Electron Spin Resonance (ESR) is one type of modern absorption spectroscopy. The radiations having a frequency in the microwave region (0.04 – 25 cm) in this device can be absorbed to induce transitions between magnetic energy levels of electrons with unpaired spins of various paramagnetic type samples.
The main factor of ESR relates to the fact that atoms, ions, molecules, or molecular fragments which have an odd number of electrons show the characteristic of magnetic properties. We know that an electron has a spin that has the ability to show the magnetic moment. It was invented in 1944 by E.K. Zavoisky. This device has a sensitive and informative technique for investigating various kinds of paramagnetic species in solid or liquid states.
The Working Principle of Electron Spin Resonance (ESR)
The working principle of Electron Spin Resonance (ESR) relates that an electron is a charged particle. It acts as a tiny bar magnet when it spins around its axis. The spin of unpaired electron of a molecule or compound has the ability to align in two different ways by creating two spin states ms = ± ½.
This alignment can parallel to the magnetic field which relates to the lower energy state ms = – ½. This alignment can also be antiparallel to the magnetic field which relates to the higher energy state ms = + ½
Different energies can be created by these alignments and this difference in energy lifts the degeneracy of the electron spin states. The difference in the energy is given by the following equation:
∆ E = E+ – E- = hv = gmßB
Where,
h = Planck’s constant which value is 6.626 x 10-34 J S-1
v = the frequency of radiation
ß = Bohr magneton which value is 9.274 x 10-24 J T-1
B = the magnetic field strength which is expressed in Tesla
g = the g-factor which is a unit less measurement of the intrinsic magnetic moment of the electron whose value for a free electron is 2.0023.
An unpaired electron has the ability to move between the two energy levels by either absorbing or emitting a photon of energy which is the fundamental equation of EPR spectroscopy.
Instrumentation of Electron Spin Resonance (ESR)
Klystron tube: This tube of Electron Spin Resonance (ESR) acts as the source of radiation. It can be stabilized against temperature fluctuation by immersion in an oil bath or by forced air cooling. It is possible to determine the frequency of the monochromatic radiation by the voltage applied to the klystron. An automatic control circuit of this part helps to fix the level of frequency which provides a power output of about 300 milliwatts.
Wave meter: A wave meter stays in between the oscillator and the attenuator. The shape of this meter is a hollow, rectangular brass tube that is used to convey wave radiation to the sample and crystal. This meter must be calibrated in frequency unit (megahertz) instead of wavelength.
Attenuators: This part of Electron Spin Resonance (ESR) is used in varying the power of the selected sample from the full power of the klystron tube to one attenuated by a force of 100 or more.
Isolators: This part of Electron Spin Resonance (ESR) reduces vibrations in the frequency of microwaves produced by klystron oscillators. It prevents any type of reflection of microwave power back into the radiation source. The frequency of the klystron tube is stabilized by this part.
Sample cavities: This part of Electron Spin Resonance (ESR) contains the sample. Two types of cavity are used namely rectangular TE120 cavity and cylindrical TE011 cavity. Generally, dual sample cavities are used at the present time to observe a sample and a reference material properly.
Couples and matching screws: These are used for making of irises or slots of various sizes.
Crystal detectors: These are made of silicon which uses to convert the radiation in D.C.
Magnetic system: An electromagnetic system is used which should be stable and uniform over the sample volume. The stability of this system can be achieved by energizing the magnet with a highly regulated power supply. The ESR spectrum can be recorded easily by slowly varying the magnetic field through the resonance condense by sweeping the current supplied to the magnet system after applying the power supply.
Modulation coil: This coil should be mounted inside the resonant cavity or cavities constructed of a non-metallic material to get higher modulation frequencies. On the other hand, the coils should be mounted outside the cavity and even on the magnet pole pieces to get the lower modulation frequencies
Display device: This part is very important to observe the signal of a system.
Applications of Electron Spin Resonance (ESR)
Electron Spin Resonance (ESR) is one type of modern spectrometry that is used in different fields to know the active site geometry, determine the rate of catalysis, and investigate enzyme-ligand interaction. It helps to evaluate the characteristics of inorganic compounds as well as the study of free radicals.
References
1.Schweiger, A., & Jeschke, G. (2001). Principles of pulse electron paramagnetic resonance. Oxford university press.
