Electron-impact dissociative ionization of the molecular ion ${\mathrm{HDO}}^{+}$: A global view

We present a combined experimental and theoretical study of the fragmentation of ${\mathrm{HDO}}^{2+}$ molecular ions produced by electron-impact ionization of ${\mathrm{HDO}}^{+}$ in the collision energy range 20–2500 eV. Experimental absolute partial inclusive cross sections for the production of ${\mathrm{OD}}^{+},{\mathrm{OH}}^{+}$, and ${\mathrm{O}}^{+}$ are reported and compared successfully to theoretical predictions. Ab initio methods are used to calculate the electron-impact ionization cross sections of the cationic ground state and first excited state leading to the first seven dicationic states. Dissociation probabilities of each channel are obtained by performing classical molecular dynamics on fitted dicationic potential energy surfaces. The predictive character of the theoretical modeling allows us to estimate that the nonmeasured dissociation channel giving a neutral oxygen atom contributes to 30% of the total ionization cross section. The isotopic ratio ${\mathrm{OD}}^{+}/{\mathrm{OH}}^{+}$ deduced from the experiment is $\left(3.1\pm 0.2\right)$ on average, constant in the 30–2500 eV energy range. The calculated isotopic ratio is found to be strongly dependent on the vibrational excitation of the target. Good agreement with the experimental value is obtained for a vibrational excitation corresponding to a temperature of about 2500 K, which is compatible with typical characteristics of electron cyclotron resonance (ECR) ion sources.

Figure 1

Upper panel: Squares and triangles with error bars: Experimental electron-impact dissociative ionization cross sections for the production of ${\mathrm{OD}}^{+}$ and ${\mathrm{OH}}^{+}$. Dashed and dashed-dot lines are their theoretical equivalents at $T=2000$ K, with $\Sigma \left[\alpha \right]$ being defined in Eq. (4). Solid line: Theoretical cross section for the production of O. Lower panel: Computed ionization cross sections of vertical transitions from the ground state $\tilde{X}{}^2{B}_1$ of ${\mathrm{HDO}}^{+}$ to the first seven states of ${\mathrm{HDO}}^{2+}$ (an electron is removed from one of the four highest molecular orbitals, as indicated in the inset). Solid squares with error bars: Experimental cross section for the production of ${\mathrm{O}}^{+}$. Solid line: Theoretical cross section corresponding to the creation of a vacancy in the $2a_1$ orbital.

Figure 2

Electron-impact dissociative ionization of ${\mathrm{HDO}}^{+}$: Isotopic ratio ${\sigma }_{{\mathrm{OD}}^{+}}/{\sigma }_{{\mathrm{OH}}^{+}}$ (full dots with error bars) versus the electron-impact energy. The other curves correspond to the theoretical predictions $\rho (E,T)$ for several vibrational temperatures.