We have succeeded in observing vibrationally induced inelastic structures in the photoemission spectra of carbon monoxide or oxygen covered Cu(001) surfaces excited by laser light. The structure appears as a step at the vibrational energy below the Fermi level. We found that the CO/Cu(001) spectra contain two step structures arising from the C-O internal stretching mode and the frustrated rotation mode. These steps shift to lower energy for 13C18O. In the O/Cu(001) spectra we have observed the inelastic structures originating from not only adsorbate-substrate vibrations but also substrate surface phonons.
