Multiple electron transfer processes in collisions of ${\text{N}}^{6+}$ and ${\text{O}}^{7+}$ with methane

Recent experiments on collision processes of ${\text{O}}^{7+}$ and ${\text{N}}^{6+}$ ions colliding with methane at the same velocity show unexpected differences in the fragmentation cross sections of the methane. Despite the expected similarity of these two processes, as both projectiles are hydrogenic, the mechanisms of electron transfer are different and lead to different fragmentation cross sections. In the present work, the collisions between ${\text{N}}^{6+}$ and ${\text{O}}^{7+}$ ions and methane are investigated theoretically at equal velocities corresponding to projectile energies of 30 and 35 keV, respectively. Electron-nuclear dynamics is used to study multiple electron transfer processes occurring in these collisions. Several multiple charge transfer probabilities are calculated and results, averaged over various orientations of the methane molecule, are reported. The collisions proceed in two stages: a fast stage of electron transfer from methane to the ion, and a much slower stage of breakup of the methane. We find and explain the intuitively unexpected result that the total charge transfer cross section for ${\text{N}}^{6+}$ is slightly larger, but that the ${\text{O}}^{7+}$ leaves the methane in a higher charged state with higher probability, leading to more fragmentation in the collisions with ${\text{O}}^{7+}$.

Figure 1

(Color online) Orientationally averaged one-electron transfer probability for ${\text{N}}^{6+}-{\text{CH}}_4$ (solid) and ${\text{O}}^{7+}-{\text{CH}}_4$ (dash) collisions as a function of the impact parameter at $E=30\text{keV}$ and $E=35\text{keV}$, respectively.

Figure 2

(Color online) Orientationally averaged two-electron transfer probability for ${\text{N}}^{6+}-{\text{CH}}_4$ (solid) and ${\text{O}}^{7+}-{\text{CH}}_4$ (dash) collisions as a function of the impact parameter at $E=30\text{keV}$ and $E=35\text{keV}$, respectively.

Figure 3

(Color online) Orientationally averaged three-electron transfer probability for ${\text{N}}^{6+}-{\text{CH}}_4$ (solid) and ${\text{O}}^{7+}-{\text{CH}}_4$ (dash) collisions as a function of the impact parameter at $E=30\text{keV}$ and $E=35\text{keV}$, respectively.

Figure 4

(Color online) Orientationally averaged all-electron transfer probability for ${\text{N}}^{6+}-{\text{CH}}_4$ (solid) and ${\text{O}}^{7+}-{\text{CH}}_4$ (dash) collisions as a function of the impact parameter at $E=30\text{keV}$ and $E=35\text{keV}$, respectively.

Figure 5

(Color online) Orientationally averaged Mulliken population change in the dynamic END electronic wave function for ${\text{N}}^{6+}-{\text{CH}}_4$ (solid) and ${\text{O}}^{7+}-{\text{CH}}_4$ (dash) collisions as a function of the impact parameter at $E=30\text{keV}$ and $E=35\text{keV}$, respectively. The weighted sum Eq. (13) is shown for ${\text{N}}^{6+}-{\text{CH}}_4$ (squares) and ${\text{O}}^{7+}-{\text{CH}}_4$ (asterisks) to show convergence of the projection of the final wave function.

Figure 6

(Color online) Overlaps squared between dynamic END orbitals and SCF orbital-spaces for the ${\text{N}}^{6+}-{\text{CH}}_4$ collision at $E=30\text{keV}$ as a function of the time. Orbital 2 Eq. (15) (solid), orbital 3 Eq. (16) (long dash), orbital 4 Eq. (17) (short dash), orbital 5 Eq. (18) (dotted).

Figure 7

(Color online) Overlaps squared between dynamic END orbitals and SCF orbital-spaces for the ${\text{O}}^{7+}-{\text{CH}}_4$ collision at $E=35\text{keV}$ as a function of the time. Orbital 2 Eq. (15) (solid), orbital 3 Eq. (16) (long dash), orbital 4 Eq. (17) (short dash), orbital 5 Eq. (18) (dotted).