Probing ultrafast electronic motions in atoms with the attosecond pump-probe

Through full-dimensional numerical simulations with using our recently-developed efficient and accurate parallel solver for the time-dependent Schr\"odinger equation, we have demonstrated that an attosecond pulse can effectively {\it probe} the extremely fast motion of an electronic wave packet in atoms. Pumped by a broadband femtosecond UV pulse, one electron of ground-state Helium can be launched into a superposition of low-lying excited states, thus forming a wavepacket that begins to orbit the atomic core. A time-delayed attosecond EUV pulse (probe) then ionizes the atom causing three-body breakup. Measuring either the energy sharing of the ionized electrons or the total ionization probability as a function of the time delay displays the internal motion of the excited electron. Our simulation has shown that an ultrashort Kepler period of 2 $fs$ can be followed for several cylces. This opens the prospect of a wealth of similar pump-probe experiments to examine {\em{electronic}} motion.