Mossbauer spectroscopy

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)

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

Emission

Absorption

Recoil

Energy of recoil

2


E
ER =
2
2mc

γ-ray energy
Mass of atom

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

Emission

Absorption

2


E
ER =
2
2Mc
Mass of particle;
Very large

No recoil



oxidation and spin state of nuclear resonance probe.



molecular symmetry



magnetic properties of material under investigation.



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.


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.

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: 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.

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.


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.

 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.

Mossbauer spectroscopy

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Mossbauer spectroscopy