Time-resolved two-color interferometric photoemission of image-potential states on Cu(100)

We describe an interferometric time-resolved photoemission technique that makes it possible to simultaneously observe the decay of optical induced polarizations and populations at surfaces in a two-color excitation scheme. In this scheme initially unoccupied electronic surface states are coherently excited by the interaction of laser pulses with frequency omega(a) and the two-photon polarization which is induced by laser pulses with frequency omega(a)/2. Interference is observed by changing the delay between both laser pulses using an actively stabilized two-color Mach-Zehnder interferometer. We demonstrate this technique for excitation of the n=1 image-potential state on a Cu(100) surface.