| The interaction between laser and material is a forefront in the research of atomic and molecular optical physics. With the development of modern science and technology, many new strong field physical processes arise in the interaction between ultrafast intense laser with atoms and molecules, such as Non-Sequential Double(or Multiple) Ionization, High-Order Above-Threshold Ionization, High Harmonic Generation, Dissociation and Coulomb explosion. Particularly, strong-field ionization is the most basic and important step in these strong-field molecular physical phenomena. Study on the strong field ionization of atoms and molecules, is helpful to understand and reveal the new physical mechanism of the interaction between strong laser field with atoms and molecules. Moreover, it has important significance to the development of many scientific aspects, such as molecular orbital tomography, attosecond physics, and the novel ultra-short EUV light source.Comparing to atoms, the interaction of molecules(especially the polyatomic molecules) with strong laser fields is much complicated and exhibits many peculiar behaviors, due to the complexity in molecular structure, additional nuclear degree-of-freedom, molecular orbital, the molecular size, the space alignment and orientation etc. In this dissertation, we systematically studied the ionization and the related process of linear triatomic molecule CS2 in strong 800-nm 50-fs femtosecond laser fields. The research content mainly includes the Non-Sequencial Double Ionization of CS2 molecules, the angular distributions of different fragment ions formed by the ionization, dissociation and Coulomb explosion, along with the enhanced ionization for the vibrational CS2 molecules. This dissertation is of importance for understanding the ionization mechanism of CS2 molecules, and further researching the ionization of the polyatomic molecules in the ultrafast intense laser field and related physical processes.We obtained the following main results:1) We measured the DI yield of CS2 in both linearly(LP) and circularly polarized(CP) laser fields. For the first time, a “knee” structure around 60 TW/cm2 is identified in the laser intensity dependence of CS22+ yield in LP field, which is characterized as a signature of strong-field atomic NSDI. We further present the ellipticity dependence of CS22+/CS2+, and the result shows that the ratio strongly depends on the laser polarization. We use the full classical ensemble method to establish the two-dimensional CS2 model under the two electron approximation, and simulate the double ionization processes of the CS2 molecule in LP and CP laser field. The dependence of electron momentum, energy and trajectory on laser cycle is calculated, and the results confirm the recollision between the electron and the nuclear. Analysis indicated both RII and RESI may contribute to the NSDI of CS2 molecules.2) The strong-field ionization/dissociation and Coulomb explosion of the CS2 has been investigated with laser intensity of more than 1014W/cm2. The angular distributions of CS2+ and CS+ are isotropic, indicating that CS+ is from the dissociation of CS2+. The anisotropic in angular distribution of Sn+ and Cm+ fragment increase with the increase of the charge of these ions. It can be seen that the peak direction of Sn+ and Cm+ fragments is quite different. The maxima of the Cm+ distributions are observed with time-of-flight axis perpendicular to the laser polarization direction. On the contrary, the maxima of the Sn+ distributions are parallel to the laser polarization direction. Such anisotropic character in the angular distribution of fragments could be attributed to the geometric alignment of the molecules in strong laser fields before dissociation. And the different maxima distributions of the Cm+ and Sn+ show that the geometry of CS2 molecule will become bent by the strong laser field before the momentum transfer in Coulomb explosion.3) Using a temperature controlled molecular beam device, we prepare the molecules with vibrational population, then measure the ionization yield for different molecular temperature(corresponding to different initial vibrational population) to reveal the underlying mechanism for strong-field ionization of vibrational excited molecules. The experiment shows that the initial vibrational population of molecules would enhance the ionization yield, and the enhancement on molecular temperature is not linear. In the meanwhile, with the increase of laser intensity, the enhancement is gradually decreases. Combining with the developed theories, we have analyzed the enhanced ionization for the vibrational excited CS2 molecules. The results show that not only the effective IP but also the change of Frank-Condon factors influences the strong-field ionization of vibrational-excited CS2 molecules. |
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