# Dissociative ionization of the H${_2}$O molecule induced by medium energy singly charged projectiles

## TL;DR

This study investigates how water molecules fragment when impacted by medium-energy singly charged ions, revealing differences in ionization and fragmentation channels, and comparing experimental results with theoretical models.

## Contribution

It provides new experimental data on water molecule fragmentation by different ions and evaluates the accuracy of existing theoretical models for ionization cross sections.

## Key findings

- N$^{+}$ impact yields higher fragmentation cross sections than H$^{+}$.
- Different projectiles produce distinct fragmentation channels.
- Theoretical models agree at low ionization degrees but overestimate at higher degrees.

## Abstract

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.