Photoelectron Diffraction Imaging for $C_2{H}_2$ and $C_2{H}_4$ Chemisorbed on Si(100) Reveals a New Bonding Configuration

A new adsorption site for adsorbed acetylene on Si(100) is observed by photoelectron imaging based on the holographic principle. The diffraction effects in the carbon $1s$ angle-resolved photoemission are inverted (including the small-cone method) to obtain an image of the atom's neighboring carbon. The chemisorbed acetylene molecule is bonded to four silicon surface atoms. In contrast to the ${\mathrm{C}}_2{\mathrm{H}}_2$ case, the image for adsorbed ${\mathrm{C}}_2{\mathrm{H}}_4$ shows it bonded to two Si surface atoms.