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Abstract

Using time-resolved two-photon photoemission spectroscopy, we have studied the dynamics of image potential states in an adsorbate heterolayer system on Cu(111), consisting of 1 monolayer (ML) N₂ on top of 1-2 ML Xe. The image potential states show long lifetimes of up to 1.6 ps, which is attributed to the negative electron affinity of the N₂ layer that acts as a barrier between the image-state wave functions and the metal substrate. The most striking dynamical feature is an intraband relaxation within the two-dimensional band of the first
image state, resulting in a pronounced dependence of the rise and decay times of the electron population on the wave vector parallel to the surface. As underlying mechanism of the intraband relaxation, the interaction with the Cu electron system can be excluded on the basis of theoretical estimates concerning the effect of e-e scattering and the long-range interaction envisaged as “image charge friction”. On the other hand, a threshold for the intraband decay of (6±2) meV parallel kinetic energy indicates that the process is linked
to the excitation of low-energy phonons. The most likely channel for energy transfer from image-state electrons to nuclear degrees of freedom is the excitation of frustrated rotations (librations) in the N₂ layer. This is supported by theoretical estimates for freely rotating N₂ molecules.