Half-Cycle Pulse Assisted Electron-Ion Recombination

Abstract

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...

Multiphoton Assisted Recombination

“…However, as with other nonresonant multiphoton phenomena, the germane physics is most easily understood in terms of momentum transfer between a classical field and a continuum electron. In fact, the process is analogous to electron-ion recombination stimulated by isolated half-cycle pulses [18].…”

“…However, as with other nonresonant multiphoton phenomena, the germane physics is most easily understood in terms of momentum transfer between a classical field and a continuum electron. In fact, the process is analogous to electron-ion recombination stimulated by isolated half-cycle pulses [18].We excite electrons with well-defined energies jE 0 j < 15 cm À1 relative to the ionization threshold, in the presence of a pulsed 38.8 GHz microwave field. A novel scheme, based on radiative stabilization of autoionizing Rydberg states, facilitates pulsed field-ionization detection of only those atoms with electrons bound by <1 cm À1 following the microwave pulse [19,20].…”

Coherent population transfer in four-level ladder systems by single frequency-chirped laser pulse

“…Meerson and Friedland [8] suggested that using a laser pulse, initially at the Kepler frequency that is chirped to lower frequency later, would transfer atoms to a higher n state, leading to ionization at a lower microwave field. Recent work of Bensky et al [9] and Wesdorp et al [10] suggested that it might be more interesting to chirp the frequency in other direction. They have demonstrated that it is possible to induce electron-ion recombination into high-lying Rydberg states with half cycle pulses, and the technique could be a way to produce antihydrogen [11].…”

All-optical control of unipolar pulse generation in a resonant medium with nonlinear field coupling

“…In particular, this property can be an advantage if one wants to control the ultrafast charge motion in the pump-probe experiments. Specifically, unipolar pulses can efficiently deliver a kinetic momentum to the charged particles in order to control their motion, for instance, to ionize the atoms or ions in the medium [5][6][7] or to measure the quantum dynamics of electron and ionic wavepackets [8][9][10][11]. Unipolar pulses can efficiently accelerate the charge particles and thus be used for producing coherent beams for particle injectors and charge-particle accelerating devices [12,13].…”