Velocity Map Imaging of Ultrafast Molecular Dynamics Using Coulomb Explosion Imaging

能够产生强烈的、相干的飞秒光脉冲的激光器的发展允许在其原生时间尺度上对化学动力学进行广泛的详细研究。本项目拟利用库仑爆炸成像技术研究气相分子的结构和动力学。在这里，一个强烈的超短激光脉冲被用来从目标分子中快速去除几个电子。由此产生的高电荷阳离子在碎裂方面非常不稳定-称为库仑爆炸。如果爆炸发生在分子的任何显著变形或重排之前，则可以通过测量产生的碎片的动量以及它们之间的任何相关性来确定分子的原始结构。这在Brouard组中最近已经用于直接确定气相分子的手性（J. Chem. Phys.，2016，144，161105）。该技术可以扩展到泵探测制度，允许结构动力学成像的时间分辨的方式。该小组最近使用该方法研究联苯衍生物中的光诱导扭转运动（Phys. Rev. Lett.，2014，133，073005）。本研究旨在将上述工作扩展到研究一系列光诱导过程，如光异构化和电荷转移。这将通过一种新的飞秒激光器（Ti：Sa）来促进，该激光器最近已安装在牛津的Brouard集团实验室中。该激光系统将用于在目标分子中诱导库仑爆炸。这些实验将利用一种称为速度图成像（VMI）的技术，其中以相同速度产生的离子被聚焦到位置敏感探测器上的同一点上。将使用新型像素成像质谱（PImMS）或Timepix传感器记录检测到的离子命中。PImMS是在牛津开发的事件触发检测器，其包括允许记录事件的位置（x，y）和时间（t，精确到12.5ns）的像素阵列（J. Inst.，2012，7，C08001）。Timepix是一个类似的探测器，最新的传感器将计时精度提高到约1 ns。这些传感器提供了显着的改进，在VMI实验中传统使用的CCD相机，因为它们允许所有的离子撞击被记录在一个单一的实验周期。在单个实验中记录多个离子撞击（由它们的特征飞行时间分开）允许使用重合或协方差成像技术来阐明不同离子的动量之间的相关性。这些相关性可以提供大量的结构信息，从而可以更详细地研究结构和动力学。使用新的牛津飞秒激光器的实验最初将集中在单色库仑爆炸。将针对一系列小分子研究其库仑爆炸动力学，以期建立结构更复杂的目标。这些单色实验也将使有前途的分子的时间分辨研究到他们的光动力学被确定。时间分辨泵浦探测实验的重要部分将在汉堡DESY的FLASH自由电子激光器上合作进行。这种合作包括广泛的团体，如Rolles和Rudenko（堪萨斯州），Stapfeldt（奥胡斯）和巴里和波尔（DESY）。这项合作的目的包括开环反应的研究，以及肽的电荷转移动力学研究。计划与Neumark和Leone小组（加州大学伯克利分校）合作进行的实验旨在将新型阿秒激光器与PImMS探测器联合收割机结合起来，研究超快（亚飞秒）电荷迁移。该项目属于EPSRC化学反应动力学和机理研究领域的福尔斯。

The development of lasers capable of producing intense, coherent, femtosecond pulses of light has allowed for a wide range of detailed studies into chemical dynamics on their native timescales. This project proposes to investigate the structures and dynamics of gas phase molecules using a technique known as Coulomb explosion imaging. Here, an intense, ultrashort laser pulse is used to rapidly remove several electrons from the target molecule. The resulting highly charged cation is very unstable with respect to fragmentation - known as a Coulomb explosion. If the explosion occurs before any significant distortion or rearrangement of the molecule, the original structure of the molecule can be determined through measuring the momenta of resulting fragments, and any correlations between them. This has been used recently in the Brouard group to directly determine the chirality of gas phase molecules ( J. Chem. Phys., 2016, 144, 161105). The technique can be extended to a pump-probe regime, allowing structural dynamics to be imaged in a time resolved manner. The group recently used this to study photoinduced torsional motion in a biphenyl derivative (Phys. Rev. Lett., 2014, 133, 073005). The proposed research aims to extend the above work to study a range of photoinduced processes, such as photoisomerisations and charge-transfers. This will be facilitated by a new femtosecond laser (Ti:Sa) which has very recently been installed into the Brouard group lab in Oxford. This laser system will be used to induce Coulomb explosions in the molecules targeted. These experiments will make use of a technique known as velocity map imaging (VMI), in which ions generated with the same velocity are focussed onto the same point on a position sensitive detector. Detected ion hits will be recorded using the novel Pixel Imaging Mass Spectrometry (PImMS) or Timepix sensors. PImMS is an event-triggered detector developed in Oxford, which comprises an array of pixels allowing the logging of an event's position (x,y) and time (t, to a precision of 12.5 ns) (J. Inst., 2012, 7, C08001). Timepix is a similar detector, with the latest sensor improving the timing precision to approximately 1 ns. These sensors provide significant improvements over the CCD cameras conventionally used in VMI experiments, as they allow for all ion hits to be recorded in a single experimental cycle. Recording multiple ion hits (separated by their characteristic time of flight) in a single experiment allows for correlations between momenta of different ions to be elucidated, using coincident or covariance imaging techniques. These correlations can provide a great wealth of structural information, allowing for structure and dynamics to be studied with far greater detail. Experiments using the new Oxford femtosecond laser will initially be focussed on one-colour Coulomb explosions. A range of small molecules will be targeted to investigate their Coulomb explosion dynamics, with a view to building towards more structurally complex targets. These one colour experiments will also enable promising molecules for time-resolved studies into their photodynamics to be identified. A significant portion of the time-resolved pump-probe experiments will be carried out in collaboration at the FLASH free electron laser in DESY, Hamburg. This collaboration incorporates a wide range of groups, such as Rolles and Rudenko (Kansas State), Stapfeldt (Aarhus) and Bari and Boll (DESY). The aims of this collaboration include studies of ring-opening reactions, and studies into the charge-transfer dynamics of peptides. Planned experiments in collaboration with the Neumark and Leone groups (UC Berkeley) aims to combine novel attosecond lasers with the PImMS detector to study ultrafast (sub femtosecond) charge migration. This project falls within the EPSRC Chemical Reaction Dynamics and Mechanisms research area.