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

We report on the fragmentation of the water molecule by 1 MeV ${\mathrm{H}}^{+}$ and ${\mathrm{He}}^{+}$ and 650 keV ${\mathrm{N}}^{+}$ 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 ${\mathrm{N}}^{+}$ are found to be more than an order of magnitude higher than that for ${\mathrm{H}}^{+}$. The relative ratios of the fragmentation channels are also different for the three projectiles. Additional fragmentation channels were observed in the spectra for ${\mathrm{He}}^{+}$ and for ${\mathrm{N}}^{+}$ impact, which are missing in the case of ${\mathrm{H}}^{+}$. From the analysis of the kinetic energy of the fragments, the maximum observed degree of ionization was found to be $q_{max}=3$, 4, and 5 for $\mathrm{H}{}^{+}$, $\mathrm{He}{}^{+}$, and $\mathrm{N}{}^{+}$ 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.

Figure 1

Absolute double-differential fragmentation cross-section spectra for the ${\mathrm{H}}_2\mathrm{O}$ molecule measured at $45{}^{\circ }$ observation angle. Open triangles stand for ${\mathrm{H}}^{+}$ impact, open circles for ${\mathrm{He}}^{+}$, and solid squares for the ${\mathrm{N}}^{+}$ projectile. The yields above 15 eV are due to multiple-ionization ($q>2$) processes. Their relative contribution is small for ${\mathrm{H}}^{+}$ and strongly increases with increasing perturbation (see text).

Figure 2

Absolute double-differential fragmentation cross-section spectra of the ${\mathrm{H}}_2\mathrm{O}$ molecule induced by 650-keV ${\mathrm{N}}^{+}$ impact. The presented spectrum was measured at $45{}^{\circ }$ observation angle. The identified fragmentation channels and the regions of the different ionization degrees are indicated.

Figure 3

Fragment ion spectra of ${\mathrm{H}}_2\mathrm{O}$ induced by (a) ${\mathrm{H}}^{+}$, (b) ${\mathrm{He}}^{+}$, and (c) ${\mathrm{N}}^{+}$ projectiles (symbols). The peaks represent Gaussian fit for the fragmentation channels listed in Tables 1 and 2. Channel positions are indicated by vertical lines with numbers.

Figure 4

The ${\sigma }_{{\mathrm{N}}^{+}}/{\sigma }_{{\mathrm{He}}^{+}}$ ratios for the individual fragmentation channels (the colors of the lines are the same as the colors of the Gaussians in Fig. 3). The ratios are presented starting from the ${\mathrm{OH}}^{+}+{\mathrm{H}}^{+}$ channels. The sequence of the lines is almost the same as the sequence of the energy centers of the Gaussians. The ionization degrees of the ${\mathrm{H}}_2\mathrm{O}$ molecule and the O fragments for the displayed channels are indicated.

Figure 5

(a) Ionization and (b) single capture + ionization probabilities as a function of the impact parameter for the ${\mathrm{H}}^{+}$ + ${\mathrm{H}}_2\mathrm{O}$ collision system. The number of vacancies produced in the target molecule, $n$, is indicated at the curves.

Figure 6

(a) Ionization and (b) single capture + ionization probabilities as a function of the impact parameter for the ${\mathrm{N}}^{+}$ + ${\mathrm{H}}_2\mathrm{O}$ collision system. The number of vacancies produced in the target molecule, $n$, is indicated at the curves.

Figure 7

Pure ionization (${\mathrm{i}}^n{\mathrm{c}}^0$) and single capture + ionization (${\mathrm{i}}^{n-1}$ ${\mathrm{c}}^1$) cross sections for (a) ${\mathrm{H}}^{+}$ and for (b) ${\mathrm{N}}^{+}$ projectiles. Pure ionization is presented as solid circles for CTMC and solid triangles for CDW-EIS calculations. The capture + ionization process is denoted by open circles and open triangles for CTMC and CDW-EIS, respectively. The lines are a guide for the eye.

Figure 8

Multiple-ionization cross section as a function of the ionization degree of the target for (a) $\mathrm{H}{}^{+}$, (b) $\mathrm{He}{}^{+}$, and (c) $\mathrm{N}{}^{+}$ bombardment. The theoretical predictions for the different ionization degrees are also shown.