Studying the Dissociation Dynamics of Carbon Dioxide

Abstract: Small molecules, and their fragmentation to smaller constituents upon excitation, are often used to understand how the bonds in a molecule are formed and how they break. One of such widely studied model molecule is CO2. This study aims at extending the knowledge of dissociation of carbon dioxide by measuring also the negatively charged fragments. In addition, these anions are linked to positive ions (cations) which are more abundant, and their production pathways are widely studied. The measurements were carried at Elettra Synchrotron Radiation Laboratory in Trieste, Italy. Provided that anion production at core-level excitation is far less abundant than that of electrons and cations, a new spectrometer was developed precisely for the purpose of detecting the negative ions. It works in tandem with a positive ion spectrometer, mounted together for coincidence measurements. The properties and advantages of the present instrument, such as high acceptance angle and magnetic deflection of unwanted electrons are described. CO2 molecule is excited by photons which are energetic enough to promote core electrons of the oxygen atom to unoccupied molecular orbitals in CO2 - the following dissociation pathways are then studied by time-of-flight based mass spectroscopy. Both singly-charged and doubly-charged cations are found, in varying proportions, at different photon energies. Also, negative ions are seen, with C– being present only at the O1s edge but not at lower excitation energies. Furthermore, metastable states and triply-charged cations are a possibility, however, their complete analysis requires more attention. They are nonetheless discussed and analyzed to some extent.