Pulsed electron paramagnetic resonance

With a vast variety of pulse sequences it is possible to gain extensive knowledge on structural and dynamical properties of paramagnetic compounds.

Because of the specific relation between the magnetic parameters, electronic wavefunction and the configuration of the surrounding non-zero spin nuclei, EPR and ENDOR provide information on the structure, dynamics and the spatial distribution of the paramagnetic species.

R. J. Blume reported the first electron spin echo in 1958, which came from a solution of sodium in ammonia at its boiling point, -33.8˚C.

The first microwave electron spin echoes were reported in the same year by Gordon and Bowers using 23 GHz excitation of dopants in silicon.

In the first decade only a small number of groups worked the field, because of the expensive instrumentation, the lack of suitable microwave components and slow digital electronics.

The first observation of electron spin echo envelope modulation (ESEEM) was made in 1961 by Mims, Nassau and McGee.

In the 1980s, the upcoming of the first commercial pulsed EPR and ENDOR spectrometers in the X band frequency range, lead to a fast growth of the field.

Differences can be found in the relative size of the magnetic interactions and in the relaxation rates which are orders of magnitudes larger (faster) in EPR than NMR.

For example, a +y π/2 pulse means that a B1 field, which has been 90 degrees phase-shifted out of the +x into the +y direction, has rotated M0 by a tip angle of π/2, hence the magnetization would end up along the –x-axis.

That means the end position of the magnetization vector M0 depends on the length, the magnitude and direction of the microwave pulse B1.

This microwave signal generated by the rotating magnetization vector is called free induction decay (FID).

In reality EPR spectra have many different frequencies and not all of them can be exactly on resonance, therefore we need to take off-resonance effects into account.

In broad EPR spectra where Δω > ω1 it is not possible to tip all the magnetization into the xy-plane to generate a strong FID signal.

However, in reality the electron spins interact with their surroundings and the magnetization in the xy-plane will decay and eventually return to alignment with the z-axis.

The spin-lattice relaxation results from the urge of the system to return to thermal equilibrium after it has been perturbed by the B1 pulse.

The frequency spectrum is reconstructed using the time behavior of the transverse magnetization made up of y- and x-axis components.

To obtain more information one can recover the disappeared signal with another microwave pulse to produce a Hahn echo.

Different frequencies in the EPR spectrum (inhomogeneous broadening) cause this signal to "fan out", meaning that the slower spin-packets trail behind the faster ones.

Changing the times between the pulses leads to a direct measurement of TM as shown in the spin echo decay animation below.

ESEEM [3][5] and pulsed ENDOR[4][5] are widely used echo experiments, in which the interaction of electron spins with the nuclei in their environment can be studied and controlled.