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Abstract
Introduction
Protocol
Results
Discussion
Disclosures
Acknowledgements
Materials
References
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Chemistry

由 F− 310 Nm 支队所示的阴离子光电子成像

Published: July 27, 2018
doi:
10.3791/57989
, , ,
1Department of Chemistry,Washington University in St. Louis

Summary

在这里, 我们提出了一个关于阴离子物种光电子成像的协议。利用速度映射光电子成像技术, 对真空中产生的阴离子和质谱分离, 提供了负离子和中性能级、负离子和中性结构以及负离子电子态性质的详细资料。

Abstract

负离子光电子能成像是研究束缚负离子、中性种和非约束电子与中性分子/原子相互作用的一种非常有效的方法。真空阴离子产生技术的最新进展允许应用于广泛的原子、分子和簇阴离子系统。这些分离和选择使用飞行时间质谱法。电子被线性极化光子 (照片剥离) 除去, 使用桌面激光源, 它提供了从红外线到近紫外的激发能量的现成通道。用速度映射成像透镜和位置灵敏探测器检测光电子, 这意味着, 在原则上, 每一种光电子都能到达探测器, 并且检测效率对所有动能都是一致的。利用逆阿贝尔变换进行数学重建, 从图像中提取出的光电子能谱, 揭示了负离子内部能量状态分布和由此产生的中性能量状态的细节。在低电子动能下, 典型分辨率足以揭示几个 millielectron 伏特阶的能级差异,即分子物种的不同振动水平或原子的自旋轨道分裂。从逆阿贝尔变换中提取的光电子角分布代表了束缚电子轨道的特征, 允许对电子结构进行更详细的探测。光谱和角分布也编码了输出电子与剩余中性种在激发后的相互作用的细节。该技术应用于原子负离子 (F−), 但也可应用于分子负离子光谱学的测量、低负离子共振的研究 (作为散射实验的替代方法) 和飞秒 (fs) 对阴离子动态演化的时间解析研究。

Introduction

负离子光电子成像1是光电子能谱的一个变体, 代表了原子/分子电子结构的强大探针和电子与中性物种之间的相互作用。所获得的信息对于发展对边界和亚稳态 (电子-分子散射共振) 负离子态、门态的化学还原、游离附着过程和离子分子的理解是至关重要的。相互 作用。此外, 这些结果为高层次的从头算理论方法提供了重要的测试, 特别是那些旨在处理高度相关系统和/或非静止状态的研究。

该技术结合离子生产, 质谱和带电粒子成像2,3,4到灵敏探针电子 (和小分子, 振动) 结构。使用阴离子物种可以通过飞行质谱的时间进行良好的质量选择性。可见光/近紫外线 (UV) 光子有足够的能量去除多余的电子, 允许使用表顶激光源。使用阴离子的另一个好处是能 photoexcite 低说谎, 不稳定的阴离子状态代表能量机制, 电子和中性原子/分子强烈相互作用。利用速度映射成像5 (VMI) 提供均匀的检测效率, 即使在低电子动能, 监测所有弹射光电子, 同时揭示其速度的大小和方向。

实验结果是包含光电子谱的光电子图像 (父负离子内部能量分布的细节和子中性内部状态的能量) 和光电子角分布 (与电子轨道在支队之前)。在 fs 时间分辨研究中发现了该技术的一个特别有趣的应用。初始超快激光脉冲 (泵) 激发到游离阴离子电子状态, 第二个世俗地延迟超快脉冲 (探针) 然后分离电子从兴奋负离子。泵探针时差的控制遵循系统的能量状态的演化和系统轨道在原子运动时间刻度上的变化特性。例子包括光 i2−和其他 interhalogen 种类6,7,8,9, 分裂并且/或者电子适应在 I−·uracil10,11,12,13, i−·thymine13,14, 我−·adenine15, i−·nitromethane16, 17和 I−·acetonitrile17聚类阴离子和迄今意想不到地长的时间刻度的启示为铜−原子阴离子的生产在 photoexcitation 以后厝2−18。

图 1显示了圣路易斯的华盛顿大学 (WUSTL) 负离子光电子成像光谱仪19。该仪器由三个差异泵浦区域组成。离子在源腔内产生, 在 10−5乇的压力下运行, 含有放电离子源20和静电离子萃取板。离子在威利-迈凯轮飞行中由质量分离-MS21 (在飞行管的压力是 10−8乇)。离子检测和探测发生在检测区域 (10−9乇的压力), 其中含有 VMI 透镜5和带电粒子探测器。该仪器的主要部件是图解图 1b , 其中阴影区域代表所有的元素包含在真空系统中。气体通过脉冲喷嘴引入放电。为了抵消高进气压力, 源腔在真空下使用油基扩散泵进行维护。在图 2a中更详细地说明了放电区域。电极之间有很高的电位差, 这是由一系列聚四氟乙烯垫片与喷嘴表面绝缘的。事实上, 聚四氟乙烯作为氟原子的来源为以后显示的结果。

放电产生阴离子、阳离子和中性种的混合物。离子萃取板, 离子加速栈, 电位开关和微通道板 (MCP) 探测器 (图 1b) 形成了2米长的威利迈凯轮飞行时间-MS. 离子提取通过应用 (负) 电压脉冲的离子萃取板和然后所有离子被加速到同样动能。萃取脉冲幅度的变化集中于 VMI 透镜的到达时间, 而 einzel 透镜减小了离子束的空间截面。阴离子被重新引用到地面使用一个潜在的开关22, 时间作为一个质量鉴别器。通过同步可见光/近紫外光子脉冲的到达和 VMI 透镜中负离子的到达时间来实现阴离子的选择。离子分离和检测区域使用无油涡轮泵来保护成像探测器。

阴离子和光子相互作用, 产生光电子在整个斯坦梅茨固体的空间体积, 代表了离子和激光束之间的重叠。VMI 透镜 (图 2b) 由三个开放电极组成, 其目的是确保所有光电子到达探测器, 并保持光电子的动量空间分布。为了实现这一点, 不同的电压被应用到提取器和驱, 这样, 无论空间点的起源, 电子与相同的初始速度矢量检测在同一点上的探测器。该探测器由一组雪佛龙匹配的 MCPs 作为电子倍增器组成。每个通道的直径都有几个微米的顺序, 定位增益并保持初始撞击位置。MCPs 后面的荧光粉屏幕表明, 通过放大电子脉冲作为光的闪光, 这是使用电荷耦合器件 (CCD) 相机记录。

使用一对数字延迟发生器 (DDG,图 3) 控制所需的各种电压脉冲的定时和持续时间。整个实验是重复的拍摄基础上, 重复率为10赫兹。对于每一个镜头, 几个离子和光子相互作用, 产生了一些检测事件, 每个相机帧。几个千帧被积累成一个图像。图像中心代表动量空间的起源, 因此从中心 (r) 的距离与电子的速度成正比。角θ (相对于光子极化方向) 代表电子速度的方向。图像包含检测事件密度的分布。因此, 它也可以被视为代表的概率密度检测 (在给定点) 的电子。援引波浪函数 (ψ) 的诞生的解释形象代表 | ψ |2为光电子23。

3D 电子概率密度与随之而来的信息置乱, 是关于辐射的电矢量极化的圆柱对称的。原始的发行的重建在数学上达到了24,25,26,27。重构中的径向分布 (电子) 是动量 (速度) 域光电子谱, 通过适当的雅可比变换的应用转化为能量域。

在这些实验中使用的负离子光电子成像光谱仪 (图 1) 是一个定制的仪器28。《议定书》表 1和表 2中的设置专门用于生产 F−和成像其光电子分布的仪器。设计的几个相似的版本在各种各样的研究实验室被使用6,29,30,31,32,33,34,35,36,37,38,39,40,41,42, 但没有两个仪器是完全一样的。此外, 仪器设置是强烈相互依存和高度敏感的小变化的条件和仪器尺寸。

Protocol

注意: 这里提出了一个通用的实验协议, 具体的 WUSTL 仪器。图 4a中所示的 F−图像的特定仪器设置可以在表 1-2中找到。 1. 离子生成 要产生阴离子, 在脉冲喷嘴后面应用一个后备气体或气体混合物 (F−, 40 psig, O2), 并在10赫兹上操作喷嘴。 将喷嘴持续时间设置在数字延时发生器 1 (DDG1) 上, 通道 A (A1) 并触发?…

Representative Results

通过 centroiding43的数据记录在640×480像素 CCD 阵列的相机, 一个网格分辨率的6400×4800是可能的。然而, 光谱和角分布的提取涉及到数据的逆阿贝尔变换, 要求图像强度变化相对平稳。作为折衷, centroided 数据是 “作废”, 通过求和 n×n 块点。同样的治疗也需要显示成像结果。 图 4a的显示图像和?…

Discussion

两个因素对于所描述的协议的成功尤为重要。最好的速度映射条件必须确定, 更关键的是, 必须产生一个足够的和相对时间不变的期望负离子产量。对于 VMI 聚焦步骤, 应重复步骤5.2 和 5.3, 并与图像分析相结合, 以确定提供最锋利 (最窄) 图像特征的条件。电极电压 (V5 和 V6) 的微调受离子和激光束交叉口的大小和位置的影响, 但一旦给定系统达到最佳条件, 仪器就保持稳定。最重要的是产生特定负离?…

Disclosures

The authors have nothing to disclose.

Acknowledgements

该材料是根据1566157在国家科学基金会的支持下的工作。

Materials

Digital Delay Generators Berkeley Nucleonics Corp. 565-8c DDG1
Digital Delay Generators Berkeley Nucleonics Corp. 577-8c DDG2
HV Power Supplies Stanford Research Systems PS325 V3
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Photoelectron ImagingAnionsElectron ScatteringMolecular SpectroscopyReaction DynamicsMolecular OrbitalsElectronic StructureVibrational Energy LevelsScattering ResonancesAuto-detachment ResonancesPulsed NozzleGas MixtureDischargeTime-of-flight Mass SpectrometryImaging ModeLaser TriggerRepeller And Extractor Electrodes
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Lyle, J., Chandramoulee, S. R., Hart, C. A., Mabbs, R. Photoelectron Imaging of Anions Illustrated by 310 Nm Detachment of F−. J. Vis. Exp. (137), e57989, doi:10.3791/57989 (2018).
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