mossbauerspectroscopy

1. BY: Yogesh Kumar

2.  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 ismeasured.
 discovered by Rudolf Mossbauer in 1957-58
(German Physicists 1929-2011)

3. Just as gun recoils when bullet is fired, conservation of momentum requiresa
free nucleus to recoil during emission or absorption of gammarays.
 If nucleus at rest emit gamma ray , the energy of the gamma rayis
slightly less than the natural energy of the transition, but in order for anucleus
at rest to absorb a gamma ray,the gamma ray's energy mustbe
slightly greater than the natural energy, because in both cases energy is lostto
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”

4. Emission Absorption
Recoil
E2
ER =
2mc2

Free emitting and absorbing nuclei/ atoms
Energy of recoil
γ-ray energy
Mass of atom

5. Emission Absorption
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 indiscrete
packets called phonones.
Emitting and absorbing atoms fixed in a lattice
No recoil
E = 2
R
E2
2Mc

Mass of particle;
Very large

6.  oxidation and spin state of nuclear resonanceprobe.
 molecular symmetry
 magnetic properties of material under investigation.

8.  solid sample exposed to beam of gammarays
 detector measures the intensity of transmitted rays through thesample.
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 levelsto
shift .
 Tobring the two nuclei back into resonance Doppler effect is used.
the source is accelerated through a range of velocities using alinear
motor to produce a Dopplereffect.

9. Several conditions have to be satisfied:
 Energy of transition have to be large but not larger than latticevibrations.
(10-150 ev)
 substantial proportions of excited state nuclei should bethere.
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 bestable.
 cross section of absorption should be high.

11. 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 (Re
2 – Rg
2) {[Ψs
2(0)]a – [Ψs
2(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 isnegative
 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.

12. 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 asa
'doublet'. Quadrupole splitting is measured as the separation between these
two peaks and reflects the character of the electric field at thenucleus.

14. 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 magneticfield.
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 aspectrum
produced by a hyperfine splitting nucleus.

16. It is formed by three main parts:
 a source that moves back and forth to generate a dopplereffect.
a collimator that filters out non-parallel gamma raysand,
 a detector.

20. 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 hexagonaland
cubic shapes.