Femtosecond Laser-induced Nonadiabatic Alignment Of Molecules

To control the spatial alignment and orientation of gas-phase molecules is of special interest in multiple areas of both chemistry and physics. Commonly, the direction of molecules in gas-phase is randomly distributed in space, making the study of molecules under molecular frame impossible for scientists. Since1990s, different ways to achieve the alignment and orientation of molecules have been proposed, such as using static electric field, long pulsed intense laser, and short pulsed intense laser. The method that employs short laser pulse to align the molecules impulsively can confine the spatial direction of molecules under field-free condition, which is perfect for further study on these aligned molecules.In this thesis, the nonadiabatic alignment process of gas-phase molecules induced by ultrafast laser pulse is studied experimentally and theoretically. In addition, the possible effection of quantum-state selection on the degree of alignment is also discussed, by taking methyl iodide (CH3I) as example. Firstly, the relevant concepts and theory background of nonadiabatic alignment are introduced. Secondly, the basic principle of quantum-states selection by hexapole is described. Thirdly, the experimental alignment revival curves of CH3I seeded in Ar with hexapole on and off are both displayed in detail. And after the experimental results are compared with the theoretical results, we can conclude that quantum-state selection can enhance the alignment of molecules dramatically. At the end of this thesis, the relation between the rotational temperature and the alignment of molecules is briefly studied.