  We report the detection of electron spin resonance (ESR) in individual dimers
of the stable free radical 2,2,6,6-tetramethyl-piperidine-1-oxyl (TEMPO). ESR
is measured by the current fluctuations in a scanning tunnelling microscope
(ESR-STM method). The multi-peak power spectra, distinct from macroscopic data,
are assigned to dimers having exchange and Dzyaloshinskii-Moriya interactions
in presence of spin-orbit coupling. These interactions are generated in our
model by interfering electronic tunneling pathways from tip to sample via the
dimer's two molecules. This is the first demonstration that tunneling via two
spins is a valid mechanism of the ESR-STM method.
