Femtosecond Photoemission Studies of Electron Relaxation at Cu Surfaces

Abstract

A number of recent studies have illustrated the importance of photoexcited electrons in surface reactions at metal surfaces. This work focuses on directly determining the relaxation dynamics for photoexcited electrons at Cu(100) surfaces. This particular surface is important as recent observations for photoinduced desorption of CO have found quantum yields as high as 15% for the hot electron reaction channel at high laser fluences.1 Equal pulse correlation studies employing 3 eV pulses (50-60 fs) with both parallel and crossed polarization and low energy (<100 nJ/cm2) have been used to characterize the intrinsic electron relaxation dynamics in the absence of excess lattice heating or transient collective effects. This low energy limit corresponds to photoinduced surface chemistry employing conventional low flux CW light sources such as arc lamps. These results are shown in Figure 1. The inset shows the observed photoemission spectrum. The highest energy electrons give a pulse width limited response function which can be used to deduce the slower relaxation dynamics of the lower energy electrons. The slower relaxation dynamics are clearly resolved as a broadening in the FWHM of the two pulse correlation of the photoemitted electron at specific energies. The deconvolved relaxation times for the electron energy distribution are shown in which the solid line through the data is a fit to a calculation based on Fermi liquid theory for electron relaxation scaled by a factor of 2.45. The extent of this agreement for the electron relaxation dynamics at a single crystal surface is similar to the relaxation dynamics observed previously for polycrystalline gold surfaces at higher fluences.2 At lower energies above EF, hot electron transport effects become noticeable and can be understood in terms of a ballistic transport model. These results fully characterize the non-radiative relaxation of electrons at low fluence and peak power.