We have measured the very low energy photoelectron spectra of the clean and CO covered Cu(0 0 1) surfaces at low temperature (15 K). A step at 34.5 meV below the Fermi level appeared in the CO/Cu(0 0 1) spectra, while no structure was found at the same position in the clean Cu(0 0 1) spectrum. Furthermore, we have succeeded in measuring the spectral shift of the step to 33.2 meV below the Fermi level by isotopic substitution of the CO molecule. We have concluded that this step originates from the inelastic scattering of photoelectrons through excitation of the frustrated rotation mode of the CO molecule.
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