Electron Spin Resonance Spectrometer

Features

• FET based marginal R.F. Oscillator
• Digital diaplay of frequency
• Excellent peaks display
• Digital display of Helmoltz Coil Current
• Compatible with general pupose CRO in X-Y mode

Introduction

In recent years Magnetic Resonance has developed into a very useful and powerful tool in solid state research. In this method, use is made of the Zeeman interaction of the magnetic dipoles associated with the nucleus or electron, when placed in an external magnetic field. Accordingly, they are identified as NMR (Nuclear Magnetic Resonance) or ESR (Electron Spin Resonance). This form of spectroscopy finds many applications in the investigation of crystal structures, environmental effects, dynamic effects, defects in solids and in many diverse branches of Physics, Chemistry and Biology.

Elementary Magnetic Resonance

We know that the intrinsic angular momentum (spin) of the electron S couples with the orbital angular momentum of the electron  to give a resultant  and this coupling gives rise to the ‘fine structure' of the spectra. Further, under the influence of an external magnetic field (H) each of the level will split into (2j+1) sublevels (Zeeman effect) and the splitting of a level will be

DE = (gµ₀H)m_j

where µ₀ is the Bhor magneton, g is the Lande' g-factor and mj is the magnetic quantum number. As can be seen, the splitting is not same for all levels; it depends on the  and  of the level (s=½ always for one electron). However, the sublevels will split equally by an amount

DE = gµ₀H ₀r = hn₀

where n₀ is the frequency of the system. Now if the electron is subjected to a perturbation by an oscillating magnetic field with its direction perpendicular to the static magnetic field and its frequency n₁such that the quantum n1 is equal to E=hn₀, we say that there is a resonance between n1 and n₀. This willinduce transition between neighbouring sublevels (mj=±1) and in turn will absorb energy from oscillating field. Thus, at resonance, we get a peak due to the absorption of energy by the system

Experimental Technique

If we consider a free electron and substitute the proper value of constants in the equation: g=2.00, µ₀=0.927X10-20 erg/gauss & h=6.625 X l0^-27 erg sec, we get

That is ESR can be observed at radio frequencies in a magnetic field of a few gauss or in the microwave region in a magnetic field of a few kilogauss. The latter alternate has many advantages: improved signal-to-noise ratio, high resolution etc. and is always preferred for accurate work, though it is very sophisticated and expensive. However, if the basic understanding of the subject is the main criteria as is usually the requirement of class room experiments, the observation of ESR in low magnetic field and in a radio frequency region makes it a lot simple, inexpensive and within the reach of every post-graduate laboratory.