Pulsed Electron Spin Paramagnetic Resonance Laboratory

Objective

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.

Equipment & Details

Pulsed Electron Paramagnetic Resonance (EPR) Spectrometer (X-band)
Manufacturer: Bruker
Model No.: ELEXSYS 580
Year of installation: 2016

Specifications:

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

Sample Details

A. For Solid Sample:i) Sample has to be filled in glass capillary tube with both side sealed.ORii) Sample has to be filled in standard quartz (NMR/EPR) tube (250 mmlong and diameter < 4 mm).B. For Liquid Sample:i) Non-aqueous (Non H-bonded): Sample has to be filled in standard EPRquartz tube / glass capillary with both side sealed (if both side sealing is notpossible, then other side has to be sealed well with paraffin wax).ii) Aqueous/H-bonded: Sample has to be filled in thin glass capillary withboth side sealed (if both side sealing is not possible, then other side has tobe properly sealed with paraffin wax). Total sample volume must be less than10 microlitter.Reason: Aqueous/H-bonded samples absorb lot of microwaves. We cannottune the microwave bridge with excess sample volume due to this absorption.****** If above guideline is not followed, then we might not be able toaccept samples.

Assigned Technician:


Dr. Debamalya Banerjee

Arijit Maity

Junior Technician/Junior Laboratory Assistant

arijit.maity@iitkgp.ac.in

Pulsed Electron Spin Paramagnetic Resonance Laboratory marker Location: OB / GF / *, CRF administrator-male Facilitator: Dr. Debamalya Banerjee
Dr. Debamalya Banerjee

Dr. Debamalya Banerjee

Physics

debamalya@phy.iitkgp.ac.in

+91-3222-283804


Utility and Working Principle

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 ina 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,Nlow-NupNlow+Nup=ΔNN=1-exp?(-ΔEKT)1+exp?(-ΔEKT)……………………….(2)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 [5] i.e.IESR∝ΔN=Nlow-Nup≈N.gμBB02KT, whenΔEKT?1……………(3)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.
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