【作者】 李东旭；

【导师】 丁大军；

【作者基本信息】 吉林大学 ， 原子与分子物理， 2017， 硕士

【摘要】 本论文以异核双原子分子一氧化氮为例,通过速度成像技术在实验上探测了飞秒强激光场与分子作用电离产生的光电子速度成像,分析了激光光强对其共振激发电离路径的影响,讨论了其内部的不同里德堡态在电离过程中的贡献,同时提取了相关里德堡态的角度分布特性信息。通过分析逸出电子的动能和角度分布信息,为进一步深入地理解分子的电离过程提供了实验依据。相对原子而言,分子的能级结构更为复杂,在分子的电离解离过程研究中仍有很多现象还未得到准确充分的解释。为解决这些问题不仅需要发展更接近分子实际情况的理论模型,在实验技术手段上也要不断的发展和创新。由此,本实验室搭建了六极杆装置和电子速度成像装置,分子束通过六极杆实现聚焦后与飞秒强激光场作用产生电子,再通过速度成像装置对电子进行速度成像探测。本文工作中提取并获得了一氧化氮分子电离光电子的动能分布及角度分布。在本论文的实验工作中对六极杆施加高压的主要目的是将分子束聚焦以获得更强的产物信号,实验中所使用的激光光源是线偏振800 nm和400 nm的飞秒激光,当波长锁定在800 nm时,测量得到了不同激光光强(从1.1×10¹³ W/cm²到7.8×10¹³ W/cm²范围内)的光电子速度成像,通过对光电子动能谱结构及其随光强的通道切换现象进行分析,将出现的共振峰全部进行了归属,确定了在不同的光强作用下参与电离的里德堡态,同时还获得了电离过程中各里德堡态的逸出电子角度分布信息。当激光波长为400 nm时,测得了激光光强从2.0×10¹²W/cm²到1.4×10¹³W/cm²范围内的不同光强下光电子速度成像。与800 nm的光电子动能谱不同,400 nm下的能谱结构较单一,只存在一个较为清晰的共振峰及其对应的ATI结构。我们将800 nm出现的通道切换现象归结为激光外场引起一氧化氮分子内电子激发态能级的斯塔克移动,即随着激光光强的改变在电离过程中共振电子激发态的贡献也会发生变化,某特定电子激发态由于斯塔克效应而正好步入多光子共振区,导致动能谱中对应的峰值信号增强;同时部分电子激发态则由于斯塔克移动而逐渐远离多光子共振区,所以对应的峰值信号发生相应减弱。进一步通过对比不同激光光强下共振电离过程中各里德堡态所对应的电子角度分布,我们发现电离光电子角度分布基本体现的是里德堡态的本质属性。一氧化氮分子光电子成像的实验探测研究,有助于我们对分子内部里德堡态的结构建立更全面的认识,更深入的理解一氧化氮分子的飞秒激光诱导共振增强多光子电离过程及过程中明显的通道切换现象和电子角度分布,本论文工作中关于激光光强对分子电子激发态影响的分析,为实现强场下分子过程的量子调控提供了实验数据及相关依据。更多还原

【Abstract】 In this thesis,taking heteronuclear diatomic molecules as an example,we experimentally measure the photoelectron velocity images from nitric oxide molecules ionized by a strong femtosecond laser using velocity-map-imaging technique.Based on this measurement,we analysis the influence of laser intensity on resonant excitation ionization paths and discuss the contribution of different Rydberg states in the ionization process,meanwhile extract the characteristic information about the photoelectron angular distributions of these Rydberg states involved.The kinetic energy and angular distributions of released electrons can provide direct experimental evidence for getting a further understanding of the molecular ionization processes in laser field.Compared with atoms,the energy structures of molecule are more complex,there is still a lot of phenomena unexplained in strong field ionization and dissociation processes of molecules.For solving these problems,the innovation of experimental techniques and new theoretical models,which are more close to the actual situation of the molecule,are required.Our laboratory has built up an experimental apparatus combined a hexapole with a velocity map imaging of electrons system.After focusing be hexapole,molecular beam interact with the femtosecond laser to produce electrons.Then,we perform velocity-map imaging on the photoelectrons of the nitric oxide molecules.Nitric oxide molecules ionized by a linearly polarized laser field have been measured at different laser intensities ranging from 1.1×10¹³ to 7.8×10¹³W/cm² at 800 nm or 2.0×10¹² to 1.4×10¹³ W/cm² at 400 nm,respectively.Through the analysis of the kinetic energy spectra and their switching information with various laser intensities,we have identified the source of electrons and certained the contribution of different Rydberg states at different laser intensities.Compared with 800 nm,the structure of photoelectrons energy spectra for 400 nm is more oversimplified,there is only one obvious resonant peak with its corresponding ATI peaks.As varying the 800 nm laser intensity,we observe the channel switching phenomenon as well as the changed contribution rate of different electronic Rydberg states in corresponding photoelectron images.The reason for these phenomena is the stark shift of electronic Rydberg states so that certain Rydberg state is easily moved into the resonance region which induced stronger signal and vice versa.While,by comparing the photoelectron angular distribution of each Rydberg state at different laser intensities,it is found that angular distribution reflects the natural characteristics of Rydberg states.It would help us to get an overall view of the structure of Rydberg states involved in molecule,an in-depth understanding of complex resonance enhanced multiphoton ionization process,as well as obvious chinnel switching phenomenon and photoelectron angular distributions through detecting photoelectron images of nitric oxide molecules.This work provides experimental basis for achieving the control of molecular quantum state under strong laser field by analyzing the field impact on the molecular Rydberg states involved.更多还原