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The excited electron dynamics in ytterbium is investigated by means of the time-resolved two-photon photoemission spectroscopy and ab initio GW+ T-matrix approach. We show that the standard GW approach with core 4f orbitals fails to describe the experimental energy trend of the lifetimes. However, a fine agreement with the experimental data is achieved within the GW+ T-matrix approach when 4f orbitals are included in a valence basis set. In contrast to previous scanning tunneling spectroscopy data, we show that at small excitation energy, the energy dependence of lifetimes strongly deviates from that for the free-electron gas. We also argue that the e-ph coupling in Yb should be weak.Understanding the dynamics of hot electrons in metals is a fundamental issue for the interpretation of many physical and chemical phenomena, for example, chemical reactions on surfaces, transport, and molecular-surface interactions. 1 This subject attracted great interest both from the experimental 2-5 and theoretical points of view. [6][7][8][9] Up to now, noble and transition metals have primarily been under investigation. In these systems, the key role in the low-energy electron relaxation dynamics is played by the electronic structure of the system close to the Fermi level. For example, in transition metals, the high density of d bands in the proximity of the Fermi level leads to a very fast electron relaxation. 10 In this context, the exact energy position and the shape of the d bands have to be considered in detail in order to achieve a complete understanding of the relaxation processes. 11 Compared to d-electron metals, a basic knowledge of more complex systems such as rare earth ͑RE͒ metals is still lacking. Experimentally, only a few investigations of lifetimes of excited electrons and holes in rare earth metals have been published. 4,12-15 In Refs. 12-15, the surface states of RE's were studied. The only attempt to elucidate the lifetimes of electrons in a bulk 4f metal has been undertaken by Wegner et al. 4 who have extracted the lifetimes from scanning tunneling spectroscopy ͑STS͒ measurements of the linewidth of quantum-well states in thin Yb͑111͒ films. The authors attributed these lifetimes to an Yb d-bulk state. One of the issues of Ref. 4 was the ͑E − E F ͒ 2 dependence of the lifetime broadening of this state, which is a feature of the electron dynamics in a free-electron gas. This surprising result suggests that 4f states of Yb play negligible role in the low-energy electron relaxation dynamics. However, the energy of the 4f 7/2 states in Yb is only about −1.4 eV with respect to the Fermi energy. 16 A priori, it is not clear if the 4f states located in proximity to the Fermi level can affect polarization ͑via creation of electron-hole pairs͒ and consequently lifetimes of low-energy excited electrons. Hence, new efforts are desirable in order to elucidate the applicability of the free-electron gas theory and the impact of the 4f states on electron dynamics in bulk Yb. Another issue of Ref. 4 was the unusually stron...

In this paper we give a detailed analysis of the difference between the lifetimes of Yb quantum-well states measured by scanning tunneling spectroscopy (STS) in Wegner et al (2005 Phys. Rev. Lett. 94 126804) and the lifetimes of bulk Yb obtained by means of time-resolved two-photon photoemission spectroscopy (TR-2PPE). In particular, we show that in spite of a seeming disagreement with the TR-2PPE measurements, the inelastic lifetimes yielded in STS experiments are close to the inelastic lifetimes of bulk states, emphasizing the complementarity of both methods. Our approach is supported by ab initio electron self-energy calculations performed within the GW and GW+T approximations. Moreover we analyze the impact of the 4f-states on the lifetimes. We show that the GW-term of the inverse lifetime (linewidth) is markedly smaller than the experimental linewidth. The agreement with experimental data is recovered when the T-matrix is included in the linewidth calculations.