Transient excitons at metal surfaces

TL;DR

This paper provides direct evidence of transient excitons at a silver surface, showing how electron-hole interactions influence multi-photon photoemission before Coulomb screening fully occurs, revealing new quasiparticle dynamics.

Contribution

It demonstrates the existence of transient excitonic responses at metal surfaces during ultrafast photoemission, a phenomenon previously implicit in theory but not directly observed.

Findings

Transient excitonic response observed at Ag(111) surface

Electron-hole interactions affect multi-photon photoemission

Excitonic effects dominate before Coulomb screening completes

Abstract

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.