Exploring molecular structure and dynamics through Coulomb Explosion Imaging

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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.