Spectroscopic evidence against tetra-σ adsorption configurations of C2H2 on Ge(1 0 0)

► We study C2H2 adsorption on Ge(1 0 0) at room temperature using UPS. ► In the UPS spectra, molecular orbitals of the C2H2 along with Ge-C bonding states are identified. ► After partial desorption of the adsorbed C2H2 molecules, all the MOs remain intact. ► Persistence of the πC-C orbitals is firm evidence against the tetra-σ bonded structure.