Experimental and theoretical investigation of the spectroscopy and dynamics of multiply charged CO cations.

Abstract

The spectroscopy and the dynamics of ${\mathrm{CO}}^{2+}$ and ${\mathrm{CO}}^{3+}$ in the 35--150-eV energy range have been studied using various complementary experiments, such as photoionization with synchrotron radiation, ion-ion coincidence technique, and ${\mathrm{H}}^{+}$ double-charge-transfer spectroscopy. In addition, ab initio calculations have been performed in order to obtain the potential-energy curves of the ${\mathrm{CO}}^{2+}$ electronic states lying between 35 and 60 eV. The lowest-energy state of ${\mathrm{CO}}^{2+}$ is shown to be predissociated. The metastable molecular doubly charged ions that appear at 40.75 eV energy correspond to transition(s) to the lowest-energy singlet state(s) of ${\mathrm{CO}}^{2+}$ $^{1}\mathrm{\ensuremath{\Pi}}$ and/or $^{1}\mathrm{\ensuremath{\Sigma}}^{+}$. The formation of the ${\mathrm{C}}^{+}$${(}^{2}$${\mathrm{P}}_{\mathrm{u}}$)+${\mathrm{O}}^{+}$ ${(}^{4}$${S}_{u}$) ion pair is shown to have a threshold energy of 38.4\ifmmode\pm\else\textpm\fi{}0.5 eV and is ascribed to highly excited (${\mathrm{CO}}^{\mathrm{*}}$${)}^{+}$ states, which autoionize into the double-ionization continuum. For most of the observed states, the partial cross section is reported from threshold up to 130 eV. No metastable ${\mathrm{CO}}^{3+}$ can be observed. The dominating dissociative path of triply charged molecular ions is the formation of the ${\mathrm{C}}^{2+}$+${\mathrm{O}}^{+}$ ion pair, for which we report the cross section up to 140 eV. The appearance energy is 81\ifmmode\pm\else\textpm\fi{}2 eV. The comparison of the double-photoionization cross section for the gas-phase molecule with previous photodesorption results is discussed.