Emission Mossbauer functions

Fig. 2.8 (a) Fractional absorption of a Mossbauer absorption line as function of the effective absorber thickness t. (b) The depth of the spectrum is determined by fs. The width for thin absorbers, t 1, is twice the natural line width F of the separate emission and absorption lines (see (2.30)). AE is the shift of the absorption line relative to the emission line due to chemical influence... 

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...

Fig. 8.21 (a, lifi) Pe Mossbauer absorption spectra of [ Fe/Co(phen)3](C104)2 as a function of temperature versus Co/Rh as source (which was kept at 295 K). (b, right) Time-integral Fe MossbauCT emission spectra of [ Co/Co(phen)3](004)2 source as a function of temperature... 

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]. 

Recently a new technique has come into use which avoids radioactivity as well as after-effects conversion electron spectroscopy This method uses the fact that in most Mossbauer transitions not only 7-rays but also conversion electrons are emitted. In the case of Co the electron conversion is the major decay mode. Thus, instead of measuring the 7-rays, absorbed or reflected from an absorber, one measures the emission of electrons from the absorber as a function of the velocity... 

A Mossbauer spectrometer consists of a radioactive Co source on a transducer that continuously scans the desired velocity range, an absorber consisting of the catalyst and a detector to measure the intensity of the gamma radiation transmitted by the absorber as a function of the source velocity. This is the common mode of operation, called Mossbauer absorption spectroscopy, sometimes abbreviated as MAS. It is also possible to fix the Co containing source and move a single-line Fe absorber, in order to investigate Co-containing catalysts. This technique, called Mossbauer emission spectroscopy (MES), has successfully been applied to study Co-Mo hydrodesulphurization catalysts [42]. 

Let both the emission and absorption line be centered at Eq and have the same natural line width F = Ff, = F. The distribution of the ground-state Mbssbauer atoms in question (e.g., Fe) is taken to be uniform in both the source and the absorber. The distribution of the parent atoms (e.g., Co) producing the excited Mossbauer atoms ( Fig. 25.7) (and thus the y photons) is described by the density function p x) along the x-axis (see Fig. 25.6 for the notation). In the derivation of the peak shape of the Mossbauer spectrum, nonresonant and resonant absorption processes are to be considered in both the source and the absorber as illustrated schematically in Fig. 25.6. 

In order to measure a Mossbauer spectrum one has to detect recoilless absorption or emission of a selected y radiation as a function of the Doppler velocity of the sample and a reference material relative to each other. In the most common case of transmission geometry, absorption of the y rays is measured, and the reference material is a standard source, which is moved. Thus, the most important components of a Mossbauer spectrometer are a Doppler velocity drive system and an energy selective y detection chain with appropriate recording system. 

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. 

Now the pSK experiment involves the determination of the spin polarisation of the muon at the time of its decay by observing the direction of emission of the positron. The situation is therefore analogous to the PAC case. Since the spin polarisation of the muon is known precisely at the time of its creation, any change in this produced as a result of the magnetic hyperfine interaction between the muon and its environment can be detected by measuring the direction of emission of the positron. The PAC and the pSK experiment are both performed as a function of time whereas the Mossbauer spectra are recorded as a function of the hyperfine interaction frequency (or enei gy). 

A controversy arose about the interpretation of these two lines. As can be seen in Fig. 6.10, in Co/ Fe emission experiments the line intensities varied as a function of implantation fluence. In Fe(Si) absorber experiments, after implantation at fluences higher than to the highest fluences used in Fig. 6.10, a fairly symmetric doublet was observed. In the in-beam Mossbauer experiments, at implantation fluences lower than the lowest fluences shown in Fig. 6.10, an asymmetric doublet was observed. Was there a common interpretation possible ... 

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