X-ray-induced intermolecular reactions in mixed clusters

Abstract: The aim of this project was to study what mixed cluster fragments were formed in the interaction between radiation and H2O-CO2 clusters formed by adiabatic expansion, both experimentally and using simulations. The experiment was done using a mass spectrometer at the MAX IV synchrotron facility and by studying the time-of-flight of the ionic fragments. All peaks in the mass spectrum were identified by converting the time-of-flight to mass/charge, and the spectrum for two different photon energies (307 eV and 550 eV ) and two different stagnation pressures (1.5 bar and 2.2 bar) were compared. The data showed that mixed clusters were formed, including fragments with extra hydrogen atoms. The mass spectra showed a larger change in peak height and width with increasing pressure than with increasing photon energy. Molecular dynamics simulations were used to study the stability of the identified clusters. The simulated clusters contained CO2, H2O and H2O+. As a measure of stability, the largest centre of mass distance between the H2O+ molecule and any other molecule in the cluster in every time step of the simulation was determined. The structure of the more stable clusters was also studied visually. The simulations showed that the clusters vary greatly in stability, but generally clusters with many H2O molecules were more stable. The stable structures had the H2O+ molecule on one end of the cluster, with its hydrogen atoms directed towards the rest of the molecules. Although the clusters with many H2O molecules were more stable in the simulations, clusters with many CO2 molecules were more abundant in the experimental data. This was most likely due to a high amount of CO2 in the gas used to create the clusters.