Ultrafast Measurement And Control Of Electronic Processes In Molecular Ionization: H₂,O₂ And OCS

Any biological and chemical reactions in nature cannot happen without the forming and breaking of chemical bonds in molecules from their electronic bonding,and these chemical bonds are closely related to the movement of atomic nuclei.In fact,the time scale of the nuclear movement is in the order of hundreds of femtoseconds,and the manipulation of electronic and nuclear motions in molecular ionization and dissociation by ultrashort laser pulses provides a powerful support for exploring the ultrafast dynamics of the interaction between light and matter at the microscopic scale.When the laser peak intensity reaches10¹⁴ W/cm²,the interaction will lead to the electronic excitation or ionization,and the bond breaking,i.e.,the dissociation.In this dissertation,two experimental techniques,cold-target recoil ion momentum spectroscopy?COLTRIMS?and velocity map imaging?VMI?,are employed to measure and manipulate the products of the moelcules for the purpose of studying the electronic and nuclear dynamics in the ionization and dissociation of the molecules in intense ultrafast laser fields.Starting from the simplest molecules in nature,we investigate the electron localization in dissociative ionization of H₂ molecule in 800 nm,40 fs linearly polarized laser pulses by using COLTRIMS.The electron interference and localization in this nonpolar molecule in a multi-cycle symmetric intense laser field are observed for the first time by a sub-cycle"pump-probe"method.It is confirmed that the electron localization is closely related to the interference path,which provides a means to trace the localization path of multi-electron diatomic molecules or even polyatomic molecules.Combined with the phase controlled two-color laser field and the velocity-map imaging?VMI?,we further investigate these electronic interference and localization in H₂ molecule in an asymmetric laser field,and observe three dissociation channels,i.e.one-photon,two-photon and three-photon above threshold dissociative channel in the low kinetic energy release region.The electron localization for these dissociation channels is modulated periodically with the relative phase of the two-color field.It is found that phase shifts of asymmetric electron localization in the dissociative ionization.We propose a multi-pathway interference model to describe the asymmetric dynamics,and confirme that the phase shift arises from the different evolution processes of the wave packets involved in the excitation and dissociation.The results provide an insight into quantum interference inside molecules and pave the way to control electron localization and molecular reaction dynamics by ultrafast laser pulses.For multielectron diatomic or triatomic molecule,multiple electrons can be drived coherently by the intense laser fields,and thus the interaction between the laser pulses and the molecules is more complex.In the second part of the dissertation,we study the ionization and excitation effects of diatomic molecule and triatomic molecule in the intense laser field.Firstly,we focus the multi-orbit effect and quantum interference of O₂ molecules in 800 nm linearly polarized femtosecond laser pulses.By using COLTRIMS,the process of molecular fragmentation induced by multi-orbital tunneling ionization is observed and the molecular orbits of fragment and parent ions during tunneling ionization are distinguished.The destructive and constructive interferences between different molecular orbits?HOMO and HOMO-2?are expored from the measured photoelectron momentum,which prove that this interference is originated from the different symmetry of the orbits.Next,by using a weak electrostatic field in the spectrometer to induce the population among molecular high Rydberg states,we further study the Rydberg excitation effect of O₂ molecule in the intense laser field.The measured photo-electron-photo-ion coincidence distribution characterizes the excitation mechanism of frustrated single ionization,frustrated double ionization,and dissociative Rydberg excitation of O₂molecule,respectively.It is found that the ionization of highly Rydberg state mainly comes from two way,one is due to the over-barrier or tunneling ionization induced by the electrostatic field of the spectrometer,the another is caused by the blackbody radiation ionization.Three mechanisms of Rydberg excitation obviously depend on the laser intensity but with some distinct differences,which proves that the multi-orbital effect of molecules is also very important in the excitation process.Finally,we have also studied the resonance excitation effect in triatomic molecule OCS at different wavelengths?800 nm and 400 nm?by using the VMI device.Three resonant states have been identified by measuring the energy shifts in ATI electron spectra of OCS under the intense laser fields with different intensities and wavelengths.It is proved that the resonance excitation effect cannot be ignored in the strong field ionization of OCS molecule.In the last part of the dissertation,we investigate the electronic-nuclear coupling dynamics of H₂ molecules by the Coulomb explosion in a multi-cycle elliptical polarized laser field.By using COLTRIMS,the electron momentum distribution is measured for tracing the two-electron emission dynamics on femtosecond time scales during H₂ double ionization.We exploit the rotation of the electric field vector of elliptically polarized light as a time reference,and the energy of ion fragments depending on the molecular geometry as a clock for nuclear motion.It is confirmed the sub-cycle dynamic process of the nuclear wave packet in the sequence ionization of two electrons in H₂ Coulomb explosion,which opens the way for the study of the influence of the Coulomb potential of two central molecular ions on the two electron orbits.