Two-photon photoemission (2PPE) is a variant of photoelectron spectroscopy which allows to investigate unoccupied electronic states located between the Fermi level and the vacuum level of a metal or semiconductor. A first (pump) laser pulse with photon energy ℏωa populates the unoccupied state |i>, a second (probe) laser pulse with photon energy ℏωb photoemits the electron. The measurement of its kinetic energy by
Ekin = ℏωb – Ei
provides a straightforward means to determine the binding energy Ei of the unoccupied state with respect to the vacuum level.
Dynamical information can be obtained with time-resolved 2PPE. The intensity recorded from a given intermediate state as a function of the variable delay between the sufficiently short pump and the probe pulses is determined by the lifetime of the intermediate state.
Both, spectroscopic and time-resolved 2PPE from surfaces benefit from angle-resolved detection of the photoelectrons. The photoemission process preserves the parallel momentum of the electron. It is simply given by
Since electronic states of well-ordered surfaces are fully characterized by their energy and parallel momentum, time- and angle-resolved 2PPE are able to provide extremely detailed information about the time-evolution of electronic excitations from two-dimensional systems.
Further expansion of the capabilities of 2PPE is possible by detecting the spin of the photoelectrons and by exploiting coherence phenomena.