Ionization and electron-capture cross sections for single- and multiple-electron removal from ${\mathrm{H}}_2\mathrm{O}$ by ${\mathrm{Li}}^{3+}$ impact

In this work, we report experimental and theoretical ionization and electron-capture cross sections for single-, double- and triple-electron removal from ${\mathrm{H}}_2\mathrm{O}$ by ${\mathrm{Li}}^{3+}$ impact at energies ranging from 0.75 to 5.8 MeV. The experiment was carried out by selecting both the final charge state of the projectile and the ejected fragments in coincidence to obtain cross sections associated with ionization and electron-capture channels. The ionic fragments and the emitted electrons produced under single-collision conditions were collected by a time-of-flight spectrometer with single-hit (e.g., ${\mathrm{OH}}^{+}+{\mathrm{H}}^0)$ and double-hit events (e.g., ${\mathrm{OH}}^{+}+{\mathrm{H}}^{+})$ properly discriminated. For the one- and two-electron removal cases, the calculations based on the basis generator method for orbital propagation agree well with the experiment for most of the collision channels studied. Auger-electron emission after vacancy production in the inner $2a_1$ orbital of ${\mathrm{H}}_2\mathrm{O}$ is shown to have a substantial effect on the final charge-state distributions over the entire impact-energy interval.

Figure 1

Coordinate system for the coincidence experiment in the gas-jet assembly. The positive $X$ axis is defined in the axial direction of the spectrometer toward the recoil detector, the positive $Y$ direction along the beam direction, and the negative $Z$ direction in the direction of the gas jet. The electron, recoil-ion, and projectile-ion detectors indicated as SB (surface barrier detector), PS (position-sensitive detector), DT (dynode time-of-flight detector), FC (Faraday cup), and the TOFMS (e-side for electron side and R-side for recoil side) are included in the figure for clarity. The double arrows indicate the movement direction of the detectors.

Figure 2

Geometries of the ${\mathrm{Li}}^{3+}\text{−}{\mathrm{H}}_2\mathrm{O}$ collision system. (a) The collision plane and the molecular plane coincide. (b) The molecular plane is perpendicular to the collisional plane.

Figure 3

Single-electron removal probabilities for the active MOs $\{1b_1,3a_1,1b_2,2a_1\}$ as functions of the impact parameter for ${\mathrm{Li}}^{3+}\text{−}{\mathrm{H}}_2\mathrm{O}$ collisions at (a) $E_P=20$ keV/u and (b) $E_P=1000$ keV/u. The geometry corresponds to the configuration depicted in Fig. 2.

Figure 4

Partial, charge-state-correlated cross sections ${\sigma }_{kl}$ for $k$-fold capture and $l$-fold ionization as functions of impact energy for ${\mathrm{Li}}^{3+}\text{−}{\mathrm{H}}_2\mathrm{O}$ collisions: (a) pure ionization $\left(k=0\right)$, (b) single capture $\left(k=1\right)$, and (c) double capture $\left(k=2\right)$. Solid lines correspond to Eqs. (7) and (8), i.e., they include Auger processes, and dashed lines correspond to Eq. (5), i.e., Auger processes are ignored.

Figure 5

Total single-capture (SC) cross section as a function of impact energy for ${\mathrm{Li}}^{3+}\text{−}{\mathrm{H}}_2\mathrm{O}$ collisions. Experimental data (solid squares) are obtained via the growth-rate method. The error bars are smaller than the size of the symbols. Theory: the dashed line corresponds to the complete sum ${\sum }_l{\sigma }_{1l}$, the dash-dotted line to the restricted sum ${\sigma }_{10}+{\sigma }_{11}+{\sigma }_{12}+{\sigma }_{13}$, and the solid line to ${\sigma }_{10}+{\sigma }_{11}+{\sigma }_{12}$. The calculations include Auger-electron emission.

Figure 6

Total double-capture (DC) cross section as a function of impact energy for ${\mathrm{Li}}^{3+}\text{−}{\mathrm{H}}_2\mathrm{O}$ collisions. Experimental data (solid squares) are obtained via the growth-rate method. The error bars are smaller than the size of the symbols. Theory: the dashed line corresponds to the complete sum ${\sum }_l{\sigma }_{2l}$, the dash-dotted line to the restricted sum ${\sigma }_{20}+{\sigma }_{21}+{\sigma }_{22}$, and the solid line to ${\sigma }_{20}+{\sigma }_{21}$. The calculations include Auger-electron emission.

Figure 7

Pure-ionization cross sections ${\sigma }_{01},{\sigma }_{02}$, and ${\sigma }_{03}$ as functions of impact energy for ${\mathrm{Li}}^{3+}\text{−}{\mathrm{H}}_2\mathrm{O}$ collisions. The symbols represent the experimental results (most error bars are smaller than the size of the symbols) and the lines represent the calculations (including Auger-electron emission).

Figure 8

Single capture cross sections ${\sigma }_{10},{\sigma }_{11}$, and ${\sigma }_{12}$ as functions of impact energy for ${\mathrm{Li}}^{3+}\text{−}{\mathrm{H}}_2\mathrm{O}$ collisions. The symbols represent the experimental results (most error bars are smaller than the size of the symbols) and the lines represent the calculations (including Auger-electron emission).

Figure 9

Double-capture cross sections ${\sigma }_{20},{\sigma }_{21}$, and ${\sigma }_{22}$ as functions of impact energy for ${\mathrm{Li}}^{3+}\text{−}{\mathrm{H}}_2\mathrm{O}$ collisions. The symbols represent the experimental results and the lines represent the calculations (including Auger-electron emission).

Figure 10

Pure total ionization cross section ${\sum }_l{\sigma }_{0l}$ divided by the squared projectile charge $q^2$ as a function of impact energy for ${\mathrm{H}}_2\mathrm{O}$ targets. Experimental data: black circles - ${\mathrm{Li}}^{3+}$ (present work), triangles - ${\mathrm{H}}^{+}$: left filled [4], bottom filled [5], top filled [6], open [7], and full [9]. Calculations: black line - ${\mathrm{Li}}^{3+}$ (present work), blue line Refs. [36, 37, 38].