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Time-resolved two-photon photoemission; ultrafast electron dynamics; Cu(111); adsorbates; electronic surface states; image potential states; dielectric continuum model; intraband relaxation
Abstract:
The electron dynamics of the Cu(111) surface with the physisorbed adsorbates Xe, N2, and O2 and the chemisorbate benzene have been studied with time-resolved two-photon photoemission (2PPE) spectroscopy.
The unoccupied electronic states in the physisorbate systems resemble the image potential states of the clean metal surface. They are very sensitive to the adsorbed species and the number of adsorbed layers, as shown for Xe and N2. The influence is due to the continuum properties of the adsorbates (the electron affinity, the dielectric constant, and the conduction band dispersion) as described by the dielectric continuum model.
The studies are extended to the heterolayer systems N2/Xe and O2/Xe on Cu(111). The lifetime of the first unoccupied state of O2/Xe/Cu(111) displays a resonance-like dependence on the number of Xe spacer layers. This is attributed to coupling with the lowest unoccupied O2 orbital. The unoccupied states in N2/Xe/Cu(111) are image potential states with long lifetimes of up to 1.6 ps. In addition to the decay to the substrate bulk they exhibit intraband relaxation parallel to the surface. This is attributed to the excitation of low-energetic N2 vibrations and is one of the first examples for energy transfer from electrons in image potential states to nuclear coordinates of adsorbates.
In benzene on Cu(111), the image potential states display a pronounced dependence on the morphology of the C6H6 layers. An unoccupied state, which acts as a final state in the 2PPE process, is assigned to the b2g level of benzene.
It is additionally demonstrated that the exact excitation pathway of the 2PPE process can be identified by the dependence of the signal on the laser light polarization. Analytic expressions for the spectra and the temporal behavior have been developed for various scenarios of direct excitation where additional scattering processes are negligible.
