Dissociative ionization of the H2 O molecule induced by medium-energy singly charged projectiles

We report on the fragmentation of the water molecule by $1$ MeV H$^{+}$, He$^{+}$ and 650 keV N$^{+}$ ion impact. The fragment-ion energy spectra were measured by an electrostatic spectrometer at different observation angles. The obtained double-differential fragmentation cross sections for N$^{+}$ is found to be more than an order of magnitude higher, than that for H$^{+}$. The relative ratios of the fragmentation channels are also different for the three projectiles. Additional fragmentation channels were observed in the spectra for He$^{+}$ and for N$^{+}$ impact, which are missing in the case of H$^{+}$. From the analysis of the kinetic energy of the fragments, the maximum observed degree of ionization was found to be $q\rm{_{max}}=3$, $4$, and $5$ for H${^+}$, He${^+}$ and N${^+}$ impact, respectively. Absolute multiple ionization cross sections have been determined. They are compared with the predictions of the classical trajectory Monte Carlo and continuum-distorted-wave eikonal-initial-state theories. At lower degrees of ionization, theories provide reasonable agreement with experiment. The systematic overestimation of the cross section by the theories towards higher degrees of ionization indicates the failure of the independent particle model.