Research On Correlated Electron-nuclear Dynamics In Strong-Field Molecular Dissociation

The interaction of atoms and molecules with an intense laser pulse has been extensively studied.When the molecular system is exposed in high-intensity laser pulse,the equilibrium motion between the particles in the molecular system will be destroyed.This is because the laser field is equivalent to the Coulomb field inside the molecule.Then,the nuclei of the system will be separated influenced by the combination of internal force and external force.This dynamics described above is the so-called molecular dissociation in strong laser field?usually referred to as strong-field molecular dissociation?.Studying molecular dissociation in strong fields is of great scientific significance since it is the basis of the control of the electronic wave package and the chemical reaction at the atomic and molecular scale.Usually,the electron and nuclear dynamics in strong-field molecular dissociation are analyzed in isolation.However,the electron and nuclear dynamics in molecules are correlated essentially and must be considered on an equal footing.The correlated electron-nuclear dynamics in molecular dissociation determines the direction of the dissociation of molecules and the break and formation of chemical bonds to some extent,which is of great significance to understand many strong-field ultrafast phenomena.Generally,the electrons of the system maybe stay in two possible cases after the dissociation of the molecule.On the one hand,the motion of the electrons is restricted around the nuclei throughout.In this case,electrons may be localized on different nuclei,i.e.,the electron localization.The study of electron localization may provide a way to detect and control the break and formation of the chemical bond in chemical reaction in the attosecond time scale.On the other hand,the electrons of the system are ionized by the intense electric field and become free electrons.This process is so-called dissociative ionization.In this case,an essential question is how the photon energy is deposited to the electron and nuclei during the laser-molecule interaction.This provides a way to deeply understand the physical process of dissociative ionization.The above two processes,the electron localization and the electron-nuclear-energy sharing,are closely related to the correlated electron-nuclear dynamics.In order to further understand the correlated electron-nuclear ultrafast dynamics in molecular dissociation,several efforts have been done in this thesis.They include:?1?We propose a scheme to steer the electron localization in dissociating H₂⁺ by using the combination of a few-cycle mid-infrared laser pulse and a low-frequency field.The mid-infrared pulse triggers the dissociation of H₂⁺.Then,the low-frequency field is used to manipulate the population dynamics of the two lowest-lying states.Our results show that the distributions of the nuclear kinetic energy release spectra of these two states are surprisingly close to each other,indicating that most of the dissociative wave packets along two pathways end at the same final kinetic energy.By adjusting the carrier-envelope phase,the electron localization is significantly enhanced compared to that in a single mid-infrared field.Furthermore,we show that the electron localization can also be controlled by simply changing the intensity of the low-frequency field,markedly relaxing the experimental requirements.?2?We propose and demonstrate a scheme for controlling the electron localization in the dissociating H₂⁺ in highly excited states.Two sequential ultraviolet pulses are utilized in our scheme.The first 134 nm ultraviolet pulse excites the electronic wavepackets from the 1s?g to the 2s?g through two-photon excitation.The second 400 nm ultraviolet pulse,which arrives after a proper time delay,manipulates the electronic transfers between the 2s?g and 3p?u states in the one-photon coupling region of the two states.After the pulses are off,the quantum interference of wavepackets in the 2s?g and 3p?u states results in the asymmetric electron localization.By adjusting the time delay between the two ultraviolet pulses or the carrier-envelope phase of the second pulse at an appropriate time delay,the control over the electron localization in highly excited states can be achieved.This provides the guidance for steering the electrons in the dissociation of the bigger molecules.?3?By numerically solving the time-dependent Schrodinger equation,we investigate electron-nuclear-energy sharing in strong-field fragmentation of the H₂⁺ molecule.By quantitatively analyzing the joint electron-nuclear-energy spectrum,we revisit the energy sharing rule given by previous work.A counterintuitive energy shift in the joint electron-nuclear-energy spectrum is found.With the decrease of the nuclear energies,the energy shift increases.By tracing the time evolution of the electron wave packet of bound states,we find that the origin of the energy shift of H₂⁺ is the Stark effect induced by the strong coupling of the two lowest-lying states in strong fields.Additionally,we calculate the Stark-induced shift using an analytical method,which is in good agreement with the ab initio result.The joint electron-nuclear-energy spectrum offers a wealth of information about the ultrafast electron and nuclear dynamics of the molecules,which could provide an alternative approach to detect molecular structure and ultrafast electron dynamics in molecules.?4?We theoretically study the correlated electron-nuclear dynamics of the asymmetric HeH₂⁺ molecule.By nu1erically solving the time-dependent Schrodinger equation,we study the electron-nuclear energy sharing in strong-field dissociative ionization of HeH₂⁺.Compared with the joint electron-nuclear-energy spectrum of symmetric molecules,we find an anomalous energy shift in the joint electron-nuclear-energy spectrum of HeHG2+,inconsistent with the electron-nuclear energy sharing rule predicted by previous work.By tracing the time evolution of the electron wave packet of bound states,we identify that the energy shift originates from the joint effect of the Stark shift,which is associated with the permanent dipole of HeH₂⁺ molecule,and the Autler-Townes effect induced by the coupling of the 2pa and 2s? states of HeH₂⁺ in strong laser fields.We further demonstrate that the electron-nuclei energy sharing can be controlled by varying the laser intensity for asymmetric molecules.