Water-molecule dissociation by proton and hydrogen impact

Time-of-flight-based mass analysis of charged water fragments have been used to measure the dissociative and the nondissociative reaction pathways of water formed during collisions with $15\text{to}100\mathrm{keV}$ and $500\text{to}3500\mathrm{keV}$ ${\mathrm{H}}^{+}$ projectiles and with $8\text{to}100\mathrm{keV}$ ${\mathrm{H}}^0$ projectiles. The fragmentation pathways resulting from the ionization and the electron capture collisions with the incident ${\mathrm{H}}^{+}$ and ${\mathrm{H}}^0$ projectiles, as well as collisions involving projectile electron loss by the incident ${\mathrm{H}}^0$ projectiles, were separately recorded by detecting the target product ions in coincidence with either the ejected target electrons or the charge-analyzed projectiles. The fragmentation profile shows that at high collision energies the ionization of water arises mainly through outer shell processes. At lower energies valence electron capture and ionization dominate and transfer ionization leads to substantially different fragmentation patterns. ${\mathrm{H}}^0$ and ${\mathrm{H}}^{+}$ projectiles are found to be equally efficient at ionizing the water molecule. These results are of particular interest to workers in astrophysics and those involved in cancer therapy with heavy particle ion beams.

Figure 1

TOF spectrum for $2\mathrm{MeV}$ protons incident on water vapor taken with the experimental setup at the Catholic University of Rio de Janeiro.

Figure 2

${\mathrm{H}}^{+}$ impact capture cross sections for combined ${}_{10}\sigma _{01}+{}_{10}\sigma _{02}$ leading to the formation of the target products ${\mathrm{H}}_2{\mathrm{O}}^{+}$, $\mathrm{O}{\mathrm{H}}^{+}$, ${\mathrm{O}}^{+}$, and ${\mathrm{H}}^{+}$. Present data—filled symbols; Gobet et al. [11, 12]—open symbols. Key: squares—${\mathrm{H}}_2{\mathrm{O}}^{+}$; up triangles—${\mathrm{H}}^{+}$; circles—$\mathrm{O}{\mathrm{H}}^{+}$; inverted triangles—${\mathrm{O}}^{+}$. Error bars are not shown for the sake of clarity.

Figure 3

${\mathrm{H}}^{+}$ impact cross sections leading to the formation of the target product channels ${\mathrm{H}}_2{\mathrm{O}}^{+}$, $\mathrm{O}{\mathrm{H}}^{+}$, ${\mathrm{O}}^{+}$, and ${\mathrm{H}}^{+}$. Pure ionization ${}_{10}\sigma _{11}$: present high-energy data—filled symbols; Gobet et al. [11, 12]—open symbols. Combined pure ionization and transfer ionization, ${}_{10}\sigma _{11}+{}_{10}\sigma _{02}$: present low-energy data—filled symbols; Werner et al. [10]—left-filled symbols. Key: squares—${\mathrm{H}}_2{\mathrm{O}}^{+}$; up triangles—${\mathrm{H}}^{+}$; circles—$\mathrm{O}{\mathrm{H}}^{+}$; inverted triangles—${\mathrm{O}}^{+}$. Error bars are not shown for the sake of clarity.

Figure 4

Ternary graph for the single ionization of water by $15–3500\mathrm{keV}$ proton (squares) and $45–1500\mathrm{eV}$ electron (circles) impact. Proton data is from this work and electron data is from Scully et al. [28] (see also Montenegro et al. [25]).

Figure 5

${\mathrm{H}}^{+}$ impact total cross sections summed over all target product channels. One-electron capture cross sections ${}_{10}\sigma _{01}+{}_{10}\sigma _{02}$: present—full circles; Gobet et al. [11, 12]—open circles. Pure ionization cross sections ${}_{10}\sigma _{11}$: present high-energy data—filled squares; Gobet et al. [11, 12]—open squares. Combined transfer ionization and pure ionization ${}_{10}\sigma _{11}+{}_{10}\sigma _{02}$: present low-energy data—filled squares; Werner et al. [10]—half-filled squares. Also included are the total Rudd et al. [9] cross sections for capture ${\sigma }_{10}$—star symbol, and total target positive ion production ${\sigma }^{+}$—cross symbols, as well as present combined cross sections for the capture and the ionization channels summed over all target products—triangles. Error bars are not shown for the sake of clarity.

Figure 6

${\mathrm{H}}^0$ impact ionization cross sections for the combination ${}_{10}\sigma _{11}+{}_{10}\sigma _{02}$ leading to the formation of the target products ${\mathrm{H}}_2{\mathrm{O}}^{+}$, $\mathrm{O}{\mathrm{H}}^{+}$, ${\mathrm{O}}^{+}$, and ${\mathrm{H}}^{+}$. Black squares—${\mathrm{H}}_2{\mathrm{O}}^{+}$ channel; black circles—$\mathrm{O}{\mathrm{H}}^{+}$ channel; black up triangles—${\mathrm{H}}^{+}$ channel; black inverted triangles—${\mathrm{O}}^{+}$ channel; bottom filled circles—summed ionization cross sections over all target product channels. Error bars are not shown for the sake of clarity.

Figure 7

${\mathrm{H}}^0$ impact one-electron capture cross sections ${}_{00}\sigma _{-11}$ leading to the formation of the target products ${\mathrm{H}}_2{\mathrm{O}}^{+}$, $\mathrm{O}{\mathrm{H}}^{+}$, ${\mathrm{O}}^{+}$, and ${\mathrm{H}}^{+}$. Symbols are same as in Fig. 6. Only few error bars are shown for the sake of clarity.

Figure 8

${\mathrm{H}}^0$ impact cross sections ${}_{00}\sigma _{11}$ for projectile electron loss with target ionization leading to the products ${\mathrm{H}}_2{\mathrm{O}}^{+}$, $\mathrm{O}{\mathrm{H}}^{+}$, ${\mathrm{O}}^{+}$, and ${\mathrm{H}}^{+}$. Symbols are same as in Fig. 6. Error bars are not shown for the sake of clarity.

Figure 9

Comparison of present ${\mathrm{H}}^0$ impact total cross sections for ionization, one-electron capture and projectile electron-loss collisions obtained by summing individual cross sections for the ${\mathrm{H}}_2{\mathrm{O}}^{+}$, $\mathrm{O}{\mathrm{H}}^{+}$, ${\mathrm{O}}^{+}$, and ${\mathrm{H}}^{+}$ target channels. Right-filled circles—ionization, ${}_{00}\sigma _{01}$; right-filled triangles—one-electron capture, ${}_{00}\sigma _{-11}$; right-filled squares—projectile electron loss, ${}_{00}\sigma _{11}$. Present cross sections for projectile electron loss measured in noncoincidence mode $({}_{00}\sigma _{10}+{}_{00}\sigma _{11})$—bottom-filled circles, are included for comparison. Also included are the ${\sigma }^{+}$ cross sections for total positive target ion production by ${\mathrm{H}}^0$ projectiles—solid line, and by ${\mathrm{H}}^{+}$ projectiles—crosses. Error bars are not shown for the sake of clarity.

Figure 10

Comparison of present ${\mathrm{H}}^0$ impact cross sections for total projectile electron loss with previous measurements. Sum ${}_{00}\sigma _{10}+{}_{00}\sigma _{11}$ obtained through measurements without coincidences with target ion products: bottom-filled circles, present (same as in Fig. 9); open circles, Gobet et al. [29]; black stars, Dagnac et al. [30]; black crosses, Toburen et al. [8]. ${}_{00}\sigma _{11}$ using coincidence methods: right-filled squares present (same as in Fig. 8); open black squares, Gobet et al. [29].