  Excitons, electron-hole pairs bound by the Coulomb potential, are fundamental
quasiparticles of coherent light-matter interaction energizing processes from
photosynthesis to optoelectronics. Excitons are observed in semiconductors, and
their existence is implicit in the quantum theory of metals, yet their
appearance is tenuous due to the screening of the Coulomb interaction on few
femtosecond timescale. Here we present direct evidence for the dominant
transient excitonic response at a Ag(111) surface, which precedes the full
screening of the Coulomb interaction, in the course of a three-photon
photoemission process with <15 femtosecond laser pulses. Electron-hole pair
interaction through the excitonic response introduces coherent quasiparticle
correlations beyond the single-particle description of the optics of metals,
which dominate the multi-photon photoemission process.
