On the Energy Deposition of Cesium-Beam Gas-Phase Ionization (CBI)

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Cesium-beam gas-phase ionization (CBI) mass spectra measured for several organic molecules demonstrated a considerable fragmentation suggesting excitation energies in CBI higher than those in electron ionization (EI). A methodology to extract excitation energy distribution resulting from CBI has been developed using the knowledge of breakdown curves of product ions, and the energy deposition function (P) for the molecular ion prior to fragmentation has been experimentally evaluated using the well understood breakdown curves of hexacarbonyl chromium ions. It was found that energies of effective excitation are higher than 17 eV and the excitation probability decreases with decreasing the internal energy of the molecular ion (E). Additionally, these excitation energy distributions observed were not significantly influenced by the acceleration voltage of cesium beam. A plot of ln(P/E) against E showed a linear relationship with a positive slope although such the plot should result in a negative linearity for collision-induced dissociation processes. These results have here been interpreted not by collision excitation directly due to translational energy transfer caused by fast-atom bombardment but by a resonant ionization due to the formation of fast super-excited Rydberg cesium atoms attributed to an incomplete recombination of a fast Cs+ and an electron in a Cs-FAB source.
日本質量分析学会の論文