page_title: Electron paramagnetic resonance
section_title: Field modulation
hierarchical_section_title: Electron paramagnetic resonance / Theory / Field modulation
caption_attribution_description: English: Depicts field modulation in the continuous wave electron paramagnetic resonance experiment.
context_page_description: Electron paramagnetic resonance or electron spin resonance spectroscopy
  is a method for studying materials with unpaired electrons. The basic concepts of EPR are analogous to those of nuclear magnetic resonance, but it is electron spins that are excited instead of the spins of atomic nuclei. EPR spectroscopy is particularly useful for studying metal complexes or organic radicals.
  EPR was first observed in Kazan State University by Soviet physicist Yevgeny Zavoisky in 1944, and was developed independently at the same time by Brebis Bleaney at the University of Oxford.
context_section_description: As previously mentioned an EPR spectrum is usually directly measured as the first derivative of the absorption. This is accomplished by using field modulation. A small additional oscillating magnetic field is applied to the external magnetic field at a typical frequency of 100 kHz. By detecting the peak to peak amplitude the first derivative of the absorption is measured. By using phase sensitive detection only signals with the same modulation (100 kHz) are detected. This results in higher signal to noise ratios. Note field modulation is unique to continuous wave EPR measurements and spectra resulting from pulsed experiments are presented as absorption profiles.
caption: The field oscillates between B1 and B2 due to the superimposed modulation field at 100 kHz. This causes the absorption intensity to oscillate between I1 and I2. The larger the difference the larger the intensity detected by the detector tuned to 100 kHz (note this can be negative or even 0). As the difference between the two intensities is detected the first derivative of the absorption is detected.
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