The Mossbauer Effect

The Mossbauer effect, also known as the recoilless nucleus resonance absorption of gamma rays, was first discovered and explained by Rudolf L. Mossbauer in 1957, while he was working on his doctoral thesis at Heidelberg. In the following years, more thorough understanding concerning principles, methods, and applications has been achieved by a myriad of research. 

Edited by Chunying Chen, Zhifang Chai and Yuxi Gao 

X is the wavelength of the gamma ray, Ig and Ig are nucleus spin quantum numbers of the excited and the ground state, respectively, and a is the internal conversion coefficient.  

Ideally, the absorption profile would overlap the emission profile however, due to the wave-particle duality of the gamma quantum, the excited free nucleus which emits the gamma quantum suffers a recoil hence, the energy of the emitted quanta Ey becomes 

Due to recoil the position of emission line shifts to smaller energy and, similarly, the absorption line shifts to a larger energy as a result of the same amount of energy lost during absorption, which requires that the energy of the absorbed quanta Ey should be 

This can be much greater than the natural broadening which comes from the finite lifetime of nuclear processes that give rise to the y-ray emission in the first place. When the emission of y-rays takes place within a solid, the presence of phonons can change things significantly. Specifically, much of the photon momentum can be carried by phonons with low energy, near the center of the BZ, and the emission 

The transition probability between initial ( )) and final ((/I) states is given by 

We are now in a position to calculate the probability of recoil-less emission in the ground state of the crystal, at r = 0, so that all oscillators are in their ground state before and after the transition. This probability is given by 

But there is a simple relation between 1 /2a and the average value of Q] in the 

Now we will assume that all the phonon modes I have the same average (0q ) which, following the same steps as in Eq. (6.99), leads to 

In this chapter, we will first describe what the Mossbauer effect is, and then explain why it can only be observed in the solid state and in a limited number of elements. Next, we discuss the so-called hyperfine interactions between the nucleus and its environment, which make the technique so informative. After some remarks on spectral interpretation, we will pass systematically through a number of examples which illustrate the type of information that Mossbauer spectroscopy yields about catalysts. 

Let us consider the following experiment (see Fig. 5.1). Suppose we have two identical atoms, one with its nucleus in the excited state, and the other with its nucleus in the ground state. The excited nucleus decays to the ground state by emitting a photon with energy of typically some 10 to 100 keV. This photon falls on the nucleus of the second atom, which is in the ground state. The question is, 

The conclusion of all this is the following. If we place our two atoms in a lattice and perform the experiment under conditions where recoil energy of the photon emission and absorption are significantly smaller than the energy of the lattice vibrations, a fraction of the photons emitted by the source nucleus will be absorbed by the nucleus in the absorber. This is the Mossbauer effect, named after Rudolf L. Mossbauer, who discovered it in 1957 and subsequently received the Nobel Prize in 1961 [10]. 

The intensity of the Mossbauer effect is determined by the recoil-free fraction, or f factor, which can be considered as a kind of efficiency. It is determined by the lattice vibrations of the solid to which the nucleus belongs, the mass of the nucleus, and the photon energy, E0 and is given by  

Er is the recoil energy of the nucleus upon emission of a y-quantum k is Boltzmann s constant  

The most prominent nuclide for Mossbauer speetroseopy is Fe, with over 90% of the 50000 or so publications that make use of this technique referring to this isotope. Reports range from compound identification in complex chemistry, through the bioinorganie chemistry of iron, to industrial studies of corrosion of metals and even water detection on Mars (in water-containing iron minerals [1,2]). However, the Mossbauer Effect is applicable to more than forty elements, as indicated in the Periodic Table inside the front cover. 

However, this simple picture holds only for the hypothetical case in which the two nuclei are in the same chemical surroundings and are held in fixed positions. In reality, the emitted 7 photon has momentum E lc, which leads to recoil of the emitting nucleus due to conservation of momentum. 

Structural Methods in Molecular Inorganic Chemistry, First Edition. David W. H. Rankin, Norbert W. Mitzel and Carole A. Morrison. 2013 John Wiley Sons, Ltd. Published 2013 by John Wiley Sons, Ltd. 

The recoil effect, (a) Schematic diagram of an atom recoiled by emission of a y quantum, which leads to a reduced -y energy Ey. (b) A large recoil effect, resulting in absorption and emission energies that do not coincide, (c) A small recoil effect, which results in absorption and emission energies that do coincide. 

For free atoms, or atoms in molecules, these recoil effects are typically about five to six orders of magnitude larger than the natural line width, and so there is no possibility of a resonance phenomenon (but having said that, under very special conditions resonances for gases have in fact been achieved). Even in the liquid state, atom or molecule movements are generally too large. However, when both emitter and absorber atoms are bound in solid samples recoilless nuclear resonance absorption becomes observable. But even here the atomic movements due to lattice and molecular vibrations lead to broadening of emission and absorption lines. We must therefore expect temperature-dependent effects in Mossbauer spectroscopy. 

Multi-channel analyzer -(+ amplifiers, power units, etc.) 

For a given source material we can simplify this expression to  

The Mossbauer nucleus can, therefore, be used as an observer or probe to obtain information about site symmetries and field gradients, much as in NQR spectroscopy. However, whereas in NQR transitions are observed directly between ground-state sublevels, in Mossbauer spectroscopy transitions between sublevels of the excited state and the ground- 

As noted in Section 2.5.2, a nucleus with a spin I has a nuclear magnetic moment (p ) given by  

if a magnetic field is applied to this nucleus in some way, there is an interaction between the field and the moment Pn such that (2/ -r 1) energy levels result with energies  

---

N. R. Smyrl and Gleb Mamontov The Mossbauer Effect in Supported... 

The Mossbauer effect, discovered by Rudolf L. Mossbauer in 1957, can in short be described as the recoil-free emission and resonant absorption of gamma radiation by nuclei. In the case of iron, the source consists of Co, which decays with a half-life of 270 days to an excited state of Fe (natural abundance in iron 2%). The latter, in turn, decays rapidly to the first excited state of this isotope. The final decay generates a 14.4 keV photon and a very narrow natural linewidth of the order of nano eV. 

The Mossbauer effect can only be detected in the solid state because the absorption and emission events must occur without energy losses due to recoil effects. The fraction of the absorption and emission events without exchange of recoil energy is called the recoilless fraction, f. It depends on temperature and on the energy of the lattice vibrations /is high for a rigid lattice, but low for surface atoms. 

Up to the present time, the Mossbauer effect has been observed with nearly 100 nuclear transitions in about 80 nuclides distributed over 43 elements (cf. Fig. 1.1). Of course, as with many other spectroscopic methods, not all of these transitions are suitable for actual studies, for reasons which we shall discuss later. Nearly 20 elements have proved to be suitable for practical applications. It is the purpose of the present book to deal only with Mossbauer active transition elements (Fe, Ni, Zn, Tc, Ru, Hf, Ta, W, (Re), Os, Ir, Pt, Au, Hg). A great deal of space will be devoted to the spectroscopy of Fe, which is by far the most extensively used Mossbauer nuclide of all. We will not discuss the many thousands of reports on Fe... 

Number of isotopes in which - the Mossbauer effect has been observed... 

Fig. 1.1 Periodic table of the elements those in which the Mossbauer effect has been observed are marked appropriately. (Taken from the 1974 issue of [10])...

In order to understand the Mossbauer effect and the importance of recoUless emission and absorption, one has to consider a few factors that are mainly related to the fact that the quantum energy of the y-radiation used for Mossbauer spectroscopy (Eo K, 10-100 keV) is much higher than the typical energies encountered, for instance, in optical spectroscopy (1-10 eV). Although the absolute widths of the... 

The true energy scale of the y-spectrum in units of keV usually cannot be derived directly from the pulse height spectrum because the overall amplification of the detection system is not known. Therefore, the y-lines eventually have to be identified by trial and error when a new system is set up by checking for the occurrence of the Mossbauer effect. 

So far, we have discussed only the detection of y-rays transmitted through the Mossbauer absorber. However, the Mossbauer effect can also be established by recording scattered radiation that is emitted by the absorber nuclei upon de-excitation after resonant y-absorption. The decay of the excited nuclear state proceeds for Fe predominantly by internal conversion and emission of a conversion electron from the K-shell ( 90%). This event is followed by the emission of an additional (mostly Ka) X-ray or an Auger electron when the vacancy in the K shell is filled again. Alternatively, the direct transition of the resonantly excited nucleus causes re-emission of a y-photon (14.4 keV). 

In a conventional Fe Mossbauer experiment with a powder sample, one would observe a so-called quadrupole doublet with two resonance lines of equal intensities. The separation of the lines, as given by (4.36), represents the quadrupole splitting The parameter Afg is of immense importance for chemical applications of the Mossbauer effect. It provides information about bond properties and local symmetry of the iron site. Since the quadrupole interaction does not alter the mean energy of the nuclear ground and excited states, the isomer shift S can also be derived from the spectrum it is given by the shift of the center of the quadrupole spectrum from zero velocity. 

Pure nuclear magnetic hyperfine interaction without electric quadrupole interaction is rarely encountered in chemical applications of the Mossbauer effect. Metallic iron is an exception. Quite frequently, a nuclear state is perturbed simultaneously by... 

The underlying physics and analysis of Mossbauer spectra have been explained in detail in Chap. 4. In that chapter, the principles of how a spectrum is parameterized in terms of spin-Hamiltonian (SH) parameters and the physical origin of these SH parameters have been clarified. Many Mossbauer studies, mainly for Fe, have been performed and there is a large body of experimental data concerning electric-and magnetic-hyperfine interactions that is accessible through the Mossbauer Effect Database. 

Mprup, S. In International Conference on the Applications of the Mossbauer Effect Proceedings, Science Academy, New Delhi, 1982, p. 91... 

The previous chapters are exclusively devoted to the measurements and interpretation of Fe spectra of various iron-containing systems. Iron is, by far, the most extensively explored element in the field of chemistry compared with all other Mdssbauer-active elements because the Mossbauer effect of Fe is very easy to observe and the spectra are, in general, well resolved and they reflect important information about bonding and structural properties. Besides iron, there are a good number of other transition metals suitable for Mossbauer spectroscopy which is, however, less extensively studied because of technical and/or spectral resolution problems. In recent years, many of these difficulties have been overcome, and we shall see in the following sections a good deal of successful Mossbauer spectroscopy that has been performed on compounds of... 

Kistner et al. [109] were the first to observe the Mossbauer effect in Ru. Kistner also reported the Mossbauer spectroscopy study of ruthenium compounds and alloys [110]. 

The Mossbauer effect has been observed in the following four hafnium isotopes I76jjf I77jjf I78jjf isojjf jj lear transitions of 88.36, 112.97, 93.2,... 

There are two y-transitions in Pt amenable to the Mossbauer effect - the 130 keV transition between the 5/2 excited state and the 1/2 ground state and the 99 keV transition between the first excited 3/2 state and the ground state. Figure 7.70 shows the simplified decay scheme of Pt. The relevant nuclear data may be taken from Table 7.1 (at the end of the book). 

It is much more difficult to observe the Mossbauer effect with the 130 keV transition than with the 99 keV transition because of the relatively high transition energy and the low transition probability of 130 keV transition, and thus the small cross section for resonance absorption. Therefore, most of the Mossbauer work with Pt, published so far, has been performed using the 99 keV transition. Unfortunately, its line width is about five times larger than that of the 130 keV transition, and hyperfine interactions in most cases are poorly resolved. However, isomer shifts in the order of one-tenth of the line width and magnetic dipole interaction, which manifests itself only in line broadening, may be extracted reliably from Pt (99 keV) spectra. 

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 Mossbauer effect as a spectroscopic method probes transitions within an atom s nucleus and therefore requires a nucleus with low-lying excited states. The effect has been observed for 43 elements. For applications in bioinorganic chemistry, the 57Fe nucleus has the greatest relevance and the focus will be exclusively on this nucleus here. Mossbauer spectroscopy requires (a) the emission of y rays from... 

Upon cooling, see Fig. 2, the observed Mossbauer spectra of [Fe(HB (pz)3)2] are very different from those observed upon the initial heating. Indeed, the dramatic difference is immediately apparent through a comparison of the 380 and 400 K spectra shown in Fig. 2 for the initial heating and initial cooling. The spectra shown in this figure are very typical of rapid relaxation on the Mossbauer effect time scale between the high-spin and the low-spin iron(II) states. As a consequence, all of the Mossbauer spectra of [Fe(HB(pz)3)2] obtained above 295 K were fitted with a relaxation model de-... 

When I obtained my Masters Degree in experimental physics at the Free University in Amsterdam in 1978,1 was totally unaware that as interesting an area as catalysis, with so many challenges for the physicist, existed. I am particularly grateful to Adri van der Kraan and Nick Delgass who introduced me, via the Mossbauer Effect in iron catalysts, to the field of catalysis. My Ph.D. advisors at Delft, Adri van der Kraan, Jan van Loef and Vladimir Ponec (Leiden), together with Roel Prins from Eindhoven, stimulated and helped me to pursue a career in catalysis, now about seventeen years ago. 

Mossbauer spectroscopy is one of the techniques that is relatively little used in catalysis. Nevertheless, it has yielded very useful information on a number of important catalysts, such as the iron catalyst for Fischer-Tropsch and ammonia synthesis, and the cobalt-molybdenum catalyst for hydrodesulfurization reactions. The technique is limited to those elements that exhibit the Mossbauer effect. Iron, tin, iridium, ruthenium, antimony, platinum and gold are the ones relevant for catalysis. Through the Mossbauer effect in iron, one can also obtain information on the state of cobalt. Mossbauer spectroscopy provides valuable information on oxidation states, magnetic fields, lattice symmetry and lattice vibrations. Several books on Mossbauer spectroscopy [1-3] and reviews on the application of the technique on catalysts [4—8] are available. 

---