  Stable organic radicals serve as model systems for investigating metal-free
magnetic phenomena at (sub)nanometer length scales. We have investigated at the
single-molecule level the stable $\pi$-radical 2,2-diphenyl-1-picrylhydrazyl
(DPPH) adsorbed at the elbow sites of the $22\times\sqrt{3}$ reconstructed
Au(111) surface. Low-temperature scanning tunneling microscopy (STM) and
-spectroscopy under ultrahigh vacuum conditions at 8~K reveal structural as
well as frontier-orbital related electronic details of DPPH/Au(111). A
Kondo-like spectroscopic signature indicates preservation of the unpaired
electron spin state.
