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Detection of unpaired electron spins and their interaction with neighbouring lattice. This instrument is capable of detecting electron spin concentration as low as 1017/cc.
Model No.: ELEXSYS 580
Year of installation: 2016
|Microwave frequency range||X-band (8-12 GHz)|
|Magnetic field scan range||-1 to 1 Tesla|
|Temperature range||4.2 to 160 K and room temperature|
|Maximum microwave power||150 mW|
|Modulation field amplitude||10 G max|
|Modulation field frequency||10-100KHz|
|Quadrature detection||Real and imaginary|
|Sample state||See details afterwards|
Any unpaired spin has a net magnetic moment. EPR spectroscopy exploits the net magnetic moments of systems having unpaired electron spins. Spin hamiltonian of an unpaired electron in an external magnetic can be written as:
H = ge ????B B0 .?
= ge ????B B0 Sz (B0 ? ? direction)
= ge ????B B0 Ms (MS=±1/2)……………………………..(1)
Here ge (=2.20023) is g-factor of free electron, ????B (=9.27?10-24 J/T) is Bohr’s magneton,
Ms is spin projection quantum number. If the electron is bound e.g. an electron captured in
a dangling bond state, then ge would be replaced by g.
The energy difference between two Zeeman energy levels is, ????E= g ????B B0. Now, if a microwave of frequency ???? is applied, then a resonant absorption of microwave will occur if energy of microwave photon becomes equal to energy difference between the two Zeeman levels i.e.,
h????= g ????B B0 ; with selection rule ???? MS=±1 for ESR transition.
At thermal equilibrium the fractional population of upper and lower spin level can calculated by Boltzmann’s distribution law as,
Where N is total number of spins in the system, Nlow and Nup are spin population in upper and lower spin energy level, K is Boltzmann’s constant, and T is temperature.
Now microwave absorption intensity (IESR) is proportional to the population difference between two energy levels  i.e.
Since IESRis proportional to resonant magnetic field B0 (and hence resonant frequency) and inversely proportional to temperature, the signal to noise ratio can be improved by measuring ESR at low temperature and at high microwave frequency. Equations (2) and (3) is only valid for isolated spins such as dangling bond states which are not coupled.
A. For Solid Sample:
i) Sample has to be filled in glass capillary tube with both side sealed.
ii) Sample has to be filled in standard quartz (NMR/EPR) tube (250 mm
long and diameter < 4 mm).
B. For Liquid Sample:
i) Non-aqueous (Non H-bonded): Sample has to be filled in standard EPR
quartz tube / glass capillary with both side sealed (if both side sealing is not
possible, then other side has to be sealed well with paraffin wax).
ii) Aqueous/H-bonded: Sample has to be filled in thin glass capillary with
both side sealed (if both side sealing is not possible, then other side has to
be properly sealed with paraffin wax). Total sample volume must be less than
Reason: Aqueous/H-bonded samples absorb lot of microwaves. We cannot
tune the microwave bridge with excess sample volume due to this absorption.
****** If above guideline is not followed, then we might not be able to
Prof. Debamalya Banerjee
Department of Physics
IIT Kharagpur, Kharagpur-721302
West Bengal, India
Electrical Characterization Lab
Departmrnt of Physics, IIT Kgaragpur