Mossbauer spectroscopy BY: Yogesh Kumar

Introduction: Mossbauer spectroscopy is more aptly described by its alternative name; NUCLEAR GAMMA RESONANCE SPECTROSCOPY. Sometimes may be abbreviated as NGR. As Name suggests, nucleus is probed using Gamma rays as exciting radiation; a gamma- absorption spectrum is measured.  discovered by Rudolf Mossbauer in 1957-58 (German Physicists 1929-2011)

Basic Principle: Just as gun recoils when bullet is fired, conservation of momentum requires a free nucleus to recoil during emission or absorption of gamma rays. If nucleus at rest emit gamma ray , the energy of the gamma ray is slightly less than the natural energy of the transition,  but in order for a nucleus at rest to absorb a gamma ray, the gamma ray's energy must be slightly greater than the natural energy, because in both cases energy is lost to recoil. means nuclear resonance is unobservable with free nuclei because shift in energy is too large to have significant overlap of emission and absorption spectra. “ Nobel Prize in 1961 for PhD work of 1958”

Free emitting and absorbing nuclei/ atoms γ-ray energy Energy of recoil Mass of atom

Basic Principle…….. Emitting and absorbing atoms fixed in a lattice Nuclei in solid crystals are not free to recoil because they are bound. still some energy is lost due to recoil but in that case it will be in discrete packets called phonones. Emitting and absorbing atoms fixed in a lattice Mass of particle; Very large

General uses: oxidation and spin state of nuclear resonance probe. molecular symmetry magnetic properties of material under investigation.

How it Works….. solid sample exposed to beam of gamma rays detector measures the intensity of transmitted rays through the sample. If emitting and absorbing nuclei are in same chemical environment, the nuclear transition energies would be exactly equal and resonant absorption observed with both materials  at rest. difference in chemical environments, causes the nuclear energy levels to shift . To bring the two nuclei back into resonance Doppler effect is used.  the source is accelerated through a range of velocities using a linear motor  to produce a Doppler effect.

Suitable Source: Several conditions have to be satisfied : Energy of transition have to be large but not larger than lattice vibrations. (10-150 ev) substantial proportions of excited state nuclei should be there. lifetime of excited state should be large to have precise energy of transition, but low enough to have intense lines in spectrum.( 1-100ns) excited state of emitter should have long lived precursor . ground state isotope should be stable. cross section of absorption should be high.

Nuclear decay scheme for 57-Co for 57-Fe Mossbauer spectroscopy:

Mossbauer parameters: Chemical Isomer Shift (IS) (): Arises out of the interaction between nuclear charge density and the surrounding ‘s’ electron charge cloud. IS can give information about the spin state as well as the co-ordination number. Isomer shift (chemical shift, CS) can be expressed using the formula below: CS = K (Re2 – Rg2) {[Ψs2(0)]a – [Ψs2(0)]b}  Physical meaning of this equation: an increase in s electron density in 57-Fe spectrum gives a negative shift because the change in the effective nuclear charge is negative an increase in s electron density in 119-Sn gives a positive shift due to a positive change in overall nuclear charge Oxidised ferric ions (Fe³⁺) have lower isomer shifts than ferrous ions (Fe²⁺) because s electron density at the nucleus of ferric ions is greater due to a weaker screening effect by d electrons.

Quadrupole Splitting: Quadrupole splitting:  reflects the interaction between the nuclear energy levels and surrounding electric field gradient (EFG).  Nuclei in states with non-spherical charge distributions, produce an asymmetrical electric field which splits the nuclear energy levels. This produces a nuclear quadrupole moment. In the case of an isotope with a I=3/2 excited state, such as 57Fe or 119Sn, the 3/2 to 1/2 transition is split into two sub-states mı =±1/2 and mı =±3/2. These appear as two specific peaks in a spectrum, sometimes referred to as a 'doublet'. Quadrupole splitting is measured as the separation between these two peaks and reflects the character of the electric field at the nucleus.

Chemical shift and quadrupole splitting of the nuclear energy levels and corresponding Mössbauer spectra

Magnetic Splitting: Magnetic splitting (hyperfine splitting): is a result of the interaction between the nucleus any surrounding magnetic field. A nucleus with spin, I, splits into 2I + 1 sub-energy levels in the presence of magnetic field. transition between excited state and ground state only occur if m₁ changes by 0 or 1. six possible transitions for a 3/2 to 1/2 transition. In the majority of cases only six peaks can be monitored in a spectrum produced by a hyperfine splitting nucleus.

Magnetic splitting of the nuclear energy levels and the corresponding Mössbauer spectrum

Mössbauer spectrometers: It is formed by three main parts: a source that moves back and forth to generate a doppler effect. a collimator that filters out non-parallel gamma rays and, a detector.

Common isotopes for Mossbauer spectroscopy

57-Fe mossbauer spectra of three Fe(II) selected compunds:

Three possible molecular structure of Fe₃(CO)₁₂

Aquaspirillum Magnetotacticum (Fe₃O₄ nanoparticals): Hexagonal and cubic shaped Fe₃O₄ nanoparticles identified in the magnetotactic bacteria Aquaspirillum Magnetotacticum. behaves as a biomagnetic compass. follow the weak geomagnetic field due to the presence of magnetic nanoparticles (40–120 nm) of hexagonal and cubic shapes.

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