Nature Phys. 10, 505–509 (2014)

Although excitons, which are electron–hole pairs bound by the Coulomb potential, are expected to arise in metallic systems, it is non-trivial to experimentally observe these quasiparticles in metals because of the screening effect, which occurs on a femtosecond timescale. Now, Xuefeng Cui and co-workers in the USA and Croatia have reported evidence for a transient excitonic response on a silver surface in the time window preceding the full onset of screening. The response was revealed by interferometric pump–probe delay scanning and photoelectron imaging. A multiphoton photoemission process was excited by a non-collinear parametric amplifier system pumped by a fibre-laser oscillator–amplifier system (average power, 80–100 mW; repetition rate, 1.25 MHz, pulse length, 15 fs). The pulses were focused onto the silver sample at an incidence angle of 45° and with transverse magnetic polarization. Measurements were performed with the single-crystal silver cooled to 100 K. The team notes that the transient exciton response of the metallic system may help explain multi-exciton generation in organic films. Additionally, strong electron–hole correlations shown in the metal system may fuel studies of non-equilibrium quasiparticle dynamics in strongly correlated materials.