【作者】 张栋栋；

【导师】 丁大军；

【作者基本信息】 吉林大学 ， 原子与分子物理， 2011， 博士

【摘要】 本文研究了分子在外场中的行为和性质,试图得到详细和系统的相关分子与飞秒激光场相互作用以及分子能级结构和分子选态操控的相关动力学信息。多原子分子在飞秒激光场中的电离解离过程由于其量子态复杂性和多粒子关联一直是人们研究的热点,尤其在中等强度飞秒激光场中分子的电离解离过程,目前还有很多现象没有很好的解释。这些问题要求发展新实验探测技术手段。我们发展了一套速度成像探测实验平台,能量分辨率好于0.1 eV。在此平台上我们研究了CH₃I分子在与飞秒激光相互作用过程中的解离电离和库仑爆炸过程。实验上探测到CH₃I分子与飞秒激光作用后产生的不同碎片离子的角度分布和速度分布。通过对速度分布的研究,确认了一些碎片离子不同速度分量产生的解离电离或者库仑爆炸通道。在光强3.7×10¹⁴ W/cm²下,测量和分析表明,碎片离子I+产生于5个通道,其中有2个解离电离通道和3个库仑爆炸通道;I2+产生于4个通道,其中有1个解离电离通道和3个库仑爆炸通道;I³⁺产生于2个库仑爆炸通道。实验表明,在光强范围2×10¹⁴ 6×10¹⁴ W/cm²下CH₃I分子的碎片离子角度分布有很强的各向异性,这是由于飞秒激光与CH₃I分子的相互作用中的几何准直效应造成。不同的解离电离和库仑爆炸通道产生的碎片离子的角度分布差别很大,库仑爆炸通道产生的碎片离子的角分布的各向异性要比解离电离通道产生的碎片离子的角分布的各向异性要强很多。根据实验结果,我们提出了CH₃I分子在飞秒激光场中的高次电离是通过序列电离过程发生的,这个模型很好地解释了我们观察到的产生于不同反应通道的碎片离子动能随光强增加而增加的现象。利用飞秒泵浦‐探测技术,结合飞行时间质谱探测方法,研究了CS₂分子和SO2两种分子解离电离超快动力学过程。通过仔细控制波长为266 nm的泵浦光和400 nm的探测光的光强,在实验上得到了CS₂分子的母体离子CS₂⁺和碎片离子S⁺和CS⁺随泵浦探测延迟时间的演化信号。观察到在时间零点之后,CS₂分子吸收两个266 nm的光子激发到里德堡6s态和在时间零点之前CS₂分子吸收三个400 nm的光子激发到里德堡4f态的演化规律。对实验数据拟合分析得到了这两种里德堡态的时间演化的信息。我们还首次研究了碎片离子随泵浦-探测延迟时间的演化情况,通过碎片离子分支比S+/CS+随时间的演化的探测,推断这些碎片离子是从母体离子的C 2?g+态解离产生的。实验中还观察到碎片离子的比值S⁺/CS⁺依赖于两束激光的延迟时间,这说明母体离子CS₂⁺的解离过程与里德堡中间态随时间的演化相关联。?实验上我们还首次研究了SO₂分子里德堡F态的超快解离动力学。结果表明,SO₂分子经过里德堡F态的解离倾向于沿着反应坐标S-O发生,解离可能沿着两个不同的路径发生:其中一个为F态的预解离过程,演化时间为几百个飞秒;而另一个为F态经过内转换到E态,演化时间则为几个皮秒。实验还发现SO2里德堡F态解离也可能存在产生中性碎片原子O和S的解离通道。这些研究不仅拓展了上述分子的激发态超快动力学研究,而且深化了对于这些分子的结构、能量及其解离通道的认识。建立了六极杆静电场分子转动态选择装置并完成了对称陀螺分子CH₃I和CHCl₃转动态选择的实验。通过对比这两种分子在六极杆静电场中的聚焦曲线的理论模拟和实验结果,发现改变六极杆静电场可以通过一阶Stark效应聚焦并选择分子不同的转动态。在两种分子聚焦曲线的上升部分,我们获得了相对较单一的转动态分子束,而在聚焦曲线的平台部分,我们得到的是分子许多转动态叠加在一起的分子束。通过进一步降低转动温度或者增加分子束的速度,预期能够获得更为单一的转动态分子。这些研究为单量子态分子的结构以及动力学研究提供了基础。更多还原

【Abstract】 In this thesis, we attend to study the properties of molecules in external field and try to obtain the detailed dynamic information concerning with the interaction of polyatomic molecules with intense femtosecond laser, the energy structure of simple triatomic molecules and manipulating dynamical processes with rotational state selection of molecules.Dissociation and ionization of polyatomic molecules in intense femtosecond laser field due to their complexity of quantum states and many-particle correlation has been a hot topic for years. Especially on the subject at moderate laser intensities, there are still a lot of phenomena which need to be interpreted. For further solving these problems, the development of novel spectroscopic techniques is necessary. We have built a velocity map imaging apparatus with a kinetic energy resolution of <0.1 eV. Using this apparatus, the interaction of CH₃I molecules with intense femtosecond laser has been investigated experimentally. The kinetic energy distribution and the angular distribution of the various atomic fragment ions In+ （n=1-3） have been measured. Several dissociative ionization and coulomb explosion channels were identified for In+ （n=1-3）. In the laser field with the intensity of 3.7×10¹⁴ W/cm², five channels can produce the fragment ion I+, including two dissociative ionization channels and three Coulomb explosion channels, one dissociative ionization and three Coulomb explosion channels can lead to the fragment ion I2+, and there are only two Coulomb explosion channels contributing to the generation of the fragment ion I3+. As expected for a geometric alignment dominated the interaction process, these atomic ion fragments show the anisotropic angular recoil distributions peaked in the laser polarization direction. Also these results exhibited very different angular distribution of the fragment ions from dissociative ionization and Coulomb explosion channels. Furthermore, we have also studied the dependence of the kinetic energy release （KER） of In+ （n=1-3） upon the laser intensity. The relative weight of the various contributions from the identified dissociative ionization and Coulomb explosion channels was found to depend on the laser intensity obviously. We found that the interpretation of this experimental observation invoked a sequential ionization process of the molecules.Ultrafast dynamics and dissociative ionization of CS₂ and SO2 were studied by means of femtosecond time resolved pump-probe method combined with a time-of- flight mass spectrometer. By carefully controlling the intensities of the 266 nm and 400 nm lasers as pump or probe light, the transients of both parent ion （CS₂⁺） and fragment ions （S+ and CS+） were observed. It was found that all ion signals decay exponentially, but different for the cases with 266 nm or 400 nm light as pumping. We attributed these differences to the evolution of different Rydberg states 6s and 4f pumped by two 266 nm photons or three 400 nm photons, respectively. The lifetimes of these two Rydberg states were obtained simultaneously from one pump-probe experiment by the best fitting of the transients. It is the first time to study the transient single of fragment ions S+ and CS+ produced by dissociation of CS₂⁺. Our observation suggested that the final state of parent ionic molecules CS₂⁺ in the laser field is its C 2?g+ state, based on the measured S+/CS+ branching ratio. Another observation in the present experiment is that the S+/CS+ ratio is dependent on the delay time of the two lasers, indicating that the dissociation process of CS₂⁺ is related to the evolution of the intermediate Rydberg state of the neutral molecules. This femtosecond pump-probe method was also employed to study the dissociation dynamics of SO2. The results clearly showed that the F state of SO2 dissociates along the S-O bond. The transients of S+ and O+, however, have different behavior, which consist a fast growth and a long decay component. Possible mechanism of the fragment formation was discussed for understanding the dissociation dynamics of the F state of SO2. Our experimental results extend the previous studies on ultrafast dynamics of these molecules on excited states and also provide better understanding for structures, energies, and dissociation processes of these molecules.A molecular rotational state-selecting hexapole machine has been designed and built in our lab. We studied the focusing curves of the two symmetry top molecules, CH₃I and CHCl3, on the hexapole machine. Comparing to the theoretical simulation, we observed different rotational states could be focused at different voltage applied on the hexapole via the first order Stark effect. During the rising part of the focusing curve for CH₃I, there was only one single rotational state at some particular voltage. As for CHCl3 there were two rotational states contributing to the rising part of its focusing curve. It is expected that if the rotational temperature of the molecular beam can be further decreased or the beam velocity can be increased, more pure population on a single rotational state in molecular beam could be achieved. This in turn will be important for producing an ideal initial sample beam in the precision study of structure and dynamics of molecules on a unique quantum state.更多还原