Davies ENDOR revisited: Enhanced sensitivity and nuclear spin relaxation
  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. 
Alexei M. Tyryshkin and John J. L. Morton and Arzhang Ardavan and S.   A. Lyon