Ultrafast electron dynamics in metals: Real-time analysis of a reflected light field using photoelectrons

We propose an approach to address ultrafast charge-carrier dynamics of metals by analyzing the momentum change in photoelectrons interacting with a transient optical grating at a metal surface. Photoelectrons are excited by an ultraviolet femtosecond laser pulse which precedes an infrared pulse setting up the transient grating. We measure the kinetic energy of the photoelectrons which are accelerated by the grating's ponderomotive potential and thus sample the respective electric field. The method is capable to access phase and amplitude differences between the incoming and the reflected light fields. The latter is determined by the response of the conduction-band electrons to the light field. We report on a demonstration of such an experimental scheme using a Gd(0001) surface and calculate the reflected field by a simplified transport equation. We derive a method to determine the average electron-scattering rate and study the time-dependent evolution of amplitude and phase of the reflected electric field including memory effects in the optically induced polarization dynamics. Finally, we discuss the required steps of this approach to probe ultrafast dynamics in metals experimentally.