Time-resolved two-photon photoemission (2PPE) combines laser-pump-probe techniques with angle-resolved photoelectron spectroscopy. The method has unique capabilities in the investigation of unoccupied electronic states at surfaces and interfaces and it allows for time-domain investigations of electronic decay and transfer processes with femtosecond resolution.
I will illustrate the capabilities of 2PPE and the type of information that can be obtained with several examples from our recent work. This includes a study of the Dirac state and other unoccupied states of the p-doped topological insulators Sb₂Te₃ andSb₂Te₂Se , the characterization of interface states between graphene monolayers and Ru(0001) and the evolution of the occupied Shockley surface state of Ag(111) into an unoccupied metal/organic hybrid state at the interface to organic semiconductors.
When an electron at a metal surface is excited to an energy that is resonant with unoccupied bulk continuum states, the corresponding electron wave packet is able to delocalize into the bulk without undergoing any scattering processes. I will demonstrate with 2PPE experiments of Al(100) image-potential resonances that this type of delocalization process can be hindered by quantum mechanical interference effects and isslower than anticipated. Furthermore, by combining 2PPE with a coherent control scheme, I will show that it is possible to induce ultrashort current pulses at surfaces and detect their decay on a femtosecond time scale directly by monitoring the temporal evolution of the electron distribution in momentum space.
Finally, I will address limitations of 2PPE due to the use of photoemission probe pulsesin the visible or ultra-violet range and discuss opportunities and challenges arising from the utilization of high laser harmonics generated in rare gases.