Vibrational energy loss structure in very low energy photoelectron spectra of CO/Cu(001) surface

We have measured the very low energy photoelectron spectra of the CO/Cu(001) surface by using low energy photons (4.821 eV) as the excitation light source. The spectra for $^{12}$C$^{16}$O show a step in the photoelectron intensity at 254 meV below the Fermi level. An isotropic shift to 240 meV was observed for $^{13}$C$^{18}$O. These energies agrees with the vibrational energies of the C-O stretching mode. Furthermore such spectral feature did not appear for the clean Cu(001) surface. Thus we conclude that the observed step in the photoelectron spectra arises from the energy loss of photoelectrons through excitation of the vibrational mode.