Unipolar "half-cycle" electric field pulses (HCPs) have been used to recombine free electrons and calcium ions. The field assisted process is very similar to controlled three-body recombination in plasmas. We report on experiments that utilize HCP assisted recombination to probe the probability distribution of continuum electron wave packets and produce bound wave packets that are highly localized in three spatial dimensions.The combination of free electrons and ions to form neutral atoms is a complicated process that can proceed through a variety of different mechanisms including dielectronic, radiative, or three-body recombination [1]. Al though these mechanisms are quite different in detail, in general, recombination is made possible through the trans fer of energy and momentum from the free electron to a third body. The recombination process is inherently time dependent, and the capture of a free electron can result in the formation of a complicated coherent superposition state within each atom. Because of the random nature of electron-ion scattering events in the laboratory, the bound wave packet produced via recombination varies widely from one atom to another due to the different scattering conditions through which each was formed. Consequently, standard theoretical and experimental studies parametrize the process using time-independent recombination rates and branching ratios for capture into various excited states of the atom [1].The experiments described in this Letter are aimed at studying controlled electron-ion recombination in the time domain. The results of these experiments provide new in formation on coherent recombination processes as well as demonstrate the utility of field assisted recombination for probing continuum electron dynamics and producing novel bound wave packets. Specifically, subpicosecond "half cycle" field pulses (HCPs) [2,3] have been used to assist the recombination of a well-characterized continuum wave packet with its parent ion. Although the recombination can be formally classified as radiatively assisted [1], the unique nature of the unipolar field pulse makes the process more closely related to three-body recombination. To make the analogy with three-body recombination the HCP field is equated with the transverse field produced by a passing ion or electron. The impulse provided by the field [4,5] extracts momentum and energy from the free electron, fa cilitating its capture by an ion.In the experiments, a 1.5 psec laser pulse photoionizes a tightly bound electron in calcium, producing a continuum wave packet which travels away from the Ca 1 ion in the form of a thin shell. After the ionizing laser pulse, the ions and free electrons are exposed to a HCP whose duration is so short that the continuum wave packet is essentially frozen during the pulse. The electron receives a R momentum "kick" or impulse, A � � 2 F � HCP �t� dt, from the HCP [4,5], where F � HCP �t� is the HCP field amplitude. All equations are given in atomic units unless otherwise noted. The impulse can hal...

The oscillation between bound-state configurations in a rapidly autoionizing three-body Coulomb system has been directly observed. Using a 500-fsec laser pulse, calcium atoms are excited to the pure 4p 3/2 15d twoelectron configuration at an energy greater than 3 eV above the ionization limit. As a result of configuration interaction, the electrons scatter coherently into multiple bound and continuum configurations. The oscillation between the degenerate 4p 1/2 n�d and 4p 3/2 nd modes as well as autoionization into 4s 1/2 �l , 3d 3/2 �l , and 3d 5/2 �l continua are observed explicitly using bound-state interferometry. The measured time dependence of the 4p 3/2 15d character is in excellent agreement with the Fourier transform of the frequency domain excitation cross section. To our knowledge, this is the first experimental demonstration of the equivalence of time and frequency domain spectra in a multiconfigurational system involving bound and continuum channels.

Picosecond laser pulses have been used to produce Rydberg wave packets in calcium atoms in the presence of a strong static electric field. The dynamics of the Stark wave packets have been observed by measuring the momentum-space probability distribution as a function of time. The full precession of the electronic orbital angular momentum, the appearance of a large-amplitude, linear oscillation of the electronic dipole moment, and a pronounced, periodic up-down asymmetry in the momentum distribution are all observed directly.A large amount of theoretical and experimental work on Rydberg electron dynamics in a variety of different circum stances has been performed �1�. However, until very re cently, the lack of good experimental techniques to monitor the full time-dependent probability distribution of wave packets has severely limited the insight gained from experi mental data alone, without the aid of theoretical simulations. Nevertheless, comparisons between the results of experi ments and theory have shown that it is possible to produce well-controlled wave packets under a variety of different conditions �1�, and that with alternative methods it is pos sible to experimentally recover their full time-dependent probability distribution with high fidelity �2-4�. In fact, one can now perform dynamics spectroscopy on uncharacterized wave packets. The full electronic motion as viewed in an experiment can be used to interpret the physics behind the motion directly, without relying on theoretical simulations. The refinement of this approach is a necessary prerequisite to controlling wave-packet motion in complex systems where complete quantum mechanical calculations are not readily available.The results presented in this Rapid Communication pro vide a complete experimental view of the time-dependent dynamics of a Rydberg wave packet in combined Coulomb and uniform static electric fields. Although numerous experi mental studies of Stark wave packets have been performed over the last decade �5-7�, this paper describes experiments where the complicated multidimensional evolution of the wave packet can be seen directly. Specifically, the precession of the electronic orbital angular momentum, strong oscilla tions of the electronic dipole moment along the static field direction, and a periodic asymmetry in the momentum distri bution along the static field axis are all linked in the motion of the wave packet and are clearly identified in the measured probability distributions.In the experiment, ground-state 4s4s 1 S 0 Ca atoms in a thermal beam are promoted to an intermediate 4s4 p 1 P 1 level using a 5-nsec dye laser pulse. A 1.5-psec laser pulse then drives a fraction of the excited-state atoms into a 4snd 1 D 2 , 26�n�30 radial Rydberg wave packet �3�. The wave packet is initially localized near the Ca � ion core but immediately propagates radially outward, reflects from the Coulomb potential, and returns to the ion core after one Ke pler period, � K �2�N 3 �3.0 psec �8�. In the absence of the static field, the wave packet...

Abstract:We have developed a new instrument for monitoring elec tronic wavepacket dynamics using a single electromagnetic pulse pair. The operation of the device is analogous to that of single-shot cross correlators commonly used to monitor the temporal evolution of short laser pulses. We have used the instrument to probe wavepacket evo lution over time scales ranging from 100 psec to less than 1 fsec. The device reduces the amount of time required to collect pump-probe time delay data by orders of magnitude, greatly reducing the deleterious ef fects of experimental drifts. In addition, the single-shot feature provides real-time feedback as to the affect of various experimental parameters on the electron dynamics, allowing us to literally tune-up our equip ment to enhance desired behavior at specific times.

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