The diagram shows the energy levels for the simplest system, for example a hydrogen atom with one one electron
(S=ω) and a nuclear spin (I=ω). The two electron spin and nuclear spin eigenfunctions
are subscripted accordingly e and n. Since (to first order), when the electron changes spin state, the nucleus does not,
then there are two EMR transitions, between levels 1 and 4 and levels 2 and 3. The intensity of, say, the 1 to 4 transition
depends on the population difference between these two levels, so that applying a nuclear frequency between either
4 and 3 or 2 and 1 will cause spin flips and changes in the populations of 4 or 1. Hence a change in intensity of the
observed EMR signal. This is ENDOR. In ELDOR two EMR frequencies are used, one to observe, the other (higher one)
to saturate, and since
there are no common energy levels between the observed and pumped transitions, the population changes are carried by
spontaneous nuclear spin flips.
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Types of Double Resonance
- ENDOR & ELDOR
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