Control of electron wavepacket dynamics with intense cycle-shaped laser pulses

Abstract

Strong field control of ultrafast electron wavepacket dynamics in atoms and molecules benefits us in many aspects, such as realizing electron trajectory selection in high harmonic generation, producing highly efficient and short attosecond pulses, probing and controlling atomic and molecular dynamics with subcycle resolution, enabling the study of correlated electron dynamics in electron-impact ionization process and so on. In this thesis our aim is to investigate whether strong field control of electron wave dynamics in atoms and molecules can be achieved in attosecond time domain and in 1- or 2-dimensional (1D or 2D) space domain by waveform-controlled laser pulses. For this purpose, we carried out two experiments by laser subcycle engineering via the Cold Target Recoil Ion Momentum Spectroscopy (COLTRIMS) technique: (i) Atomic single- and double-ionization dynamics of neon atom is investigated by employing an orthogonally-polarized two-color (OTC) laser field. (ii) Strong field ionization and dissociation dynamics of H2 molecule is studied with application of a cycle-sculpted laser field produced by a parallelpolarized two-color laser field. The OTC laser field leads to spatially-shaped trajectories of tunneling electron wavepackets from atoms. Electrons generated at different birth times in one optical cycle are emitted into different emission angles, thus a subcycle mapping between the time-spatial information of the electric field and the spatial information of the electron momentum vector is established. We prove that 2D spatial control of both emitted and recolliding electrons from atoms can be achieved with attosecond precision by varying the relative phase between the two color pulses. Attosecond streaking of tunneled electrons is visualized by their 2D momentum distrbutions. We realize subcycle resolution of the influence of the long range Coulomb potential of the parent ion on the final momentum distribution of the emitted electrons. We also observe electron-impact ionization of neon in our measurement. Due to strong dependence of the recollision process which occurs within a few hundred attoseconds on the driving field, any change of the shape of the electric field leads to the change of the probability of nonsequential double ionization (NSDI) process and the characteristics of resulting products. This is proved by the spatially preferred emission directions of both two electrons from NSDI and the yield of the doubly-charged ion with the relative phase. We also extend the ability of control of electron wave dynamics with attosecond resolution to H2 molecule by the cycle-sculpted laser field. We demonstrate that spatial direction of the final momentum of the electrons from singly-ionized H2 molecule can be controlled in 1D momentum space. Furthermore control of the intramolecular electron motion via the electron localization enables us to control the emission directions of H+ ions from molecular dissociation pathways, this is proved by a very strong left-right asymmetry in the emission of H+ ions via above-threshold dissociation (ATD) pathway from our measurement. Moreover we show that control of the kinetic energy of H+ ions from recollision excitation (RCE) induced dissociation can be achieved via controlling the initial ionization and recollision process (such as the return energy and the return instant) of electron wavepackets by changing the relative phase between the two color pulses. Besides, we observe H+ ions generated from RCE dissociation carry higher energies than those from charge-resonance- enhanced ionization (CREI) pathway and their kinetic energy spectra partly overlap with each other. All these results provide a better understanding of control of attosecond electron wave dynamics in atoms and molecules by strong laser field.