Page 1 :
ELECTRON SPIN RESONANCE, , arta cence aioe 579, , SPECTROSCopy, , energy, AV, coincides the energy level se, , absorption takes place Causing th paration between the two states, the resonance, , € excitation of electron., AE=hv=g8B, , Resonance condition es), est case of ESR. Just as in NMR, the spin magnetic, gnetic field, hence resonance condition is written as, , hv=g BP Broo =2 8 (1-3) B _..(8), , (1-6) where g, is the Land litti, , aoe tting factor g of the free, lectron. The d ¢ aes, , ae eviation of g from g, = 2.0023 depends on the ability of the applied magnetic, , n the radical. Hence it provides some information about, Species present in the sample. The ratio of electron, , This corresponds to the simpl, moment interacts with the local ma, , In ESR, normally §=8,, , field to induce local magnetic field i, the electronic configuration of the, , ont ll 1, cena 9 N, 1/2) to M, = - 2 (N_ 4/2) is given by Boltzmann distribution law as, , N, +1/2 ES, = g EikaT _ .- BBB Ik, T (9), N_ 12, , , , However, resonance condition is applicable to an isotropic system (one which does not, have directional property) such as liquids. In case of anisotropic system, g value will depend, on the orientation of the crystal axes to the direction of the magnetic field., , INSTRUMENTATION, , A typical ESR spectrometer consists of following components (Fig. 2)., , 1. Electromagnet. A linear magnetic field is generated by a powerful electromagnet., Modulation coils, located on the sides of the resonant cavity, modulate the magnetic field., , 2. Source of Microwave Radiation. In most of the ESR spectrometers, the usual, source is a klystron oscillator that normally operate at 9500 MHz and produces, monochromatic microwave radiation. The wavemeter or the frequency meter measures the, microwave frequency. The radiation is transmitted into the sample cavity by means of a wave, guide, made of brass or copper tubing., , 3. Sample Cavity. The sample cavity or resonant cavity is placed between the poles of, an electromagnet. The cavities currently in use are the cylindrical or rectangular type. The, , the sample can be irradiated or z, Resonant cavity cell, , photolysed in situ. For studying, , anisotropic effects in, Amplifier, Sweep, Supply, , samples, rotatable cavity is, Fig. 2. Schematic diagram of a typical ESR spectrometer., , , , , , generally employed. Since, paramagnetic species are, often unstable, one should be, very careful in the preparation of, samples. Generally a single, crystal of a diamagnetic” host, containing a low concentration of, an oriented radical as 4a, substituent impurity is the most, useful sample., , Oscilloscope, , , , Recorder, , Computer