Field-Free Orientation Of Linear Molecules By Using Double Half-Cycle Pulses

Molecular alignment/orientation plays an important role in a variety of fields ranging from molecular reaction dynamics, gas-surface interaction, nanoscale design, quantum information, to strong field problems such as high-order harmonic generation and ultrashort laser pulse production, etc.Generally speaking, molecular alignment/orientation can be divided into two categories. When the external field is turned on/off slowly compared with the rotational period of the molecule, the molecule aligns/orients within the external field. In this case, once the external field becomes extinct, the alignment/orientation disappears. When the external field is turned on/off rapidly compared with the rotational period of the molecule, the molecule becomes aligned/oriented after the external field has died off. This is so called field-free alignment/orientation.In many applications of molecular alignment and orientation, it is required that the aligned/oriented molecules are free from any external field, because the latter will strongly disturb the system under investigation. Therefore, field-free alignment/orientation became a focus of intensive studies in recent years. It is also the main topic of the present thesis.Molecular orientation is more difficult than alignment as an additional spatial symmetry breaking scheme has to be involved. However, the rapid development of laser technology provides us with preponderant conditions to achieve the subject. In particular, the half-cycle pulse (HCP), which is highly asymmetric in its positive and negative field strengths, serves as a powerful tool to realize molecular orientation under field-free condition.In field-free molecular orientation researches, the duration of the obtained orientation is of equal importance as the orientation degree. Thus, in addition to the promotion of orientation degree, the enhancement of orientation duration becomes another primary goal. the duration of the obtained orientation have relation with phase matching of the rotational states. The orientation degree not only have relation with it but also have relation with population distribution.of rotational states.However, there is no method to control phase matching when the initial state of the molecular evolution is rotational ground state. In this thesis, we proposed a strategy in which by using double HCPs with different intensity and the initial state of the molecular evolution is rotational ground state.The strategy can control phase matching of the rotational states and population distribution.of rotational states effectively. by a proper choice of their intensities and the time delay between the pulses, the strategy enhance orientation degree and orientation duration. We study erery factor(the delay between the pulses and their intensities) that effect the phase matching and distribution.of rotational states, within the range of the time phase matching can be reached and distribution.of rotational states can be controlled. The distribution.of rotational states can reach the best distribution of complete phase matching.So,we are provided with an opportunity to maximize the orientation degree and orientation durationIn addition, we investigated concretely the regularities concerning this strategy of double HCP. We analyze the regularities about controlling phase matching and distribution.of rotational states.And we use these regularities giving two optimized example about molecules with large permanent dipole moments and smaller permanent dipole moments respectively.The double HCP's strategy can reserve distribution.of rotational ground state and restrain the distribution of high rotational excited molecule.And the effect that can reserve distribution.of rotational ground state and restrain the distribution of high rotational excited molecule is the main point that we can reach the optimized result.