Electron Spin Echo
Definition and meaning of Electron Spin Echo in chemistry.
Electron spin echo (ESE) is a pulsed electron paramagnetic resonance (EPR) signal generated when a sequence of microwave pulses refocuses the dephased magnetic moments of unpaired electron spins, producing an observable echo at a time equal to twice the delay between the first two pulses.
In more detail
In the basic two-pulse (Hahn) sequence, a π/2 microwave pulse tips the electron spin magnetization into the plane transverse to the static magnetic field, where individual spin packets dephase due to local field inhomogeneities. A second, π pulse applied after a delay τ reverses this dephasing, and the spins realign to form an echo at time 2τ. The echo amplitude decays as τ increases, and this decay defines the phase memory time (Tm), a measure of electron spin decoherence closely related to T2 relaxation. Because nearby nuclear spins modulate the echo amplitude, the related technique ESEEM extracts weak hyperfine couplings that are unresolved in ordinary continuous-wave EPR.
Key facts
| Abbreviation | ESE |
|---|---|
| Technique | Pulsed EPR (electron paramagnetic resonance) |
| Typical pulse sequence | π/2 – τ – π – τ – echo (two-pulse Hahn echo) |
| Field | Physical Chemistry |
Recording the two-pulse echo decay of a nitroxide spin label frozen in a glassy solvent to measure its phase memory time and probe its local molecular environment.
Frequently asked questions
What information does an electron spin echo provide?
Its decay with increasing pulse delay gives the phase memory time (Tm), related to spin-spin (T2) relaxation, and, via echo envelope modulation (ESEEM), reveals weak hyperfine couplings between the unpaired electron and nearby nuclear spins.
How does electron spin echo differ from continuous-wave EPR?
CW-EPR sweeps the magnetic field under continuous microwave irradiation, while ESE applies short pulses and measures the time-domain echo, giving direct access to relaxation times and couplings hidden by inhomogeneous line broadening.