  Over the past 50 years, electron-nuclear double resonance (ENDOR) has become
a fairly ubiquitous spectroscopic technique, allowing the study of spin
transitions for nuclei which are coupled to electron spins. However, the low
spin number sensitivity of the technique continues to pose serious limitations.
Here we demonstrate that signal intensity in a pulsed Davies ENDOR experiment
depends strongly on the nuclear relaxation time T1n, and can be severely
reduced for long T1n. We suggest a development of the original Davies ENDOR
sequence that overcomes this limitation, thus offering dramatically enhanced
signal intensity and spectral resolution. Finally, we observe that the
sensitivity of the original Davies method to T1n can be exploited to measure
nuclear relaxation, as we demonstrate for phosphorous donors in silicon and for
the endohedral fullerene N@C60 in CS2.
