Near-Edge X-Ray-Absorption Fine-Structure Studies of Chemisorbed Hydrocarbons: Bond Lengths with a Ruler

Starting from an established correlation between the $\ensuremath{\sigma}$ shape resonance position in nearedge x-ray-absorption fine-structure spectra and the intramolecular C-C bond length for gas-phase hydrocarbons, we derive a similar relation for chemisorbed molecules. The linear relation obtained allows us to determine the C-C bond length for chemisorbed molecules from near-edge x-ray-absorption fine-structure spectra with a ruler to an accuracy of 0.04 \AA{}. Examples are given for bonding and bond-length changes of ${\mathrm{C}}_{2}$${\mathrm{H}}_{2}$ and ${\mathrm{C}}_{2}$${\mathrm{H}}_{4}$ on Pt(111).