electron spin resonance

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A spectroscopic technique, similar to nuclear magnetic resonance, used to detect atoms having orbitals containing unpaired electrons, as found in free radicals. The molecules are exposed to high-frequency microwave radiation, causing the electrons in unpaired orbitals to resonate between the two available spin states of that orbital and to release distinct patterns of radiation.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun physics The absorption of radiation by species having unpaired electrons when placed in a magnetic field that is used spectroscopically to detect and study free radicals

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun microwave spectroscopy in which there is resonant absorption of radiation by a paramagnet