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 decoded at nu(mod). The sensitivity of this procedure can be increased by incoherently broadening the bandwidth of the applied RF through mixing of the RF carrier signal with a white-noise source of variable bandwidth. This technique has been explored by monitoring the amplitude and width of ENDOR signals as a function of the RF bandwidth and power, in the case of the Fe-57 signals from a metalloprotein and N-14, H-1 signals from two Cu(II) compounds. The RF band broadening has produced signal enhancements of over threefold. The results are interpreted in terms of a competition between (i) an increase in the number of spin packets excited within the inhomogeneously broadened ENDOR line and () a reduction in the response per packet. Simple analysis leads to equations for the variation in the ENDOR response with incident RF power and bandwidth that are scaled by a saturation RF power and an effective spin-packet width, respectively. (C) 1994 Academic Press, Inc.