ESR: Electron Spin Resonance
Precision measurements of the magnetic moment of the electron have been fundamental in the development and testing of quantum electrodynamics, and in searching for new forces or electron substructure. P. Kusch won the Nobel Prize in 1955 for early measurements of the electron magnetic moment, and ongoing measurements continue to probe for new physics beyond the Standard Model.
Electron Spin Resonance (ESR), also known as Electron Paramagnetic Resonance (EPR), is also very widely used in condensed-matter physics, chemistry, archaeology, geology, environmental science, biology, biochemsitry, pharmacology, medicine, and industry to study both static properties and dynamic processes in almost any system containing radicals (i.e. molecules with unpaired electrons). These applications include everything from detecting tumours to dating ancient material.
This experiment familiarizes students with the generation and transmission of microwave signals, and measures the magnetic moment of the electron in a solid free-radical.
Both old and new ESR equipment and components are available. The current write-up is for the old equipment. The primary reference for students using the new LEOI equipment are is the manuals below, which discuss both ESR and Ferromagnetic Resonance (which can also be studied with the same equipment). Most of the old and new components are interchangeable, which may allow comparisons that can reduce systematic uncertainties.
New LEOI apparatus manuals: ESR and FMR (Ferromagnetic Resonance). (These are for APL use only and must not be copied or distributed!)
(The experiment is currently located in MP226.)
Additional resources:
- Old Write-Up in PDF Format or Microsoft Word Format.
- Old Appendices in PDF Format or Microsoft Word Format.
- Explanation of how a Magic Tee with a null detector can be used to measure changes in impedance in one of the arms can be found in Section 9.6.2 of Microwave Engineering by R. S. Rao or Section 4.9.3 of Fundamentals of Microwave Engineering by A. Singh and S. Verma.
- Animations of the electromagnetic fields inside the ESR experiment's Magic Tee created by Engineering Science student Jeff Nicholls using HFSS in February 2013. The sample arm was modelled as a cavity with a perfectly conducting wall, and the two animations are for adjustable pin positions that create (respectively) a high or low level of power transfer to the top arm. (Click on animation to get larger version.)
