Time-resolved coherent spectroscopy of surface states

Ultrafast electronic coherence phenomena of metallic image-potential states and of silicon dangling-bond states are discussed. The image states were investigated on a Cu(100) surface by means of time-resolved two-photon photoemission (2PPE). Spectroscopical and dynamical information about states with high quantum numbers (n greater than or equal to 4) were obtained by the coherent excitation of several eigenstates of the Rydberg series and detection of the resulting quantum beats. Electron wave packets that describe the quasi-classical periodic motion of weakly bound electrons at metal surfaces could be created by the superposition of several eigenfunctions around n = 7. A surface-sensitive purely optical technique is introduced for the investigation of rapid dephasing processes of excited dangling-bond states of Si(111) 7 x 7. The measurement of the diffracted second-harmonic signal from a transient population grating yielded decoherence times of about 5 fs.