Zero field splitting

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Zero field splitting describes various interactions of the energy levels of an electron spin (S>1/2) even in the absence of an applied magnetic field. It is important in the electron spin resonance of biological molecules.

A description of the origins of this effect may be found at: http://www.ncsu.edu/chemistry/das/zero-field_splitting.pdf (Notice, Kramers doublets cannot be split by ZFS, only magnetic field (i.e. Zeeman effects) can do that! I would recommend reading regular scientific journals for complete and adequate description of ZFS. Best regards.

The value of the ZFS parameter is usually defined as D. Values of D have been obtained for a wide number of organic biradicals by EPR/ESR measurements. This value may be measured by other magnetometry techniques such as SQUID, however EPR measurements provide more accurate data in most cases.

For an S = 1 spin system, the value of D is the energy separation between the lowest-lying triplet and singlet states in the absence of an applied field. Therefore, when D is sufficently large triplets may absorp microwave radiation. One example of this is the microwave spectrum of oxygen.