In 1958, a German physicist, Rudolf Mossbauer, invented Mossbauer spectroscopy. It is a versatile technique that provides sensitive information about the structural, magnetic, chemical and time-dependent properties of a solid substance.
This article explains the fundamentals of Mössbauer spectroscopy, including the basic working principle, instrumentation, important applications, advantages and disadvantages of Mössbauer spectroscopy.
So, without any further ado, dive deeper into the article and learn everything you need to know about Mossbauer spectroscopy.
Mossbauer spectroscopy –Historical perspective
As mentioned above, Mossbauer spectroscopy was introduced in 1958. It was a success primarily because of the discovery of the Mössbauer effect.
The Mossbauer effect was initially documented as ‘recoil-free gamma-ray resonance absorption’’ by Rudolf Mössbauer, which led to him receiving the 1961 Nobel Prize in Physics.
Therefore, Mössbauer spectroscopy is also known as Nuclear Gamma Resonance Spectroscopy and its based on the Mössbauer Effect.
What is Mössbauer spectroscopy? – Definition
Mössbauer spectroscopy primarily involves the interaction of high-energy gamma rays with the atomic nuclei to obtain information about the chemical environment and oxidation states of the atoms.
Mössbauer spectroscopy is an analytical spectroscopic technique in which a solid sample is exposed to a beam of gamma radiation. Different chemical environments present around the atomic nuclei in the solid sample lead to nuclear energy shifts in different ways. The intensity of gamma rays absorbed and consequently emitted by the sample is detected.
When the atoms of an element are exposed to gamma rays (wavelength < 100 pm), absorption or emission of gamma rays causes the atomic nuclei to undergo certain energy level transitions. The atom’s surrounding electric and magnetic environment strongly influences the change in energy levels of the nucleus of a particular atom.
This resulting change is then translated to get information about the chemical environment of the atom by using resonance fluorescence. This information can be limited owing to hyperfine interactions and recoiling of the atomic nuclei.
What is Mössbauer effect?
The Mössbauer effect ensures the resonance and recoil-free emission and absorption of gamma radiations by the atomic nuclei present in a solid sample.
The Mössbauer effect shows a narrow resonance for nuclear gamma radiation absorption and emission.
Gamma rays emitted from excited nuclei should have energy equivalent to the energy difference between the excited and ground states. Contrarily, it is observed that gamma rays contain energy less than this excited and ground state energy difference. The energy reduction is logically translated as the recoiling of the atomic nucleus at the time of the ejection of gamma rays.
The emitted gamma rays possess lower energy than the nuclear transition. However, to be resonantly absorbed, it should be higher than the nuclear transition energy. Thus, to achieve resonance, the energy loss due to nuclear recoil must be overcome at all means.
Therefore, in order to increase the energy of the gamma rays, the source is vibrated with a particular velocity. The absorption of gamma rays is then achieved by changing gamma rays’ energy to a wide range, also known as the Doppler effect. Moreover, this is achieved by reducing the recoiling effect of the source nucleus as well as by providing kinetic energy to the receiver nucleus.
Hence, the gamma-ray wavelength provided by the source is tuned via the Doppler Effect. Nuclear resonance is achieved as the particular wavelength is equivalent to the wavelength of emitted gamma rays.
What is the basic working principle of Mössbauer spectroscopy?
Mössbauer spectroscopy is based on the fundamental principle of the ‘conservation of momentum’’. It requires a free nucleus to recoil during the absorption or emission of gamma radiation.
When a nucleus is at rest, the energy of the gamma-ray is usually less than the natural energy of nuclear transition. Contrarily, for a stationary nucleus to absorb a gamma ray, the energy provided must be comparatively higher than its natural energy. However, in both cases, some energy is lost in recoil.
Therefore, nuclear resonance is hardly observable with a free nucleus as the energy shift is too high to significantly overlap the emission and absorption spectra. In contrast, the nuclei in solid crystals are not free to recoil as they are tightly held with one another.
Steps to perform Mössbauer spectroscopy
- The solid sample is exposed to high-energy gammas rays.
- The intensity of transmitted gamma radiations is measured through detectors.
- If chemical environment of the source and receiver nuclei is the same, the transition energy would be exactly equal. In turn, resonant absorption will be observed, having both materials at rest.
- If the chemical environment of the source and receiver nuclei differs, it results in a shift in nuclear energy levels.
- Doppler Effect brings both (source and receiver) nuclei into resonance. To produce Doppler Effect, the source nuclei are accelerated via a wide range of velocities using a linear motor and hence the resonance is achieved.
- While in receiver nuclei, the energy levels are modified by their environment, typically in three main ways.
Let’s have a look at all the above, one by one:
An isomeric shift occurs in result of replacing one nuclear isomer with another, which shifts the whole spectrum either positively or negatively and sets the center of the spectrum. This shift provides valuable information about the atom’s nuclear structure as well as the chemical or physical environment around the nucleus.
Moreover, it is also useful to determine ligand bonding states, the effect of electronic shielding and the electron-withdrawing power of electronegative chemical functional groups, etc.
Quadrupole splitting occurs when non-radially symmetrical nuclei are exposed to an external electric field gradient, consequently producing a double in the Mössbauer spectrum by splitting a peak into two. Thus, quadrupole splitting determines how the chemical environment around a nucleus gets affected by the externally applied electric field gradient.
The nuclear spin moment experiences a dipolar interaction in the presence of an external magnetic field. There are numerous magnetic field sources to be experienced by the targeted nucleus predominantly due to the atom’s own partially filled shells. The nucleus with spin “I” splits into 2I + 1 sub-energy levels under the influence of an external magnetic field.
These three interactions, alone or in combination, provide valuable information about a nuclear system. These three thus govern the primary characteristics of different Mössbauer spectra.
Mössbauer spectroscopy -Instrumentation and equipment
The Mössbauer spectrometer is a device used to perform Mössbauer spectroscopy by fundamentally applying the Mössbauer effect.
The Mössbauer spectrometer is composed of three main parts.
The source of gamma-ray emission moves back-and-forth and generates a Doppler effect. Mossbauer drive is used to move the source relative to the sample position. Selection of a suitable gamma-ray source is very important, and usually, it comprises a radioactive parent nucleus that decays into its desired isotope.
Under ideal conditions, the parent isotope should have a long half-life to remain useful and a good decay rate to supply the required radiation intensity. The energy the incident gamma rays provide should be relatively lower to avoid the poor signal-to-noise ratio and long collection times.
A collimator used to filter out dispersed/ non-aligned gamma rays and select those reaching the detector.
In the Mössbauer spectrometer, the detector choice depends on the required gamma-ray energies. There are mainly three general types of detectors that are used for radiation detection:
- Proportional counters
- Scintillation detectors
- Semiconductor detectors made of Silicon and Germanium
Why is Mössbauer spectroscopy important? – Applications.
- Mössbauer spectroscopy is unique because of its sensitivity to detect minute changes in the chemical environment of the targeted nucleus. These include the effect of different ligands on a particular atom, oxidation state changes, and the magnetic environment of the sample.
- Mössbauer spectroscopy can distinguish between the cyano complexes of iron (Fe), such as Turnbull’s Blue and Prussian Blue.
- In bioinorganic chemistry, the Mössbauer effect of Ferredoxin studies has convincingly reported that the oxidized form with two Fe (III)- high spin centers can be differentiated from a reduced form possessing one Fe (II)-high spin center.
- Six different iron oxides (FexOy) and oxyhydroxides (FeO(OH) of iron are together referred to as corrosion products. The corrosion reactions taking place in steel iron-containing alloys may form these corrosion products under different conditions. In industrial chemical analysis, these corrosion products can be distinguished from one another with the help of 57Fe Mössbauer spectroscopy.
- In geology, Mössbauer spectroscopy is applied to study the composition of iron-containing specimens, including moon rocks, meteors, etc.
What are the limitations of Mössbauer spectroscopy?
Sample availability and purity are two primary requirements to be taken care of in Mossbauer spectroscopy.
- The sample used for Mössbauer spectroscopy must be a crystalline solid structure.
- It is a “bulk” technique, so a large amount of sample is needed for analysis.
Also, check out our article: Nuclear magnetic resonance (NMR) spectroscopy.
- Greenwood, N.N., 2012. Mössbauer spectroscopy. Springer Science & Business Media.
- Vértes, A., Korecz, L. and Burger, K., 1979. Mössbauer spectroscopy.
- Gibb, T.C., 2013. Principles of Mössbauer spectroscopy. Springer.
- May, L. ed., 2012. An introduction to Mössbauer spectroscopy. Springer Science & Business Media.
- Long, G.J. and Grandjean, F. eds., 2013. Mössbauer spectroscopy applied to inorganic chemistry (Vol.3). Springer Science & Business Media.