Optical response of pi-conjugated molecular monolayer adsorbed on the semiconductor Si(001) surface: A first-principles study

The modification of the semiconductor-surface optical response is investigated for the prototypical system of 9,10-phenanthrenequinone adsorbed on Si(001). By means of first-principles calculations we reproduce and explain recently measured reflectance-anisotropy data. The detailed analysis of the optical signal shows that intramolecular pi-pi* transitions as well as adsorption-modified Si bulk states contribute to the surface spectrum. The calculations illustrate the sensitivity of optical spectroscopy to molecular adsorption, but demonstrate clearly that a naive interpretation of the spectrum in terms of gas-phase molecular data fails.