Theoretical Study On Alignment And Orientation Of Rotational State-selected Molecules By Femtosecond Laser Pulse

Aligned and oriented molecules serve as ideal samples to study steric effects inchemical reactions and to image the structure and dynamics of complex moleculedirectly in the molecular frame, which linked to the laboratory frame of themeasurement. This would yield so-called “molecular movies” of the ongoingdynamics, conceivably without prior knowledge of the investigated system. In thisdissertation, we theoretically studied the alignment and orientation of state-selectedmolecules. The main work is as follows:(1) The time-dependent Schr dinger equation for nonadiabatic alignment andorientation are given and solved by numerical simulation. On the theritical basis wewrite program and compute the degree of alignment and orientation for linearmolecule, symmetric top molecule and asymmetric top molecule under rotationalstate-selected and conventional cold condition.(2) We propose a new method to obtain field-free orientation by using rotationalstate-selected molecules instead of the conventional cold molecule as the sample. Weselect the typical molecule, including diatomic NO, linear OCS and symmetric topmolecule CH3I as example to verify this method. The calculated orientation degreeshows that those molecules with rotational state of|JKM〉(KM0) can be effectivelyoriented by femtosecond laser pulse and those with rotational state of|JKM〉(KM=0)cannot be oriented.(3) We compare the degree of alignment between using the state-selectedmolecules and cold molecules as samples. We found the state-selected molecules canachieve higher degree of alignment and longer duration of alignment. We concludethe mechanism for the increase of alignment degree when the state-selected moleculesare used as follows: the coherent rotational wave packet is formed after the interactionbetween the laser pulse and molecule which lead to the alignment of molecules. For cold molecules under a certain temperature, molecules are populated thermally in anumber of states which are incoherent, and the alignment can be destroyed, whereasthis incoherent effect does not exist for the state-selected molecules.(4) The dependence of orientation on the shape of the laser pulse has beenresearched. Optimal pulse shapes are designed iteratively by an evolutionaryalgorithm. The results show that laser pulses designed by an evolutionary algorithmcan increase the degree of orientation compared to unshaped laser pulses of the sameenergy.(5) We studied the alignment and orientation of asymmetric top molecule H2CO.The degree of alignment and orientation are calculated and the results show a muchlower level of alignment and orientation, and no periods are observed.