Control of the Dephasing of Image-Potential States by CO Adsorption on Cu(100)

The study of the dynamics of electrons at metal surfaces is important for the understanding of photon or electroninduced chemical reactions in adsorbate overlayers [1]. Recent progress has been advanced by the generation of ultrashort laser pulses [2]. Two-photon photoelectron spectroscopy of image-potential states [3] permits the detailed study of the electron dynamics in the energy as well as in the time domain [4]. The results show a complex behavior of the measured decay time as a function of the energy analyzer detuning from the main peak and of the linewidth as a function of time delay between the two photon pulses [5]. A consistent description can be reached by the use of the Liouville-von Neumann equations [6,7]. The main parameters entering this picture are the true lifetime t of the image-potential state as well as the pure dephasing times T of the initial and intermediate states. The lifetime describes the decay of the population by inelastic scattering events and is measured in time-resolved experiments. Additional pure dephasing can be attributed to quasielastic scattering processes which do not change the population of the image-potential state but destroy the coherence between the involved states. This increases the measured energy width G compared to the value expected from the lifetime hyt. These processes are indicated in Fig. 1 together with the excitation scheme of two-photon photoemission. The true lifetime is understood fairly well theoretically by the decay of the image-state electron [8,9]. The results are closely related to the properties of hot electrons, because the decay of the image-potential states occurs mainly into metal bulk states. Using temperature-dependent measurements the quasielastic scattering by phonons could be identified and the results indicate bulklike values weighted by the probability of finding the electron inside the metal [7]. In this Letter we address surface-specific scattering processes induced by defects on the surface. As a model system we chose the adsorption of CO on Cu(100). This system shows a cs2 3 2d phase at half a monolayer (ML) coverage and a cs7 p 2 3 p 2 dR45± compression phase at