Download EPR Spectroscopy

EPR Spectroscopy
• Overview
• EPR Spectrometer Hardware
• Bridge
• Microwave Cavity
• Magnet
Last Updated: 2014‐11‐11
EPR Spectrometer
• The EPR system consists of five main components: • Microwave Bridge
• Microwave Cavity
• Magnet / Magnetic Field Controller
• Console
• Computer/Workstation
Microwave Bridge
A. Microwave Source – generates the microwave at a constant power
Microwave Bridge Block Diagram
B. Attenuator – used to block the flow of microwave radiation. This can be adjusted to precisely and accurately regulate the microwave power which the sample sees.
G
E
F

A
C. Circulator – used to direct the source and reflected microwaves into the sample cavity and detector respectively. It also ensures that the reflected microwaves do not go back to the source.
3
C
1
2
B
D
D. Microwave Cavity – location of the sample where the sample gets bombarded by the microwaves. E. Detector Diode – detects the reflected microwaves. Converts microwave power to an electrical current. F. Reference Arm – used to ensure that the diode operates in a linear region which contains a variable attenuator and phase shifter so that powers and phases can be adjusted for optimal signal.
G. Signal Output
Microwave Cavity
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•
The microwave cavity is used to amplify weak signals from the sample. The microwave cavity is a metal box or cylinder that resonates with the microwaves. This is connected to the microwave bridge via a waveguide. When at the resonance frequency, the cavity stores the microwave energy inside reducing the amount being reflected back.
The sensitivity of the spectrometer is related to how efficiently the cavity stores microwave energy. This is defined by the Q or quality factor which can be easily measured from information of the reflected microwave power spectrum. In the cavity there is a standing electromagnetic wave where the electric and magnetic field components are completely out of phase such that the minimum magnetic field occurs at the greatest electric field. By placing the sample in the maximum magnetic field and lowest electric field a optimal signal can be measured.
Reflected ‐wave Power
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Microwave Magnetic Field
microwave
cavity
waveguide
chemwiki.ucdavis.edu

Q
res
Frequency
Microwave Electric Field
Microwave Cavity
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The coupling of microwaves between the waveguide and the cavity is done by adjusting an iris. An iris is a hole between the waveguide and cavity in which the size of the opening is adjusted by a screw. This is achieved by matching or transforming the impedances (the resistance to the waves) of the cavity and the waveguide (the rectangular pie that carries the microwaves).
Iris screw
Waveguide
Iris
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Cavity
When the sample absorbs the microwave energy the Q factor decreases because of the increased losses of energy. This causes the coupling to change because the absorbing sample changes the impedance of the cavity. The cavity is no longer coupled and the microwave will be reflected back to the bridge, resulting in the EPR signal.
Magnet
•
An electromagnet is used in an EPR experiment because the field strength of the applied magnetic field can consistently be adjusted precisely, easily and quickly. This is crucial for the EPR experiment since, as mentioned earlier, the field must be adjusted to match the frequency of the electromagnetic radiation being applied to the sample.
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For electromagnets to generate a magnetic field a high amount of current runs through the electromagnetic coils. The magnetic field strength is adjusted by varying the current applied to the coils. The amount of current used generates a significant amount of heat and must be cooled by flowing water around the electromagnet coils. •
A magnetic field controller is used to adjust the magnetic field strength. To do so, there are two components that are needed: 1. a microprocessor that sets the field values and the timing of the field sweep and 2. a Hall probe that measures the field strength and feeds back a voltage reading to a regulation unit that compares the measurement to a reference voltage and adjusts the current in the windings of the magnet accordingly to attain the requested magnetic field value.
chemwiki.ucdavis.edu
Magnet
Console
Power Supply
Regulator
3475 G
Microprocessor 3476 G
3477 G
Hall Probe
Reference Voltage