Velocity map imaging

速度图和切片成像描述了密切相关的实验方法，以获得有关基本化学过程（如单分子反应）中能量处置的信息。这些反应发生在分子被碰撞或光激发时，使其包含的能量超过其储存的能量。因此，分子离解;测量能量释放到产生的碎片中的方式提供了对化学键的性质和化学变化动力学的非常详细的探测。速度图成像通过一种类型的质谱法实现这一点，其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。这使用光谱学首先激发给定的量子能级，即具有一组明确定义的旋转、振动和电子量子数的分子状态，然后随后仅激发该选定的能级。离子可以很容易地检测到，因此通过扫描激发激光频率，REMPI质谱法是获得碎片内部能量分布的非常灵敏的方法。速度图成像是REMPI质谱法的扩展，其中将离子引导到检测器的静电光学器件以这样的方式构造，即在检测器的表面上线性地映射碎片的速度（即速度和方向）。这就产生了一幅图像，它是光致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比例。通过原始3D维速度分布的切片可以借助于反演算法或者可替代地通过以仅针对通过分布的选定切片检测离子的方式执行实验来恢复。这是通过探测器的时间门控来实现的，并且被称为切片成像。它的优点是避免了在速度图图像的数学反演中引入的噪声，我们的最终目标是将速度图成像应用于分子相干控制，其中我们使用操纵超短光脉冲的光谱振幅和相位的能力来影响反应发生时的化学动力学。然而，为了评估的可行性，并帮助解释这些提出的实验的预期结果，它将是有用的，首先应用速度图成像装置，研究使用纳秒光脉冲的三原子分子的光解离动力学。我们选择二氧化氮作为可行性研究的目标分子，是因为它具有有趣的光化学性质和大气特性。

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.