Ultrafast Dynamics Of Small Molecules Driven By Ultrashort Laser Pulses

Since entering the 21st century,the development of strong-field ultrafast physics is fast,which is mainly due to two aspects:Experimentally,laser technology and experimental con-ditions are more advanced,so we can explore some new physical phenomena.Theoretically,the development of theoretical research and the improvement of computer performance make us have a deeper understanding of ultrafast dynamics.In this paper,we study the strong-field physics from simple to complex systems（atom→hydrogen molecule→carbon monoxide molecule）.The key problem is the correlation effects on the ionization and dissociation pro-cesses under strong laser fields,which contain not only the electron-electron interaction,but also the electron-nuclei interaction.Except for the introduction part,my doctoral thesis can be divided into four parts.The first part mainly describes our theoretical methods.Firstly,the solution of the time-dependent Schr?dinger equation（TDSE）is detailedly discussed including the formulas and the corresponding calculation details.Our method is based on the first principle,so we can repro-duce the real physical process and compare the results with experimental data quantitatively.What’s more,based on the Born-Oppenheimer approximation,we establish the equation of mo-tion for the nucleus in molecules.This equation lays the foundation for solving the dissociation process of molecules.At last we briefly introduce the use of quantum chemical software—Molpro.With it,we can get the potential energy curves,molecular orbits,transition matrix and other information for complex molecules,which are conducive to the study of complex molec-ular dynamics.The second part is about the correlation effect on the ionization process in atoms,and the core is the electron rescattering process.In the first work,we use a muon and proton to form a new atom with high ionization energy,and utilize two X-rays to produce high harmon-ics.We analyze the relationship between the harmonic signal and the time delay between two pulses.Finally,by synthesizing the ultra-wide harmonic spectrum,we theoretically generate a single zeptosecond pulse with a pulse width of 130 zs.The second work is in the helium atom.We establish a new rescattering process—superelastic rescattering which is different from the previous electron rescattering leading to excitation or non-sequential double ionization.The biggest difference is that free electron can absorb energy from the parent ion during rescatter-ing.We use an extreme ultraviolet（EUV）pulse to free one electron from a helium atom,while the other electron is excited to one of the excited states.The freed electron is then driven back by a time-delayed mid-infrared（MIR）pulse and rescatters with the parent ion.By analyzing the photoelectron momentum spectra,we identify this process,which also allows us to extract details of electron-electron correlation.The third part is to study the dissociation process of hydrogen molecules.The first job is observing the electron localization in H₂⁺in real time.We collaborate with Litvinyuk’s group for this job.They observe the electron localization in real time by performing a pump-probe experiment where both pump and probe are 5 fs pulses with controlled carrier-envelope phase（CEP）.By measuring the asymmetry of proton emission as a function of pump-probe delay,they demonstrate that direction of electron localization can be controlled by the probe pulse only within the first 15 fs following ionization of H₂.For larger pump-probe delays the direction of electron localization is completely determined by the CEP of the pump pulse.Our theoretical simulation repeats the main features of the experiment,supporting its explanation.What’s more,by changing the intensity of light fields we find that this 15-fs feature isnot intensity-specific.The second work is about tunneling dissociation of H₂⁺and its isotopes in the interaction with THz pulses.We show strong evidence supporting the concept of tunneling dissociation by nu-merically solving the time-dependent Schr?dinger equation.In our simulations,we observe a significant dissociation probability as a function of the driving field compatible with tunneling dissociation.We also observe nuclear rescattering,which induces high nuclear momenta,in clear analogy to the electron rescattering,that is very important in strong-field physics.The third work is the rescattering dissociation in the hydrogen molecule.We directly simulate the non-Born-Oppenheimer time-dependent Schr?dinger equation for H₂in reduced dimensional-ity.Two dissociation pathways are identified,i.e.,the dissociation of H₂⁺in the 2pσ_ustate and the dissociation of H₂in doubly excited states.The former accounts for larger proportions as the rescattering energy is larger.The kinetic energy release of dissociative fragments reflects the temporal internuclear distance at the moment the rescattering happens.The last part is about the dissociative ionization of CO,for which we mainly worked with Jian Wu’s group（East China Normal University）.The first work is about photon energy depo-sition in the single ionization of CO.They experimentally explore the electron-nuclear sharing of the absorbed photon energy in above-threshold multiphoton single ionization of CO.Vibra-tional and orbital resolved electron-nuclear sharing of the photon energy is observed.Through our theoretical simulation,different from the simplest one-or two-electron systems,we find the participation of multiple orbits and the coupling of various electronic states alters the photon energy deposition dynamics of the multi-electron molecule.The population of numerous vi-brational states of the molecular cation as the energy reservoir in the ionization process plays an important role in photon energy sharing between the emitted electron and the nuclear frag-ments.The second work is to disentangle the role of laser coupling in directional breaking of molecules.A new technique,using elliptically polarized pump and linearly polarized two-color probe pulses,is adapted to distinguish the roles of laser-induced state coupling and selective ionization.The measured photoelectron momentum distributions governed by the light polar-izations allow us to coincidentally identify the ionization and dissociation from the pump and probe pulses.Directional dissociation of CO⁺as a function of the relative phase of the linearly polarized two-color pulse is observed for both parallel and orthogonally oriented molecules.Through the theoretical calculation,we find that the laser-induced coupling of various electronic states of CO⁺plays an important role for the observed directional bond breaking.The last work is orientation-dependent strong-field dissociative single ionization of CO.In the experiment,the ionization of CO by the pump pulse and the dissociation of the created CO⁺by the probe pulse can be fully disentangled by identifying the photoelectron momentum distributions.Different from the dissociative ionization by a single pulse in which the CO molecule mostly breaks along the field polarization,in the pump-probe strategy,the CO⁺ion created from ionization by the pump pulse is favored to dissociate when it orients orthogonal to the polarization direction of the probe pulse.We explain the physical mechanism of this process by numerical simulation.The nuclear wave packets climb a ladder through X²Σ⁺→A²Π→B²Σ⁺→D²Πstates by absorbing one photon each step and dissociates into C⁺ and O fragments.