Strong-field Ionization Dynamics Of Nanoparticles And Adhereed Molecules

The laser-induced atomic,molecular,and solid-state ionization dynamics are important scientific bases for understanding many fundamental physical and chemical processes.Related research provides scientific support for the development of applications such as precision processing of materials,optical control of chemical reactions,and high-speed optoelectronic devices.Under the irradiation of ultrashort intense laser pulses,nearby the surface of the nanostructure there exists strength enhanced near-field that changes dramatically in space and time.The molecules attached to the surface of the nanoparticle will take the lead in ionization,and trigger the avalanche ionization of the nanoparticle on the femtosecond time scale.Therefore,the molecules adsorbed on the surface of nanoparticles play a decisive role in the ionization process of nanostructures in strong laser fields.Compared with the molecules adsorbed on the surface of the nano-structure,the research on the strong field ionization of isolated molecules in gas phase can help us get a deep understanding of the molecular dynamics on the surface of nanoparticles,which is of great significance for the precise control of nanoparticle ionization and practical application by means of surface molecules.In this dissertation,the strong-field excitation and ionization behaviors of nanoparticles and atoms/molecules were experimentally studied by ultrashort intense laser pulses.Using the self-developed single shot velocity map imaging system on nanosystems,the ionization dynamics of nanoparticles and surface molecules under intense laser fields were explored.The strong field excitation and ionization process of gas-phase atoms and molecules(non-dipole effect in non-sequential double ionization and electron recapture process)were also explored by electron-ion coincidence measurement technique.The main research results and innovations are as follows:1.Realizing single-shot imaging of ultrafast strong field ionization of nanoparticles and surface molecules,revealing the shock wave formation process of nano plasma,and discovering the electron/ion directional emission along the propagation direction of the optical field generated in nanoparticle dimers.Through the self-designed velocity map imaging spectrometer on nanosystems and high-voltage pulse gating technique,single-shot excitation images of silica nanoparticles were recorded by a high-speed camera.The ion momentum distributions in the experiments revealed the dynamics of particles excited on ultrafast time scales(10-12 s-10-15 s).When the laser intensity reaches the threshold to fully excite the nanoparticles,the ions propagate outward as shock waves.The formation process of the shock wave was reproduced by the classical Coulomb explosion simulation.With the decrease of the light intensity,the electrons and ions collected from the ionization process of the nanoparticles show clear forward focusing distributions.Further analysis and theoretical simulations found that the forward focusing effect was realized in the nanoparticle dimers.This study opens the door to explore the ionization dynamics of nanoparticles and surface molecules,and also provides the possibility for fine-tuning of micro-nano processing.2.Visualizing the forward momentum shift of the electrons generated by the ultrafast strong field ionization of the atoms along the propagation direction of the optical field,and revealing the non-dipole effect in the double ionization process.Electron-ion forward focusing distributions observed during molecular ionization on nanoparticle surfaces generally occur in large isolated nanoparticles as well as nanoparticle dimers.The forward shift of photoelectron momentum during the ionization is also existed in isolated atoms and molecules.By using the electron-ion coincidence detection technique,the non-dipole effect in the non-sequential double ionization of Ar atoms was experimentally observed by using the ultrashort pulses at the wavelength of 2000 nm.Based on the electron-ion coincidence spectroscopy obtained from the atomic Rydberg state excitation and ionization process,a method to precisely determine the zero point of electron momentum in the laser propagation direction is proposed,which provides an important experimental basis for measuring extremely small momentum deviations(on the order of 0.01 a.u.)in the laser propagation direction.It is found that the sum-momentum shift of two electrons of a doubly ionized Ar atom in the laser propagation direction is completely opposite to that of single ionization,and the offset is four times that of single ionization.The electron trajectories of the non-sequential double ionization were analyzed by semi-classical simulation,and it is found that the Coulomb focusing effect and the Lorentz force brought by the laser magnetic field play a decisive role in this process.The experimental results provide the possibility to control the photon momentum transfer by manipulating the waveform of the light field.3.Realizing the coincidence measurement of the Rydberg state excitation in the strong field ionization of atoms and molecules,and revealing the physical mechanism of electron tunneling and recapture process.After the ionization process of atoms and molecules,the escaped electrons may not only collide with other electrons near the parent nucleus,but also have the possibility of being trapped in Rydberg state orbitals.This physical process is called frustrated tunneling ionization.Using electron-ion coincidence and ion-ion coincidence techniques,the following two research works have been carried out:Excitation process of Rydberg states in N2-Ar dimerUsing cold target recoil ion momentum spectroscopy apparatus,the frustrated double ionization of N2-Ar dimer in strong laser fields was experimentally studied,and the charged ion fragments and neutral Rydberg fragments after excitation were measured.The results show that due to the special T-type structure of the N2-Ar molecule,the first electron is more inclined to be freed from the side of the Ar atom,while the second electron is more likely to be freed when the laser electric field point to the N2 molecule due to the localized electron-enhanced ionization effect.Thus the second electron will move towards Ar+ ion under the action of the laser field.It is experimentally observed that electrons are more easily to be captured by the Ar atoms during the double ionization process of frustrated tunneling ionization of N2-Ar dimer.Orbital effects in strong-field Rydberg state excitation of N2,Ar,O2 and XeUsing femtosecond laser pulses to excite the frustrated single ionization of N2,Ar,O2,and Xe,the generated electrons,ions and neutral Rydberg state atoms were collected by coincidence detection,and the effect of molecular orbital on the excitation process of Rydberg state is clearly revealed experimentally.Our experiment found that the excitation probability of the Rydberg state of the O2 molecule is much greater than that of the Xe atom with the similar ionization potential,while the excitation probability of the Rydberg state of the N2 molecule is lower than that of the Ar atom.Our experimental and simulation results reveal that it is the initial momentum distribution(determined by the detailed characteristics of orbitals)that finally leads to the tendency that the Rydberg state yield of O2(Ar)decreased slower than that obtained for Xe(N2)when the ellipticity of the laser field is increased.This physical phenomenon can be explained by the cancel out of initial momentum of tunneling electron and the transverse components of the laser field.