Presented By:

Anuradha Verma

What is Mössbauer spectroscopy

• Named after its discoverer Rudolf

Mössbauer in 1957.

• Received a noble prize in 1961.

• Consists of the recoil-free, resonant absorption and emission of gamma rays in solids.

• Involves transitions between energy levels within the nuclei of atoms.

Principle

• Heavier elements when formed by the radioactive decay of an isotope of the same or different element are initially produced in an excited nuclear state.

• After a very short delay, of the order of microseconds, the excited nucleus reverts to the ground state and emits energy of very high frequency, usually in the gamma-ray region of spectrum.

• Study of this gamma-ray emission and subsequent reabsorption constitutes Mössbauer spectroscopy.

Interaction of spin and magnetic field

• Charged particle spinning about an axis constitutes a circular electric current which in turn produces a magnetic dipole.

• The spinning particle behaves as a tiny bar magnet placed along the spin axis.

• The size of the dipole i.e. the stength of magnet for a point charge can be given as:

� = JT-1

where g is Lande splitting factor, � is Bohr magneton and I is spin quantum no.

• The seperation between neighbouring energy level is:

= Hz

g� N

BZ

h

Effect of a Magnetic Field

The non-spin is associated with either the excited or the ground state nucleus and usually with both will interact with a magnetic field.

Each energy state will split into 2I + 1 separate energy level.

Spacing between energy level is g� N BZ / h where B z

is the magnetic field at the nucleus. g values of excited and ground states will be

different and may have opposite signs. e.g. Fe-57 excited state g is negative.

• Ground state (1/2, g positive) will split into two sub-levels

• Excited state (3/2, g negative) will split into four sublevel.

• Sublevel of the ground state nucleus Iz = +1/2 is lower than that of Iz = -1/2

• Sublevel of the excited state increase in energy in the order Iz = -3/2, -1/2, +1/2, +3/2.

Selection Rules:

Iz = 0 or ± 1

There are six transition probabilities which are

found to occur and these are:

(1) 3/2 1/2 , -3/2 -1/2

(2)½ ½ , -1/2 -1/2

(3)-1/2 ½, ½ -1/2

Two members in each pair have the same

Probability and the relative probabilities of the

three pairs are 3: 2: 1 for (1) : (2): (3)

Fe *

Fe

+3/2

-3/2

-1/2

+1/2

-1/2

+1/2

1.Energy level in absence

of magnetic field

2. Splitting produced

by magnetic field

Spectrum for metallic iron

Magnetic field necessary to cause the energy

level splitting may be applied externally, but it

happens that internal effects within the sample

produce sufficient field to cause observable

splitting.

By using external field we can calibrate the

spectrum and can estimate the magnitude of

internal field.

Field of 20-50 T found for various compound of 57 Fe (Large compared to fields created by

superconducting magnets just 5-10 T)

Note:-

Internal fields does not extends uniformly throughout the bulk sample and is extremely localized effect.

Formed by interaction of the nucleus with surrounding electrons.

Electric Field and Magnetic Field existing simultaneously

Quadrupolar shifts are superimposed on magnetic splitting. Six line spectrum is again produced with same transition probability, but are not equally spaced. ±3/2 states are moved upwards in energy ±1/2 states are moved down. e.g. FeF 2 (iron difluoride) give such kind

of spectrum

Comparison of the obtained spectra

Refrences

• Fundamentals of molecular spectroscopy

Colin N. Banwell & Elaine M. McCash

• Principles of Physical Chemistry

Puri, Sharma, Pathania