Zeeman Dependence of Hyperfine Spectra

Thus far, we have established that the nuclear hyperfine parameters of the spin Hamiltonian are desirable for assessing the chemically interesting problem of structure-funetion eorrelation and reaction control. The advanced EMR methods known as ENDOR and ESEEM best recover this information from samples in which the ehemieal agent of interest is paramagnetic, and, in principle, there are methods that enable the spectroscopist to cope with the sometimes pathologieal behavior of spin systems, in other word, coax a spectrum out of a sample. In this section, however, we shall address the question of whether there is neeessary and sufficient information in a single ENDOR or ESEEM spectrum and how to design an experimental approaeh that enables one to fully parameterize the spin Hamiltonian. 

Multi-Frequency Measurements and Energy Level-Crossing as an Interpretative Aid 

Besides the ambiguity of fitting simulated to experimental spectra, the instrumental limits (most conventional microwave sources — klystrons, Gunn diodes, etc. — operate only a limited band width within the octave, that is, they are not tunable) of the one frequency per octave approach does not assme one of being able to obtain the requisite number of spectra to begin with. For example, it is highly problematic to record cw-ENDOR of weakly coupled N at the X-band 

Discrete ly fliS iaI + y icS ca is rewritten in a form that reflects the state admixture  

Terms fj and gjx represent expectation values (e.g., oscillator strengths) of the excitation and relaxation process, respectively. It follows that, in general, the transition rate under the condition of crossing levels is greater than the discrete, which follows from Cauchy s theorem. 

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