Atomic Physics Seminars

We have used ultrashort, nearly unipolar "half cycle" electric field pulses (HCPs) as a tool to measure the time-dependent momentum distribution of electrons in a Stark induced continuum. Ca atoms in an external field are excited from the ground state into an intermediate 4s4p state with a nanosecond dye laser. A 1 ps laser pulse with a bandwidth of approximately 20cm^-1 promotes the 4s4p atoms to the field induced continuum, with an energy just above the saddle point in the Stark potential. At a variable time delay Delta-t following the excitation of the continuum wavepacket, the system is exposed to a HCP which imparts a non-zero linear momentum "kick". The portion of the probability distribution moving antiparallel to the applied kick suffers a reduction in its total energy. The fraction of probability amplitude whose energy is below the saddle-point forms a bound wavepacket. Because the recombination probability depends on the kick strength and distribution of momentum along the kick direction, the time-dependent momentum distribution of the continuum wavepacket can be recovered from measurements of recombination probabilities versus kick strength, orientation of the momentum kick, and time delay Delta-t.