Ultrafast Dynamics Of Nuclear Motion And Electron Localization In Strong-Field Molecular Dissociation

The nuclear and electronic motions inside the molecules are of fundamental importance in determining the formation and fracture of chemical bonds. For more than two decades, many efforts have been done to study the electronic dynamics in laser-matter interactions, aiming at the control over ultrafast chemical reactions. With the recent development of laser techniques, the carrier-envelope phase (CEP) stabilized few-cycle laser pulses, the ultraviolet attosecond pulses and the intense mid-infrared pulses have been available in laboratories, helping scientists study and control the ultrafast dynamics inside molecules. In strong-field physics and attosecond sicence, the electron localization in the dissociation of molecules has recently become an interesting subject for molecular research. By steering the nuclear motion and the electron localization with the laser pulses, one can localize the electron at one of the dissociating nuclei during the breaking of the chemical bonds, achieving the direct light-field control of chemical reactions. However, there are still many unknown physical machenisms responsible for the electron localization in strong-field molecular dissociation. In order to further understand the electron localization process as well as provide some guidelines on futher expriments, several efforts have been done in this thesis. They include:(1) We theoretically investigate the laser wavelength dependence of asymmetric dissociation of H2+. It is found that the electron localization in molecular dissociation is significantly manipulated by varying the wavelength of the driving field. Through creating a strong nuclear vibration in the laser-molecular interaction, our simulations demonstrate that the few-cycle mid-infrared pulse can effectively localize the electron at one of the dissociating nuclei with weak ionization. Moreover, we show that the observed phase-shift of the dissociation asymmetry is attributed to the different population transfers by the remaining fields after the internuclear distances reach the one-photon coupling point.(2) The control of dissociation pathways of H2+through individual manipulations of nuclear vibration and electronic transfers is theoretically studied by using the two-color mid-and near-infrared laser fields. We show that the time-delayed weak near-infrared pulse can steer the electron localization by directly manipulating the electronic transfers in the one-photon coupling region with minor influence on the nuclear motion. Our further results demonstrate that the nuclear and electronic dynamics can be efficiently and individually controlled by changing the relative phase between the two laser pulses. The best balance between the nuclear and the electronic motions and, thus, the further enhancement of electron localization compared to that of a single midinfrared field, are ultimately achieved.(3) We have theoretically studied the effect of nuclear mass on electron localization in dissociating H2+and its isotopes subjected to a few-cycle3-μm pulse. Our results reveal an anomalous isotopic effect in which the degree of electron-directed reactivity can be even higher for heavier isotopes in the intense midinfrared field. We show, for the first time, the pronounced electron localization can be established through the interferences among the multi-photon coupling channels. Due to the relative enhancement of higher-order coupling channels with growing mass, the interference maxima at different kinetic energy of the spectra gradually become in phase, ultimately resulting in the larger dissociation asymmetries of heavier isotopes.(4) The electron localization in the dissociation of the asymmetric charged molecular ion HeH2+exposed to an intense few-cycle laser pulse is studied by solving numerically the3D time-dependent Schrodinger equation. By varying the carrier-envelope phase (CEP) and the intensity of the pulse, the upward shift of the localization probability and the suppression of the dissociation channel He2++H are observed. Our analysis shows that the phenomenon is attributed to the asymmetric structure of the molecule as well as the recollision-assistant field-induced ionization of the electron wave packets localized on H+in the trailing of the pulse.