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Abstract:
We report an experimental study of the electron-impact ionization (E0 = 65 eV) and fragmentation of carbon dioxide clusters for the formation of ${\mathrm{C}\mathrm{O}}_{2}^{+}$ and ${{\left({\mathrm{C}\mathrm{O}}_{2}\right)}_{2}}^{+}$ using a reaction microscope. All three charged particles, two outgoing electrons and one cation, are detected in coincidence such that the momentum vectors and, consequently, the kinetic energies for these final-state particles are determined. The reaction mechanisms correlated to ionic species are analyzed using the measured binding energy spectra over a broad range of ionization energy (8–32 eV). We observe that the ${{\left({\mathrm{C}\mathrm{O}}_{2}\right)}_{2}}^{+}$ cations with small momenta are formed mainly from the direct ionization of the outermost 1πg orbital in the carbon dioxide dimer (CO2)2. While the (1πu)−1, (3σu)−1 and (4σg)−1 states in ${{\left({\mathrm{C}\mathrm{O}}_{2}\right)}_{2}}^{+}$ are unstable and most likely dissociate into ${\mathrm{C}\mathrm{O}}_{2}^{+}$ and neutral species. The ${{\left({\mathrm{C}\mathrm{O}}_{2}\right)}_{2}}^{+}$ cations with high momenta are attributed to the ionization of small carbon dioxide clusters (CO2)n (n ⩾ 3) and subsequent dissociation processes. Compared to the appearance energy studies of carbon dioxide clusters where only the outermost 1πg orbital ionization was observed for formation of ${{\left({\mathrm{C}\mathrm{O}}_{2}\right)}_{2}}^{+}$, several additional channels are identified in this work.
