Studies of pyridine in water clusters with synchrotron radiation and 3D momentum spectrometry

Sammanfattning: By using the site-selectivity enabled by synchrotron radiation, we can choose the ionization site on a molecule. Core-ionization can result in that the molecule dissociates into charged (and uncharged) ions. With mass spectroscopy, the dissociated fragments’ mass-over-charge can be identified in a time-of-flight spectrum. Pyridine dissociation was studied for pyridine and for pyridine mixed with protonated water clusters (a cluster is a larger molecular compound consisting of multiple molecules ‘clustered together’). Fragments originating from different dissociation channels cannot be distinguished if they have the same mass-over-charge ratio in a time-of-flight spectrum, and therefore, by using momentum spectroscopy, the dissociation channels were studied qualitatively with a radius vs. time spectra in order to distinguish fragments from different dissociation channels. To correlate the different fragments (ions) with each other, an ion-ion coincidence map was used, which can be created from a 3D momentum spectrometer experiment to find out if it is a 2-body or 3-body dissociation, or neither (i.e., no or higher dissociation order). The mass and momentum spectroscopy experiments provided much data with complex structures, which requires some advance data treatment methods. I have created a GUI (Graphical User Interface) based on a command-line interface developed by the research group with which my bachelor project was conducted. The command-line interface, and hence, also the GUI, is a systematic method for treating data from a 3D momentum spectrometer experiment. I also developed an interactive tree structure within the GUI which allows for an interactive way of creating and combining filters recursively. The filters can then be used combined or individually while plotting the data, so that only data of interest is allowed through into the plot. Using the GUI to treat the data from a 3D momentum spectrometer-based experiment, we performed a case study on pyridine with protonated water clusters. Pyridine was added to protonated water clusters. We investigated the mass spectra and observed the site-selectivity enabled by synchrotron radiation ‘in action’ for the protonated water clusters. We also observed pyridinium (protonated pyridine), which means that proton transfer occurs from the protonated water to pyridine, and that high water molecules stabilize clusters with high pyridine concentration (more than 2 pyridine molecules in a single cluster). Thus, pyridine concentration determines how pyridine will interact with water. For high pyridine concentration, water molecules are more likely to be situated in the centre between the pyridine molecules to stabilize the cluster. Further analysis showed indications that pyridine molecules seem to be embedded within larger clusters, as we did not observe pyridine fragments bond with water molecules. Thus, if this is the case, water molecules can be seen as an obstacle hindering the photons from reaching pyridine.