Vibrational Excitation And Above-threshold Ionization Of Methane And Ammonia Molecules In Femtosecond Laser Field

In this paper,the above threshold ionization and vibrational excitation s of methane and ammonia are studied by using velocity imaging technology.By changing laser intensity,the analysis of the influence of molecular vibrational excitation and orbital evolution in ionization process can be done.And then the photoelectron angular distribution and kinetic energy distribution basing on the intermediate states is extracted from the images which aims to understand the molecular dynamics in the ionization process.This work provides the experimental basis,and lase induced ionization and dissociation on polyatomic molecule plays a very important role.Compared with atoms,the energy level structure of methane and ammonia molecules both are more complicated.It will be more difficult in the process of the experiment,so the laser induced ionization and dissociation of polyatomic molecules will still have a lot of phenomenon to get accurate answer.The photoelectron imaging of molecules in our work is divided into two parts.The first part studies the ionization process of NH₃ molecules under the action of linearly polarized intense femtosecond laser with a wavelength of 400nm using photoelectron velocity imaging technology.The distributions of KE and photoelectron Angular generated by multi-photon ionization of NH₃ molecules are extracted from the photoelectron images.The vibrational excitation and evolution of ionization channels under different laser intensities are analyzed.In the Freeman resonance multi-photon ionization region,each photoelectron peak induced by the same light intensity reaches the vibrational resolution,and the identification of each photoelectron peak is completed by combining the theory calculation and experiment.When the distribution of photoelectron kinetic energy induced by different laser intensity is investigated,it can be found that the position of photoelectron peak generated through same ionization channel gradually moved to the direction of low kinetic energy by tracking each ATI peak.At the same time,it was also found that there is channel switching phenomenon with vibrational resolution in the process of above threshold ionization of ammonia molecule.Finally,the photoelectron kinetic energy distributions combined with angular distributions are used to identify the ionization channels,and the vibrational excitation of intermediate Rydberg states and parent ion of ammonia molecule are analyzed in detail.The second part is about the interaction between methane molecules and linearly polarized femtosecond lasers,wavelengths at 800nm and 400nm.Photoelectron imaging with laser intensity of 4.39×10¹²W/cm~2,5.41×10¹²W/cm~2,6.42×10¹²W/cm~2,7.44×10¹²W/cm~2 and 8.45×10¹²W/cm~2 can be obtained when the wavelength is at400nm.Compared with photoelectron images with 800nm laser wavelength,photoelectron spectrum obtained under 400nm wavelength has a relatively simple structure.When the laser wavelength is 800nm,the photoelectron images under the intensities of 8.0×10¹²W/cm~2 to 10.4×10¹²W/cm~2 are analyzed according to the phenomenon of channel and orbital switching.All the peaks at different positions in the kinetic energy distribution are assigned.With wavelength of 800 nm,methane molecules easily occur channel switching phenomenon in the process of ionization,as the electron excited state also began to change according to the stark effect.Then the corresponding peak signal in the kinetic energy distribution begins to strengthen or weaken.Some of the electron excited states will be away from the multi-photon resonance region and the corresponding peak will be weakened in the kinetic energy spectrum.Finally,by changing the intensity to observe the corresponding angular distribution in the process of methane molecule ionization,the properties of intermediate Rydberg states and vibrational states can be analyzed.The study of photoelectron imaging of ammonia and methane molecules can lay a foundation role for our further study of intermediate Rydberg states and deeper understanding of channel switching in the femtosecond laser-induced resonance enhanced multi-photon ionization process.Know more on kinetic energy distribution and electron angular distribution technique.The main content of this paper is to study the dynamical mechanism on molecular ionization and electron excitation.It will provide experimental and theoretical basic research for quantum regulation of molecular ionization in strong field.