Electron spin resonance (ESR)

Electron spin resonance (ESR) can be used to detect and characterize radicals. The method is also known as EPR (for Electron Paramagnetic Resonance). Due to its high sensitivity, it is also possible to detect decay curves of the radical density on material surfaces after plasma treatment. Additionally, this method can be used for larger surface areas in imaging mode.

Radicals are characterized by unpaired electrons and thus by a spin (±1/2), which in turn is associated with a magnetic moment. In a magnetic field, the energy of the electron depends on whether it is aligned parallel or antiparallel to the magnetic field (so-called Zeeman splitting, see diagram). If the sample is exposed to microwave radiation whose quantum energy corresponds to the Zeeman splitting, resonant absorption occurs.

The local magnetic field in which a radical is located results from an external field applied to the sample, and from the magnetic field of the nuclei of the neighboring atoms whose spin is not equal to 0, such as N14. Therefore, the chemical environment of the ESR-active species (i.e., the radicals) can be inferred from the ESR spectra: similar to NMR.
Practically, the sample is placed in a microwave resonator that is in a uniform magnetic field. The MW frequency is kept constant, and the magnetic field is scanned over a certain range. During this process, the MW absorption is measured.

Particular advantages of this method include its high sensitivity and the ease of measuring the absolute number of spins in the sample.

MS-5000 (formerly Magnettech, now Bruker)

Magnettech Miniscope MS 200

Technical data/measurement capabilities

• Working frequency: X-band (approx. 9.5 GHz)
• Sensitivity: 5x1010 spins/mT (5x109 spins/G)
• Microwave power: 1 μW - 100 mW
• Temperature range: - 170 °C to + 200 °C