“Anomalous” photoelectric effect in the ultrafast electron diffraction method

Electron-pulse probing of fast laser-induced processes has allowed the direct observation of the structural dynamics in matter with a high spatiotemporal resolution. A thin gold film has appeared to be a convenient photocathode, and photoelectron emission has been induced by femtosecond ultraviolet radiation with a photon energy of about $4.65$–$4.75$ eV (in particular, $\hbar\omega\cong 4.65$ eV for the third harmonic of the Ti:sapphire laser). For the linear photoelectric effect, this energy contradicts the reference work function $W_{\text{Au}}\cong 5.1$–$5.3$ eV of pure metal. Reasons for such contradiction have been analyzed and good agreement with experimental data has been reached with a model proposed for the generation of photoelectron pulses.