Structure Detection And Imaging Of Molecular Orbitals Based On Strong-field Processes

As the ultrafast laser pulses have extremely short pulse duration and extremely high peak power, its appearance provides an unprecedented detection tool to investigate the material structure and ultrafast dynamics in extreme conditions, bringing the significant breakthrough and rapid development of strong-field physics. The recent investigation has demonstrated that, by initiating the ultrafast strong filed processes of the molecule with an ultrafast laser pulse, the emitted photonic and electronic signal carries abundant structural information about the molecular orbital of the detected molecule. Therefore, the structure detection and imaging of the molecular orbital can be achieved by analyzing the reconstructing the photonic and electronic signal. As molecular orbital is the most important information carrier reflecting the nature of the molecule, to carry out the investigation on the structure detection and imaging of the molecular orbital will benefit people to realize the physical essence of the physical processes and chemical reactions in the microscopic world more directly and clearly. This will improve the development of not only physics but also many other disciplines such as chemistry, materials science and life science. In the past decade, rapid development has been obtained aiming on the molecular imaging with ultrafast driving laser. Different imaging schemes have been proposed and applied, such as the molecular orbital imaging based on the ultrafast laser tunneling scan and the molecular orbital tomography based on the molecular high harmonic generation. Based on the previous research works, there are still many problems waiting to be solved. Generally, there are three main objectives for the development of the ultrafast molecular orbital imaging:eliminating the distortion and decreasing the error in the reconnected picture, simplifying the experimental scheme and reducing the experimental difficulty, and extending the range of application of the molecular orbital imaging. In view of the above objectives, the contents of this thesis include:(1) Based on the ultrafast laser tunneling scanning mechanism, we propose a new method to image the molecular orbital with only one-shot by circularly polarized laser pulses. This method simplifies the experimental scheme and avoids the error and distortion resulting from the multi-shot detection. This method can also be applied for the imaging of asymmetric molecules, which can not be imaged with the traditional method. In addition, we also proposed a method to detect the structure of the asymmetric molecular orbital with spatial selectivity by using elliptically polarized laser pulses.(2) We propose a new reconstruction algorithm of the molecular orbital tomography based on the high harmonic generation. This algorithm overcomes the "nodal problem" confronted by the traditional reconstruction algorithm and therefore provides a more reliable reproduction for the orbitals with two-fold mirror antisymmetry, which extends the range of application of the molecular orbital tomography.(3) We propose a new method to achieve the tomographic imaging of molecular orbitals in polar molecules by using two-color laser pulses. It is shown that, the specific permanent dipole moment in a polar molecule may adversely affect the imaging process. As a result, the existing imaging method becomes invalid. To overcome this problem, we proposed to employ two-color driving laser pulses to overcome the adverse impact of the permanent dipole, and the tomographic imaging of the molecular orbital in a polar molecule with large permanent dipole is realized. The proposal of this method further extends the range of application of molecular orbital tomography.(4) We investigate the two-center interference effect in the process of high harmonic generation from heteronuclear diatomic molecules. We theoretically predict the shift of the interference minimum in the high harmonic spectrum from a heteronuclear diatomic molecule, which is also confirmed by the numerical simulation. Furthermore, we propose that by observing the shift of this minimum, one can judge the weight coefficient of each constituent basis function and therefore judge the structure of the molecular orbital.