A scanned-energy mode photoelectron diffraction study of the surface with adsorbed has provided quantitative determination of key structural parameters previously only predicted from theoretical calculations. The N atoms are found to occupy off-atop sites at a dimerized surface Si atom with an N - Si bondlength of and bond angle relative to the surface normal of . The positions of Si atoms in the dimer to which the adsorbates are bonded are found to indicate that the marked asymmetry of this dimer on the clean surface is lost as a result of the adsorption. The conclusions are discussed in relation to published results from less direct experimental probes and the results of theoretical model calculations.