The origin of inter-dimer-row correlated adsorption for NH3 on Si(0Â 0Â 1)

We discuss the interaction between adsorbing ammonia molecules and pre-adsorbed ammonia fragments on the Si(0 0 1) surface, searching for experimental evidence of a H-bonded precursor state predicted by modelling. While correlations along dimer rows have already been identified, these mix substrate-mediated effects due to dimer buckling with ammonia-adsorbate effects. Correlations between fragments on neighbouring dimer rows are not affected by substrate effects (in this system), allowing an analysis of direct ammonia-adsorbate effects. We present an analysis of cross-row correlations in existing high-coverage STM data which shows significant correlations between NH2 groups on neighbouring dimer rows over a significant range, providing evidence for the H-bonded precursor state with a range of around 10 Å. We discuss implications for the interpretation of STM images of ammonia on Si(0 0 1).