Chemistry

Electron Paramagnetic Resonance Spectroscopy and Mössbauer Spectroscopy

By: Chelsea mckain

09/28/17

Did you know?

That the earth is 4.54 billion years old1

That iron is the forth most abundant element in the earth’s crust4

Electron Paramagnetic Resonance

This type of spectroscopy is used to analyze compounds containing unpaired electrons, particularly those with d-block and f-block elements.

The electron paramagnetic resonance, EPR, spectra is recorded using a constant microwave, CW, spectrometer.

In the CW spectrometer the compound is blasted a constant microwave with a frequency of 9 GHz with a varying magnetic field.

At 9 GHZ the CW spectrometer is known as the X-band spectrometer , which has a magnetic field of 0.3 T.

The CW Spectrometer

The X-band spectrometer is usually accompanied by an S-band (3 GHZ), Q-band (35 GHz) and a W-band (95 GHz) spectrometers.

The use of these accompanying spectrometers compliment the X-band data as they improve the resolution and simplifies it by reducing interactions between paramagentic centres.

EPR Spectroscopy math

∆E=gμBB0

∆E is the difference in energy between the ms= ½ and ms=- ½ states of the electron when an external magnetic field is applied.

μB is the Bohr magneton

B0 is the applied magnetic field

g is the g value. g= 2.0023 for free elctrons

EPR Detectability

Spectra can be obtained for systems more than one unpaired .

Compounds with an odd number of electrons are easily detected however it is difficult to obtain spectra for compounds with an even number of electrons.

Mössbauer Spectroscopy

Mössbauer Spectroscopy is based on the Mössbauer effect in which the recoilless resonant absorption and resonant emission of Υ-radiation by a nucleus is used2.

Determines the number of nuclear and condensed matter properties.

Nuclear properties – spin of the ground sate and excited states and the ratio of their spin

Condensed properties- isomer shift, nuclear electric quadrupole interaction and the magnetic level- splitting interaction in ferromagnetic materials.

Resonant Absorption

When a photon is emitted it transitions between two energy levels, if it is incident on a system in its ground state it has the possibility to be re-absorbed.

Not all the energy goes to the photon therefore it is lost to the recoil emitter and not all when the photon is absorbed so the absorber recoils.

The result is that the distributions of emission and absorption energies are separted by twice the recoil energy .

The probability of resonant aborption is proportional to the distribution overlap

Recoilless Emission

When energy is transmitted to the lattice from the nucleus by excited vibrational states, it is done via phonons

If no phonons are produced all the energy is emitted in the photon .

There is recoilless emission since the vibrational excitation is quantized, therefore there is a small probability that no energy is transferred to the lattice.

References

Ikeya, M., Zimmerman, M. R., & Whitehead, N. (2007). New applications of electron spin resonance: dating, dosimetry and microscopy. Singapore: World Scientific.

Schumacher, R. A. (2017, May). MOSSBAUER SPECTROSCOPY – www-meg.phys.cmu.edu. Retrieved September 28, 2017

Shriver & Atkins Inorganic chemistry. Oxford: Oxford University Press.

Vandenberghe, R. E., & Grave, E. D. (1970, January 01). Application of Mössbauer Spectroscopy in Earth Sciences. Retrieved September 28, 2017, from http:// adsabs.harvard.edu /abs/2013mosp.book…91V

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