Analytical Instrumentation: ESR Spectroscopy

Nikhilbinoy.CAssistant ProfessorICE DepartmentAnalytical Instrumentation:ESR Spectroscopy

BasicsElectron Spin Resonance Spectroscopy.Often called Electron Paramagnetic Resonance (EPR) spectroscopy.Supposed to be a mature field with a fully developed theory.Some surprise as organometallic problems have explored new domains.ESR is a branch of study in which electromagnetic radiation (usually microwave frequency) is absorbed by molecules, ions, or atoms possessing electrons with unpaired spins.Characterized as electron spin, S > 0.ESR is similar to NMR which deals with non-zero nuclear spin I > 0.In both ESR and NMR, the sample material is immersed in a strong magnetic field and exposed to an orthogonal low amplitude high frequency field.ESR requires microwave frequency, while NMR requires lower radio frequency.ESR can basically determine:Density of ions, number of ions in the sample.Position of insertion, symmetry of the lattice position.Couplings, line widths, dynamics and lattice vibrations.

Principle:Electron Spin

Principle:Spinning Electron in an External Magnetic Field

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Principle:Resonance

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Principle:ResonanceIn actual practice, the frequency is in the microwave region and is held at a constant value.The magnetic field strength is varied to obtain resonance condition.

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Basic ESR Spectrometer

Microwaves are generated by the Klystron tube.Power level is adjusted by Attenuator.Circulator routes the microwaves entering from the Klystron towards the cavity.The sample is placed in cavity.Under magnetic field.Microwaves reflected back from the cavity are routed to the diode detector.Any power reflected from the diode is absorbed completely by the load.The diode produces a current proportional to the microwave reflected from the cavity.Absorption of microwave by the sample is detected by noting a decrease in current in the micro ammeter.Is too noisy.Signal-to-noise ratio can be improved by introducing small amplitude field modulation.

Klystron TubeThere are three electrodes:A heated cathode from which the electrons are emitted.An anode to collect the electrons.A highly negative reflector electrode which sends those electrons which pass through a hole in the anode back to the anode.The motion of the charged electrons from the hole in the anode to the reflector and back to the anode generates an oscillating electric field.Electromagnetic radiation.The transit time from the hole to the reflector and back again corresponds to the period of oscillation.The microwave frequency can be tuned over a small range by adjusting the physical distance between the anode and the reflector, or by adjusting the reflector voltage.In practice these two methods are used.The metal tube is distorted mechanically to adjust the distance as a coarse frequency adjustment.The reflector voltage is adjusted as a fine control.

HeatedFilamentCathodeAnodeReflectorElectrodeElectronpathway

Microwave Cavity

SamplePortIris

Microwave Cavity:Excitation

ReflectorVoltageDiode current

Microwave Cavity:Magnetic Field

Microwave Cavity:Amplitude ModulationThe signal-to-noise ratio can be improved by introducing small amplitude modulation.An oscillating magnetic field is superimposed on the dc field by means of small coils, usually built into the cavity walls.When the field is in the vicinity of a resonance line, it is swept back and forth through part of the line, leading to an ac component in the diode current.The ac component is amplified by using a frequency selective amplifier.Eliminates a great deal of noise.Similar to derivative of the original diode current.

ReflectorVoltageDiode current

ReflectorVoltageDerivative

Microwave BridgeThe microwave bridge detects the ESR signal reflected from the sample cavity.The bridge contains microwave circuitry, a klystron power supply, a pre-amplifier, and an automatic frequency control circuit.The klystron generates microwave energy at 9.5GHz, which is used to irradiate the sample.The klystron is water-cooled through its mounting flange.35GHz systems are also available.The klystron output is applied to an isolator which allows signal flow only in one direction.Klystron

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SIG DetAFC DetSample CavityCirculatorReferencepowerReference arm phaseAttenuatorReferenceCouplerIsolator

Microwave BridgeThe output of the isolator is given to a four port circulator.The microwave power is directed to the sample in cavity.The circulator then directs the reflected power from the sample cavity back to the microwave detector.Klystron

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SIG DetAFC DetSample CavityCirculatorReferencepowerReference arm phaseAttenuatorReferenceCouplerIsolator

Microwave BridgeWhen all the power is absorbed and none is reflected, the detector receives no signal.At resonance, the sample absorbs microwave energy and the unbalances the impedance of the cavity.The absorption in the sample is deflected as a dip in the output of the crystal detector.The crystal output containing the ESR signal and the 70KHz AFC (automatic frequency control) signal is amplified by the pre-amplifier.Klystron

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SIG DetAFC DetSample CavityCirculatorReferencepowerReference arm phaseAttenuatorReferenceCouplerIsolator

Microwave BridgeNoise elimination is an important factor in enhancing the sensitivity of plotting the ESR spectrum.Most of the noise is eliminated in the phase-sensitive detector.The noise is further eliminated by using a long time constant filter, which averages out noise at frequencies greater than the reciprocal of its time constant.Klystron

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SIG DetAFC DetSample CavityCirculatorReferencepowerReference arm phaseAttenuatorReferenceCouplerIsolator

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