Photoemission from a solid surface provides a wealth of information about the
electronic structure of the surface and its dynamic evolution. Ultrafast
pump-probe experiments are particularly useful to study the dynamic
interactions of photons with surfaces as well as the ensuing electron dynamics
induced by these interactions. Time-resolved laser-assisted photoemission
(tr-LAPE) from surfaces is a novel technique to gain deeper understanding of
the fundamentals underlying the photoemission process. Here, we present the
results of a femtosecond time-resolved soft X-ray photoelectron spectroscopy
experiment on two different metal surfaces conducted at the X-ray Free-Electron
Laser FLASH in Hamburg. We study photoemission from the W 4f and Pt 4f core
levels using ultrashort soft X-ray pulses in combination with synchronized
infrared (IR) laser pulses. When both pulses overlap in time and space,
laser-assisted photoemission results in the formation of a series of sidebands
that reflect the dynamics of the laser-surface interaction. We demonstrate a
qualitatively new level of sideband generation up to the sixth order and a
surprising material dependence of the number of sidebands that has so far not
been predicted by theory. We provide a semi-quantitative explanation of this
phenomenon based on the different dynamic dielectric responses of the two
materials. Our results advance the understanding of the LAPE process and reveal
new details of the IR field present in the surface region, which is determined
by the dynamic interplay between the IR laser field and the dielectric response
of the metal surfaces.Comment: 18 pages, 3 figure