X-ray studies of self-assembled monolayers on coinage metals. 3. Angularly resolved near edge X-ray absorption fine structure determination of the orientation in 1-octanethiol SAMs on Ag(111) and Cu(111)

Angularly resolved near-edge X-ray absorption fine structure (NEXAFS) at the sulfur K-edge was used to probe the orientation of the S-C bond, with respect to the surface normal, in self-assembled monolayers (SAMs) of 1-octanethiol, of molecular formula CH3(CH2)(7)SH, on Ag(111) and Cu(111) formed in ultrahigh vacuum. For the first time, a structural difference is identified between SAMs on Ag and Cu. The S-C polar angle was determined to be 23 +/- 3 degrees on Cu(111) and 39 +/- 4 degrees on Ag(111) highlighting a structural difference invisible to the Fourier transform infrared (FTIR) and Raman techniques. The observed difference is explained in terms of the increased ionicity in the S - Cu bond and the relative strengths of chemisorption and interchain van der Waals interactions. The structural information derived from NEXAFS is related to that commonly obtained using FTIR and Raman methods in order to rationalize and define the adsorbate structures of alkanethiols self-assembled on Cu, Ag, and Au. A detailed interplay of adsorbate-substrate and pendant hydrocarbon chain-chain interactions is responsible for the adsorbate structures observed on Au(111), Ag(111), and Cu(111).