Direct Evidence of a Stretched C-C Distance for C2H2 and C2H4 on Cu(100) at 60 K

Abstract

Surface extended x-ray-absorption fine-structure measurements have been performed on ${\mathrm{C}}_2$ ${\mathrm{H}}_2$ and ${\mathrm{C}}_2$ ${\mathrm{H}}_4$ on Cu(100) at 60 K. Clear evidence is found for the elongation of C-C bonds of 0.13(7) Å for ${\mathrm{C}}_2$ ${\mathrm{H}}_4$ and 0.22(5) Å for ${\mathrm{C}}_2$ ${\mathrm{H}}_2$. This is the first direct evidence from an appropriate structural probe for the stretching of molecules upon chemisorption. It solves a long-standing debate and helps to calibrate other indirect information on intramolecular bond lengths as taken from vibrational spectroscopy and shifts of the $\sigma$-shape-resonance position.