Exploring molecular structure and dynamics through Coulomb Explosion Imaging

分子改变形状的方式(分子动力学)可能是它们在生物系统乃至所有生命中最重要的特性。对分子形状以及它如何改变或如何改变的科学探索一直是强大工具开发的驱动力之一，这些工具包括超短激光光源(脉冲长度类似于分子运动的自然时间刻度，十亿分之一秒飞秒)或X射线源，如同步加速器设备，它可以用一个高能光子激发分子改变形状。测量分子形状的一种特别吸引人的方法是库仑爆炸成像或CEI，在这种方法中，分子通过尽可能多的电子被电离，并被激光脉冲或X射线完全分解，真正爆炸成原子碎片。通过确定它们来自哪个方向，就有可能实际制作爆炸时分子的图像。这种方法很重要，因为它可以一次成像气体中的一个分子，并产生精确到原子尺度的图像(埃斯特罗姆)。成像过程中包含了大量与激光脉冲中的电离过程、分子内部运动(振动)、电离过程中引发的动力学以及所涉及的飞秒时间尺度有关的物理学知识。该研究计划将重点放在提高图像质量上，特别是滑铁卢大学，那里现在有一台激光专门用于这项研究。在滑铁卢，我们将解决使用大分子库仑成像方法的最大障碍之一，即离子探测设备的效率。这通常只有65%，这意味着在给出完整的碎片信息之前，许多分子必须被爆炸。一种新的探测器已经问世，它有可能达到接近100%的效率，使成像更大的具有生物意义的分子的目标成为可能。我们将在滑铁卢测试新的探测器，目标是成像到目前为止获得的最大分子。该计划将使用一些方法来帮助我们了解导致分子形状改变的物理过程，其中将包括使用加拿大光源的单X射线光子电离。使用基于激光的成像的最大动机之一是激光的可控性，这使得我们能够产生特定的波长，能够在“泵浦”脉冲和成像“探测”脉冲中引发动力学，随后成像“探测”脉冲产生库仑爆炸并创建分子图像，通过改变两个脉冲之间的时间，我们可以录制“分子电影”，这将与先进的激光光源合作，在那里我们将尝试改进我们已经令人印象深刻的电影，即质子从乙炔分子的一端移动到另一端。

The way that molecules change their shape (molecular dynamics) is perhaps their most important property at the heart of biological systems and therefore all life. The scientific quest to image molecular shape and how it changes, or can be made to change, has been one of the driving forces behind the development of powerful tools such as ultrashort laser sources (with pulses similar in length to the natural timescale of a molecular motion, one thousand million millionth of a second one femtosecond) or x-ray sources such as synchrotron facilities, which can excite a molecule to change shape with one high energy photon. A particularly appealing method for measuring molecular shape is Coulomb Explosion Imaging or CEI in which the molecule is ionized by removing as many electrons as possible and completely broken apart by either the laser pulse or the x-ray and literally explodes into atomic fragments. By determining what direction they came from, it is possible to actually make an image of he molecule at the point of explosion The method is important because it can image one molecule at a time in a gas and create an image which is accurate to the scale of atoms (Angstrom). Folded into the imaging process is a wealth of physics relating to the ionization process in the laser pulse, the molecular internal motion (vibration), dynamics initiated during ionization and the femtosecond timescales involved. The research program will focus on improving the quality of images particularly in the University of Waterloo where a laser is now dedicated to this study. In Waterloo we will tackle one of the biggest barriers to using the Coulomb imaging method with large molecules, namely the efficiency of the ion detection apparatus. This is typically only 65% which means that many molecules must be exploded before one gives complete fragmentation information. A new detector has become available which has the potential to get close to 100% efficiency, making the goal of imaging larger biologically significant molecules a possibility. We will test the new detectors in Waterloo and aim to image the largest molecules so far achieved. The program will use a number of approaches to help us understand the physical processes which lead to molecules changing shape, these will include using ionization by single Xray photons at the Canadian Light Source. One of the biggest motivations for using laser based imaging though is the controllability of lasers, which allows us to generate specific wavelengths which can initiate dynamics in a “pump” pulse followed by an imaging “probe” pulse which generates the Coulomb explosion and creates the image of the molecule, by varying the time between the two pulses we can record a “molecular movie” this will be further pursed in collaboration with the Advanced Laser Light Source, where we will attempt to improve on our already impressive movie of a proton moving from one end of an acetylene molecule to the other.