A two-dimensional sum combination frequency pulse EPR experiment

A two-dimensional electron-spin echo envelope modulation method for measuring sum combination frequencies is introduced. The approach is free of blind spots and is distinguished by diagonal- and cross-peaks in the two-dimensional spectrum that correlate basic frequencies and combination frequencies of nuclear spin transitions. Analytical formulae for the description of the experiments are derived and the features predicted by theory are confirmed by experiments on ordered and disordered spin systems.