The Imaging Of Molecular Structure Based On Above-threshold Ionization Photoelectron Spectra

Above-threshold ionization?ATI?is one of the basic phenomena of the interaction between atoms/molecules with strong laser fields.The investigations on ATI have not only deepened our understanding on the laser-matter interaction,but also provided applications on other aspects.Due to photoelectron carrying the molecular information,ATI can be used to image molecule.In this thesis,by using the quantum S-matrix theory,we study the dependence of the low energy plateau structure and the resonance-like enhancement structure on the orbital symmetry of molecule.And we propose methods to extract information of molecule based on the above two structures.In the first chapter,we introduce the basic concepts in the field of strong field physics.We also introduce the most important mechanism:the rescattering model.Then we introduce the methods to image molecule based on phenomena in strong field physics.In the second chapter,we make a detailed introduction to the quantum S-matrix theory and the saddle-point method.In the third chapter,we first study the ATI spectra of C₂H₄ and C₂H₆.Our calculations reproduce the measured molecule dependent spectra:a plateau at the low energy part for C₂H₆ but absent for C₂H₄.By analyzing the contributions of different orbitals involved in the ionization process and the components in the wave function of each orbital,we find that the low energy plateau originates from the destructive interference between two H atomic pairs in the highest occupied molecular orbital?HOMO?.Therefore,we propose that we can use the plateau structure on the low energy electron spectrum to extract the molecular orbital information.Then we propose a method to extract the information of molecular structure based on resonance-like enhancement?RLE?.First,we obtain spectra of the aligned molecule and its companion atom in certain laser intensity;second,we make ratio between spectra of the molecular and the companion atom to extract the interfering structure.We use the S-matrix theory to verify our idea.We calculate the spectra of the aligned N₂ and its companion atom respectively.We get the periodic interference structure whose oscillation period corresponds to the molecular nuclear distance.In the last chapter,we summarize the works in this thesis and prospect the future work.