  We performed STM-ENDOR experiments where the intensity of one of the
hyperfine components detected in ESR-STM is recorded while an rf power is
irradiated into the tunneling junction and its frequency is swept. When the
latter frequency is near a nuclear transition a dip in ESR-STM signal is
observed. This experiment was performed in three different systems: near
surface SiC vacancies where the electron spin is coupled to a next nearest
neighbor 29Si nucleus; Cu deposited on Si(111)7x7 surface, where the unpaired
electron of the Cu atom is coupled to the Cu nucleus (63Cu, 65Cu) and on Tempo
molecules adsorbed on Au(111), where the unpaired electron is coupled to
Nitrogen nucleus (14N). While some of the hyperfine values are unresolved in
the ESR-STM data due to linewidth we find that they are accurately determined
in the STM-ENDOR data including those from remote nuclei, which are not
detected in the ESR-STM spectrum. Furthermore, STM-ENDOR can measure single
nuclear Zeeman frequencies, distinguish between isotopes through their
different nuclear magnetic moments and detect quadrupole spectra. We also
develop and solve a Bloch type equation for the coupled electron-nuclear system
that facilitates interpretation of the data. The improved spectral resolution
of STM - ENDOR opens many possibilities for nanometric scale chemical analysis.
