Single ionization of an asymmetric diatomic system by relativistic charged projectiles

We study single ionization of a heteroatomic system by charged projectiles whose velocity $v$ approaches the speed of light $c$. The system is formed by two loosely bound atomic species, $A$ and $B$, with the ionization potential of $A$ being smaller than excitation energy for a dipole-allowed transition in $B$. In such a case, three single ionization channels occur: (i) single-center ionization of atom $A$, (ii) single-center ionization of atom $B$, and (iii) two-center ionization of $A$. While (i) and (ii) are the well known mechanism of direct impact ionization of a single atom, in channel (iii) ionization of $A$ proceeds via impact excitation of $B$ with consequent radiationless transfer of excitation energy—via (long-range) two-center electron-electron correlations—to $A$, leading to its ionization. We show that, close to the resonance energy, the two-center channel (iii) is so enormously strong that its contribution remains dominant even if the range of emission energies $\sim 1$ eV, which is orders of magnitude broader than its width, is considered. The influence of relativistic effects, caused by a high collision velocity, on the angular distribution of emitted electrons may be quite strong even at $\gamma =1/\sqrt{1-v^2/c^2}\approx 2$. However, in the energy distribution and the total cross section, these effects become substantial only at $\gamma \gg 1$. Relativistic effects arising due to a large size of the two-atomic system are shown to be very weak even for a ${}^7\mathrm{Li}\text{−}\mathrm{He}$ dimer whose mean size is about 28 Å.

Figure 1

(a) Scheme of two-center ion impact ionization and (b) schematic representation of space coordinates characterizing the collision.

Figure 2

(a) The angular distribution of electrons emitted with the resonance energy from the Li-He system in collisions with 1 GeV protons. The distribution was calculated by averaging over the size of the dimer and is shown for the parallel ($\mathit{R}\parallel \mathit{v}$, thick dashed) and perpendicular ($\mathit{R}\perp \mathit{v}$, thick dotted) orientation as well as for the orientational average (thick solid). In addition, the corresponding results in the nonrelativistic limit ($c\to \infty$) are displayed by thin dashed, thin dotted and thin solid curves, respectively. (b) The relativistic–to–nonrelativistic cross section ratio for $\mathit{R}\parallel \mathit{v}$ (dashed), $\mathit{R}\perp \mathit{v}$ (dotted) and the orientational average (solid).

Figure 3

(a) The energy distribution of emitted electrons as a function of the detuning $\delta ={\omega }_A-{\omega }_B$ for the Li-He system for two-center ionization (solid) and direct ionization of Li (dashed) in collisions with 1 GeV protons. The two-center distribution was calculated by averaging over the internuclear vector $\mathit{R}$ of the dimer. (b) The corresponding energy distribution for the Ne-He system for two-center ionization (solid) and direct ionization of Ne (dashed). The two-center distribution was obtained by averaging over the orientation of the dimer at a fixed interatomic distance $R=3$ Å.

Figure 4

Total cross section, given as a function of the projectile energy (per nucleon) $E_p$, for two-center ionization of Ne-He (thick solid) and direct ionization of Ne (thick dashed) and He (thick dotted) atoms. The two-center cross section was calculated by averaging over the orientation of the dimer for a fixed interatomic distance $R=3$ Å. In addition, we show the coresponding cross sections obtained neglecting the relativistic effects ($c\to \infty$, by thin solid, thin dashed and thin dotted curves, respectively).

Figure 5

Total cross section at $E_P=1$ GeV/amu for Li-He dimer multiplied by $R^6$ as a function of the interatomic distance $R$ using the retarded dipole-dipole interaction (3) (solid) and the instantaneous interaction (4) (dashed). The cross section was averaged over the orientation of the dimer.