Crossing, intersystem spin-free levels

Such localized states as under discussion here may arise in a system with local permutational symmetries [Aa] and [AB], If [Aa] + [S] and [Ab] = [5], the outer direct product [Aa] 0 [AB] gives rise to a number of different Pauli-allowed [A], If the A and B subsystems interact only weakly, these different spin-free [A] levels will be closely spaced in energy. The extent of mixing of these closely spaced spin-free states under the full Hamiltonian, H = HSF + f2, may then be large. Thus, systems which admit a description in terms of local permutational symmetries may in some cases readily undergo spin-forbidden processes, such as intersystem crossing. 

The theory of CIDNP depends on the nuclear spin dependence of intersystem crossing in a radical (ion) pair, and the electron spin dependence of radical pair reaction rates. These principles cause a sorting of nuclear spin states into different products, resulting in characteristic nonequilibrium populations in the nuclear spin levels of geminate (in cage) reaction products, and complementary populations in free radical (escape) products. The effects are optimal for radical parrs with nanosecond lifetimes. 

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