Relative phase measurement of a stark wave packet in the vicinity of the saddle point

A gas of Rb atoms in a static electric field has been photoexcited to just above the classical ionization threshold by a phase-locked sequence of two far infrared pulses. A single laser pulse generates a series of ejected ele ctron packets emerging at the saddle point of the potential; each of the ej ected packets is characterized by a phase and a chirp. We calculate and mea sure these phases and chirps using the time dependent interference of the e lectronic wave function controlled by the delay between the two light pulse s.