Derivative spectra of very low energy photoelectrons from CO/Cu(001) surface obtained by a lock-in technique

We have constructed a system for measuring the derivative spectra of very low energy photoelectrons using a lock-in technique. The main advantage of this system lies in determining the energy position of steps in the photoelectron spectra that arise from inelastic scattering of photoelectrons through the excitation of surface vibrational modes. With this system we have measured the derivative spectra of two isotopes, ¹²C¹⁶O and ¹³C¹⁸O adsorbed on Cu(001), and observed the small isotopic shift of ∼1 meV. This result demonstrates that one can precisely determine the vibrational energy of surface adsorbates by using the present measurement system.