Sign of zero-field splitting

The magnitude of the rfs can be obtained directly from the powder spectmm when 1st order conditions apply (D giMsB). These conditions are approximately satisfied for many triplet state organic molecules, such as biradicals and radical pairs and for several transition metal ions even at X-band. On the other hand the sign of the Tfs can only in exceptional cases (see Section 4.5) be determined at X-band. This can, however, be done more generally by measurements at high field by comparing the relative intensities of the different transitions, e.g. = 1 0 and ms = 0 -o- -1 for the triplet state. 

The probability for an electron spin to be in a specific electronic state with energy E(m ) is according to the Boltzmann distribution [31] given by  

The intensity ratio depends strongly on the TITv ratio (Fig. 4.14). 

At high temperature the intensities are equal, as usually observed at X-band. The = 1 0 transition becomes the weaker when the temperature is lowered towards 

This is due to the preferential electron spin population in the lowest ms = — ) spin level, an effect which is enhanced by the increased level separation at higher magnetic field. The term spin polarisation is sometimes used. In a temperature range about Tv the intensity ratio deviates significantly from unity. This range is at higher temperatures for high microwave frequency, see Table 4.1 and Fig. 4.14. 

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