum resonance and therefore minimum relative transmission at relative velocities where emission and absorption lines overlap ideally (cf. Fig. 4). At high positive or negative velocities the overlap of emission and absorption lines is negligible, the resonance effect being practically zero, i.e., the relative transmission yields the base line. 

When the absorber is thin, the observed transmission Mossbauer spectrum is a superposition of several Lorentz functions. 

The Mossbauer spectrum is a record of the transmission of resonant y rays through an absorber as a function of the Doppler velocity with respect to the source. It is therefore quite simply a record of transmission as a function of the energy of the incident radiation (Figure 4.45). 

A typical device for accumulating the Mossbauer spectrum is the multichannel analyser, where the count rate is a function of a definite value of the Doppler velocity. The count rate is normalized relative to the off-resonance count rate. Hence, for transmission-mode Mossbauer spectroscopy relative intensities are always less than unity (or 100%). In Mossbauer scattering experiments relative intensities always exceed 100% and can reach several hundred percent in the case of electron detection from samples with a high abundance of the resonant isotope. It is most often that the -y ,ax value corresponds to the first channel and the +y ,ax value to the last channel. The quality of a Mossbauer spectrometer is determined by how accurately the modulation of the y-quanta energy follows the chosen mode of movement. 

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 ... 

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...

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