More
than 95% of all EPR spectroscopy is performed at X-band (9 GHz), even though
EPR can be performed at any frequency. Frequencies that can be routinely
used range from 1 GHz to 100 GHz (or a field ranging from 0.03 T to 3 T).
Running samples at more than one frequency helps to interpret spectral
features that are broadened due to the interplay of field dependent and
independent terms. Basically, the EPR Hamiltonian can be broken into two
parts: a field dependent part (i.e., Zeeman terms) and a field independent
part (i.e., zero-field terms). Changing the resonance field, B, changes
the relative weighting of field dependent terms compared to field independent
terms in the spin Hamiltonian for the system under study:
H = {ßB.g.S - ßngnB.I} + hS.A.I + hI.P.I + hS.D.S + higher order terms.
This can add a large amount of information to facilitate the interpretation of spectra and often changes their appearance, especially powder spectra. If the magnetic field is high enough, both the electron and nuclear spins are quantized by the external field and a first-order spectrum is observed, in addition to a better resolved g-anisotropy. This simplifies the spectrum making for an easier analysis, but information about the nuclear quadrupole term I.P.I and the zero-field term S.D.S is lost. For many transition metal ion systems these terms provide direct information about the symmetry and magnitude of the electric field gradients at the metal ion. At lower fields, either the electronic Zeeman or the nuclear Zeeman terms can be comparable to zero-field terms. This results in state-mixing and the observation of forbidden transitions. Thus, by recording spectra at several frequencies, specific terms in the Hamiltonian can be selected for analysis.
