Experimental Study On Molecular Orbital Tomography Based On High Harmonic Generation

Imaging the molecular orbital gives an intuitionistic insight of the molecular structure and provides the opportunities for revealing and understanding the molecular dynamics and chemical reactions.In the past decades,great strides have been taken to develop the imaging methods.Even though a good spatial resolution can be achieved,the temporal resolution is limited to several tens or even hundreds femtosecond at present.In recent years,an alternative way has also been developed based on the laser-induced recollision process.It enables one to image the molecular orbital by measuring the high-order harmonics generated in the interaction of femtosecond laser and molecules,which is called molecular-orbital tomography（MOT）.It is surprising that the technique based on MOT can enable one to measure the orbital wave function itself,and the wave function itself is usually not considered to be observable.The most fascinating perspective of this approach is the potential to get the real-time evolution of the molecular orbital,i.e.,a molecular movie,with unprecedented attosecond-?ngstr?m resolutions.To access the intriguing goal of a real-time molecular movie,one crucial issue is to effectively capture the snapshot of molecular orbitals.However,there are several roadblocks at present.The first roadblock is the influence of Coulomb potential in MOT,which is not included in the previous works.The second roadblock is the measurement of phases of HHG requires sophisticated techniques and represents formidable challenges.The third roadblock is that the real-time imaging of the evolution of electron orbitals is hampered by the multi-shot measurement of HHG.Aim at the problems mentioned above,the main contents of this thesis include:（1）We experimentally demonstrate the molecule orbital reconstruction by using a Coulomb-corrected scheme.In our scheme,the molecular continuum states are described by a two-center Coulomb wave function instead of the plane-wave approximation.The molecular orbitals reconstructed from the experimental data show that the reconstructions with Coulomb corrections effectively eliminate the artificial structures induced by the plane-wave approximation.Our scheme provides a more accurate method for molecule orbital reconstruction and strengthens the theoretical basis of MOT.（2）We propose a new scheme of the molecular orbital tomography,called diffractive MOT.By combining MOT with the coherent diffractive imaging,the molecular orbital can be retrieved solely from the high-order harmonic amplitude without a priori knowledge of the phase information.We have applied this method to image the molecular orbitals of N₂,CO₂,and C₂H₂.The diffractive MOT scheme,removing the roadblock of phase measurement,significantly simplifies the MOT procedure and paves an efficient and robust way to the imaging of more complex molecules.（3）We report a single-shot MOT scheme.By controlling the two-dimensional trajectory of the electron with orthogonal two-color fields,the electron can recombine with the parent ion from different angles,which enables us to probe the target molecules with only single-shot measurement.In the proof-of-principle experiment,the molecular orbital of N₂ has been reconstructed by solely measuring the intensity of high-order harmonic generation with orthogonal two-color fields.It circumvents the hurdle of multi-shot measurements in experiment,making a substantial step towards the goal of molecular movie.