Single-line absorber

Figure 4.16. To cover all possible transitions in the absorbing nucleus, the energy of the source radiation is modulated by using the Doppler effect, such that the emitted radiation has an energy E v) = Eo(l + vjc). For Fe the required velocities fall in the range (1 to t-1 cm s k In Mossbauer emission spectroscopy, the sample under investigation is the source, and a single line absorber is...

In Mossbauer emission spectroscopy one prepares the catalyst with the radioactive source element (e.g. Co) and uses a suitable moving single-line absorber of Fe to record the spectra. In this way one can also study Co-containing catalysts, although strictly speaking the information concerns the iron in the catalyst that forms by the Co —> Fe decay process. 

Thus, the experimental Mossbauer spectmm of a thin single-line absorber is a Lorentzian line, with full-width at half maximum twice the natural Une width of the separate emission and absorption lines Texp = 2E. 

Most Mossbauer experiments are currently performed with commercially available radioactive sources. For some applications, however, a so-called source experiment may be useful, in which the sample is labeled with the radioactive parent-isotope of the Mossbauer nucleus such as Co. The y-radiation of the radioactive sample is then analyzed by moving a single-line absorber for Doppler modulation in front of the detector. 

The nuclear decay of radioactive atoms embedded in a host is known to lead to various chemical and physical after effects such as redox processes, bond rupture, and the formation of metastable states [46], A very successful way of investigating such after effects in solid material exploits the Mossbauer effect and has been termed Mossbauer Emission Spectroscopy (MES) or Mossbauer source experiments [47, 48]. For instance, the electron capture (EC) decay of Co to Fe, denoted Co(EC) Fe, in cobalt- or iron-containing compormds has been widely explored. In such MES experiments, the compormd tmder study is usually labeled with Co and then used as the Mossbauer source versus a single-line absorber material such as K4[Fe(CN)6]. The recorded spectrum yields information on the chemical state of the nucleogenic Fe at ca. 10 s, which is approximately the lifetime of the 14.4 keV metastable nuclear state of Fe after nuclear decay. 

The most direct information on the state of cobalt has come from Mossbauer spectroscopy, applied in the emission mode. As explained in Chapter 5, such experiments are done with catalysts that contain the radioactive isotope 57Co as the source and a moving single-line absorber. Great advantages of this method are that the Co-Mo catalyst can be investigated under in situ conditions and the spectrum of cobalt can be correlated to the activity of the catalyst. One needs to be careful, however, because the Mossbauer spectrum one obtains is strictly speaking not that of cobalt, but that of its decay product, iron. The safest way to go is therefore to compare the spectra of the Co-Mo catalysts with those of model compounds for which the state of cobalt is known. This was the approach taken... 

The single-line absorber is generally potassium (or sodium) nitroprusside, K4Fe(CN)g,3H20, enriched with Fe. 

A 1 at. % alloy of in iron (source) with a HfN single-line absorber shows a partially resolved five-line magnetic spectrum [33]. The known excited-state magnetic moment for Hf of = 0-71 n.m. leads to values for the internal magnetic field of 606 kG at 4-2 K and 547 kG at 78 K. By contrast, a 1 at. % alloy of i ° "Hf in iron for which /[Pg.506]

The data described above were obtained with a Te source which decays to Usually, the source is cubic ZnTe, which gives a single line. In an ingenious experiment, single line absorbers were used together with Te02 or Te(N03)2 sources, which gave eQ, 29 values for iodine atoms in... 

An interesting application of Mossbauer emission spectroscopy is illustrated by the study of Co(pyridine)2Cl2 which was used as a source against a single-line absorber. The resulting emission spectra clearly reveal the presence of the phase transition and indicate that this polymeric material is able to withstand the damage associated with the electron capture decay of the Co. 

---