Mössbauer SpectroscopyFraser Parlane

Rudolf Ludwig Mössbauer, German physicistNobel Prize in Physics, 19611929 - 2011

“Explain it! The most important thing is, that you are able to explain it! You will have exams, there you have to explain it. Eventually, you pass them, you get your diploma and you think, that's it! – No, the whole life is an exam, you'll have to write applications, you'll have to discuss with peers... So learn to explain it! You can train this by explaining to another student, a colleague. If they are not available, explain it to your mother – or to your cat!”

Rudolf Ludwig Mössbauer, 1984

Rudolf Mössbauer

The Mssbauer EffectCommon spectroscopy techniques

Has similarities to NMR spectroscopy

The Mssbauer Effectobserving resonance-fluorescence and the Compton Effect

“The Compton effect can be a nuisance.”

Jeremy Bernstein

Initially

Change in wavelength

Sponge / steel wall analogy

Energies emitted

Graph not to scale

Energies absorbed

The Mssbauer Effectobserving resonance-fluorescence

Two main issues with this technique:

1. The “recoil” of the nucleus as the gamma-ray is emitted or absorbed

2. The splitting caused in the energy levels by the electronic and magnetic environment cause “hyperfine” splitting in the energy levels

The Mssbauer EffectThe Discovery

Mossbauer discovered that when the atoms are within a solid matrix the effective mass of the nuclei is much larger.

• Mass is now effectively the massof the system

• This makes and very small

RESONANCE!

• Resolution is nowvastly increased( in )

Paper / sun comparison

The Mssbauer EffectThe Discovery

High resolution allows for detection of “hyperfine” splitting.

Requirements for high resolution:• Requires low lying excited states (lower)• Lifetime of the excited state (longer)

meets both of these requirementsand is commonly used

Resonnance!

The Mssbauer EffectThe Discovery

Introduction Mssbauer SpectroscopyFundamentals

• Hyperfine interactions (billionth of an electron volt) – this requires very small changes in the energy of the gamma-ray radiation. This is done using the Doppler effect.

Introduction Mssbauer SpectroscopyReading the Spectra

Three properties that can be read from the spectra

1. Isomer shift

2. Quadrupole Splitting

3. Magnetic Splitting

Introduction Mssbauer SpectroscopyIsomer Shift

• Shift cannot be measured directly. It needs to be measured relative to a known absorber. For instance, is often calibrated to alpha iron. Analogous to NMR.

• Measured as “centroid” of the spectrum.

• Used to determine valency states, ligand bonding states, electron shielding and the electronegativity of functional groups (the electric environment).

• Less electron shielding leads to positive isomer shift

Example

More electron shielding positive shiftLess electron shielding negative shift

Introduction Mssbauer SpectroscopyQuadrupole Splitting

• Nuclei with a quantum number have an asymmetrical charge, forming a nuclear quadrupole moment.

The principle component of the EFG

Quadrupole momentA constant

Introduction Mssbauer SpectroscopyMagnetic Splitting

• Zeeman splitting is the result of the nucleus interacting with a magnetic field

From the spin on the electrons polarizing the spin of the nucleus

From the orbital moment of those electronsFrom the dipolar field due to the spin on those

electrons

Splits the field into peaks.

Introduction Mssbauer SpectroscopyMagnetic Splitting

For , this gives six possible transitions for magnetic quantum numbers and transition.

Peak intensities

Introduction Mssbauer SpectroscopyThe Machine

Introduction Mssbauer SpectroscopyApplications

• Sensitive to subtle changes in the environment

• Detection limits in the billionth of an electron volt

• Geology• Moon• Structure and function of enzymes• Analysis of heterobimetallic complexes

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