SENSITIVITY ENHANCEMENT IN FIELD-MODULATED CW ENDOR VIA RF BANDWIDTH BROADENING

In low-temperature ENDOR studies it is common to modulate the magnetic field at nu(mod) similar to 100 kHz and to observe the ENDOR response as a change in the dispersion-mode rapid-passage EPR signal as decode d at nu(mod). The sensitivity of this procedure can be increased by in coherently broadening the bandwidth of the applied RF through mixing o f the RF carrier signal with a white-noise source of variable bandwidt h. This technique has been explored by monitoring the amplitude and wi dth of ENDOR signals as a function of the RF bandwidth and power, in t he case of the Fe-57 signals from a metalloprotein and N-14, H-1 signa ls from two Cu(II) compounds. The RF band broadening has produced sign al enhancements of over threefold. The results are interpreted in term s of a competition between (i) an increase in the number of spin packe ts excited within the inhomogeneously broadened ENDOR line and () a re duction in the response per packet. Simple analysis leads to equations for the variation in the ENDOR response with incident RF power and ba ndwidth that are scaled by a saturation RF power and an effective spin -packet width, respectively. (C) 1994 Academic Press, Inc.