Velocity map imaging

速度图成像

• 批准号: EP/G000360/1
• 负责人: John Whitaker
• 金额: $ 2.1万
• 依托单位: University of Leeds
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2008
• 资助国家: 英国
• 起止时间: 2008 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=EP%2FG000360%2F1
• 关键词: Velocity; map; imaging

Velocity map and slice imaging describe closely related experimental approaches to obtaining information on the energy disposal in an elementary chemical process, such as a unimolecular reaction. These reactions occur when a molecule is excited by a collision or by light so that it contains more energy than it can store. As a result the molecule dissociates; and measuring the way in which the energy that is released into the resulting fragments provides a very detailed probe of the nature of chemical bonds and the dynamics of chemical change. Velocity map imaging achieves this by means of a type of mass-spectrometry in which the product molecules are state-selectively ionized. State-selective ionization is generally achieved by a process called resonantly enhanced multi-photon ionization or REMPI. This uses optical spectroscopy to first excite a given quantum level, i.e. a state of the molecule with a set of well-defined rotational, vibrational and electronic quantum numbers, and then to subsequently ionize only this selected level. Ions can be easily detected so by scanning the exciting laser frequency REMPI mass-spectrometry is a very sensitive way of obtaining the internal energy distribution in the fragments. Velocity map imaging is an extension of REMPI mass-spectrometry in which the electrostatic optics that guide the ions to the detector are constructed in such a way as to map the velocity (i.e. speed and direction) of the fragments linearly across the face of the detector. This results in an image which is a 2D projection of the photofragments initial velocity, which is proportional to the kinetic energy distribution resulting from the bond-breaking process. A slice through the original 3D dimensional velocity distribution can be recovered by means of an inversion algorithm or alternatively by performing the experiment in such a way that ions are only detected for a selected slice through the distribution. This is achieved by time gating the detector, and is known as slice imaging. It has the advantage of obviating noise introduced in the mathematical inversion of the velocity map image.Our eventual aim is to apply velocity map imaging to molecular coherent control in which we use an ability to manipulate the spectral amplitude and phase of an ultrashort optical pulse to influence the chemical dynamics of a reaction as it occurs. However, in order to assess the feasibility and to help interpret the anticipated results of these proposed experiments it will be useful to first apply the velocity map imaging apparatus to study the photodissociation dynamics of a triatomic molecule using nanosecond optical pulses. The target molecule we have chosen for this feasibility study is nitrogen dioxide because of its interesting photochemistry and atmospheric interest.

速度图和切片成像描述了在基本化学过程（如单分子反应）中获取能量处置信息的密切相关的实验方法。当一个分子被碰撞或光激发，使其包含的能量超过其存储的能量时，就会发生这些反应。结果，分子解离；通过测量能量释放到最终碎片中的方式，可以非常详细地了解化学键的本质和化学变化的动力学。速度图成像通过一种质谱法来实现这一点，在这种质谱法中，产物分子被状态选择性电离。状态选择性电离通常通过称为共振增强多光子电离或REMPI的过程来实现。该方法使用光谱学首先激发给定的量子能级，即具有一组定义良好的旋转、振动和电子量子数的分子状态，然后仅电离该选定的能级。离子很容易被检测到，因此通过扫描激发激光频率REMPI质谱法是一种非常灵敏的获得碎片内部能量分布的方法。速度图成像是REMPI质谱法的延伸，其中将离子引导到探测器的静电光学元件以这样一种方式构造，即在探测器表面线性地绘制碎片的速度（即速度和方向）。这产生的图像是光碎片初始速度的二维投影，它与键断裂过程产生的动能分布成正比。通过原始三维速度分布的切片可以通过反演算法恢复，或者通过仅通过分布检测选定切片的离子的方式执行实验。这是通过对检测器进行时间门控来实现的，称为切片成像。它具有消除速度图图像数学反演中引入的噪声的优点。我们的最终目标是将速度图成像应用于分子相干控制，其中我们利用操纵超短光脉冲的光谱幅度和相位的能力来影响反应发生时的化学动力学。然而，为了评估可行性并帮助解释这些拟议实验的预期结果，首先应用速度图成像装置在纳秒光脉冲下研究三原子分子的光解动力学将是有用的。我们为这项可行性研究选择的目标分子是二氧化氮，因为它具有有趣的光化学和大气兴趣。

项目成果

Photodissociation of NO2 in the (2) (2)B2 state: the O((1)D2) dissociation channel.

• DOI: 10.1063/1.3194286
• 发表时间: 2009-08
• 期刊: The Journal of chemical physics
• 作者: I. Wilkinson;M. P. de Miranda;B. J. Whitaker