The Si(111)2x1 is one of the most studied surfaces. Its reconstruction is described by the Pandey model with a buckling of the topmost atoms. With relation to the sign of the buckling, there are two slightly different geometric structures (isomers), conventionally named positive buckling and negative buckling. STM measurements suggest that the positive buckling isomer is the stable configuration at room temperature, but a recent work, involving STS measurements, has shown the coexistence of both the isomers, at very low temperature, for highly n-doped Si(111)2x1 specimens. There is hence the necessity to deepen the study of the negative buckling isomer, almost completely neglected in literature especially for what concerns its optical properties. In this work we have studied the structural, electronic and optical properties of both the isomers of Si(111)2x1 within the Density Functional Theory in the Local Density Approximation. Our results show that the response to light of the two isomers is sizearly different; Reflectance Anisotropy Spectroscopy would be the right experimental tool to investigate the coexistence of the two isomers.
