Wave-packet excitation and quantum-beat spectroscopy of image-potential states

摘要

Abstract: The ultrafast dynamics of electrons in image-potential states on a Cu(100) surface is studied by means of femtosecond time-resolved two-photon photoemission (2PPE). By coherently exciting several eigenstates of the Rydberg series we observe periodic oscillations of the 2PPE signal as a function of the delay time between pump and probe pulses. These quantum beats allow us to determine the spacing of high-order states (quantum number n $GREQ 4) that are difficult to resolve by conventional electron spectroscopy. The superposition of several states around n $EQ 7 creates an electron wave packet that describes the quasi-classical periodic motion of weakly bound electrons. Its distance from the surface is reflected in the strength of the photoemission signal. The electron is observed to move about 100 atomic distances away from the surface and oscillates back and forth with a period of 800 femtoseconds. The results demonstrate the power of coherent laser spectroscopy for surface studies.!25