Resolution of Apparent Isotropic or Axially Symmetric, -Tensor

1 Resolution of Apparent Isotropic or Axially Symmetric g-Tensor 

In initial work principal g-values in the range 2.0024-2.0072 of phenoxyl radicals were resolved with an uncertainty of 5 10 [9]. Such small anisotropy is practically impossible to determine at low frequency, but is clearly resolved at the 2 mm band in Fig. 4.1 because of the 15 times better g-factor resolution as compared to that of the X-band. The g-factor resolution is correspondingly higher in recently constructed instruments with frequencies up to 500 GHz [10]. 

The ESR spectrum of radicals formed by illumination of photosystem II (PS II) at low temperature exhibits a single line at X-band. The D-band (130 GHz) spectrum of deuterated Mn-depleted PS n shown in Fig. 4.2 was attributed to carotenoid and chlorofyll cation radicals with slightly rhombic g-tensor components of gxx = 2.00335, gyy = 2.00251, gzz = 2.00227 and gxx = 2.00312, gyy = 2.00263, g = 2.00202, respectively [11]. 

The g-tensors of the individual species were determined by spectral simulations, and the assignments were made by comparisons of the g-tensors with data for related systems. Previously unresolvable cofactor signals in PS II could thereby be separated. It was accordingly concluded that the carotenoid and chlorophyll radicals were both generated in PS II by illumination at low temperature. 

The g-resolution can be further increased by measurements at even higher microwave frequencies and correspondingly higher magnetic fields as shown by the following recent applications. 

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