Field-swept and frequency-swept ELDOR and the manifestation of rapid-passage effects

Field-swept and frequency-swept electron-electron double resonance (ELDOR) have been quantitatively compared as a function of temperature and of modulation frequency (ωm). At ωm ? 1 kHz the ELDOR reduction factor for field-swept and frequency-swept experiments, R (H) and R (ν)o, respectively, show the normal inverse dependence on temperature. R (H) is about 2 to 4 times greater than R (ν)o for either the ΔMI = 1 or 2 transition of inhomogeneously broadened lines of a nitroxide radical. At 100 MHz modulation, ELDOR shows rapid-passage effects. The maximum frequency-swept signal lags the modulation field by 90° and tends to decrease with the decrease of temperature at low temperature. The results are interpreted in terms of present theories of ELDOR.