Single-molecule reactions and spectroscopy via vibrational excitation

Inelastic tunnelling spectra of single C4 hydrocarbon molecules adsorbed on the Pd(110) surface are presented. Experimental evidence is given that the symmetry of the. molecular orbital into which the tunnelling electron first enters determines which vibrational modes are excited. The action spectrum for cis-2-butene exhibits most of the vibrational modes that are expected to be excited except for v (C = C), which may be because the molecule is pi bonded to the substrate, thus making the lifetime of the excited state short.