Physics
Electron Spin Resonance (ESR)
Quick fact
ESR is so sensitive that it can detect a single unpaired electron in a sample of billions of atoms, making it a premier tool for studying radicals and defects.
Why this is interesting
You've probably heard of MRI scans, which use the spin of atomic nuclei. But did you know that the spin of electrons can also be harnessed—using microwave radiation—to reveal the presence of free radicals and study the magnetic properties of materials?
Read the full explanation
Understanding Electron Spin Resonance (ESR)
Imagine each electron as a tiny bar magnet due to its intrinsic 'spin' property. Normally, these magnets are randomly oriented, but when placed in a strong external magnetic field, they align either with the field (low energy) or against it (high energy). The energy difference between these two states is tiny and corresponds to microwave frequencies. In ESR, we shine microwaves on the sample and sweep the magnetic field. When the microwave energy exactly matches the spin-flip energy, electrons absorb the radiation and jump to the higher state—this is resonance. By measuring this absorption, we learn about the electron's surroundings, such as nearby atoms and bonds.
A deeper explanation
The fundamental principle behind ESR is the Zeeman effect: the energy splitting of electron spin states in a magnetic field is linear with field strength (ΔE = gμB B, where g is the g-factor, μB is the Bohr magneton, and B is the magnetic field). The g-factor is not constant; it depends on the electron's orbital environment, providing a fingerprint for the paramagnetic species. Additionally, hyperfine interactions with nearby magnetic nuclei split the resonance lines, revealing molecular structure and dynamics. ESR matters because it is one of the few methods that directly probes unpaired electrons—key in catalysis, photosynthesis, radiation damage, and even the study of quantum dots. Its applications range from identifying free radicals in biological systems to characterizing defects in semiconductors.