Transient excitons at metal surfaces

Excitons, electron-hole pairs bound by the Coulomb potential, are the fundamental quasiparticles of coherent light-matter interaction relevant for processes such as photosynthesis and optoelectronics(1-5). The existence of excitons in semiconductors is well established(2). For metals, however, although implied by the quantum theory of the optical response, experimental manifestations of excitons are tenuous owing to screening of the Coulomb interaction taking place on timescales of a few femtoseconds(6-8). Here we present direct evidence for the dominant transient excitonic response at a Ag(111) surface, which precedes the full onset of screening of the Coulomb interaction, in the course of a three-photon photoemission process with similar to 15 fs laser pulses. During this transient regime, electron-hole pair Coulomb interactions introduce coherent quasiparticle correlations beyond the single-particle description of the optics of metals that dominate the multi-photon photoemission process on the timescale of screening at a Ag(111) surface.