Resonant Electron Capture in Small-Angle Collisions ofAr+on Ar andNe+on Ne

Measurements of the electron capture probability $P_0$ have been made for single collisions of ${\mathrm{Ar}}^{+}$ ions with neutral Ar atoms and ${\mathrm{Ne}}^{+}$ ions with neutral Ne atoms, the latter measurements being an extension of work previously reported by this laboratory. The incident ions ranged in energy from 200 eV to 4 keV, the scattered particles being detected at angles ranging from 0.75° to 15° in the laboratory coordinates. A charge analysis of the scattered particles has yielded values of $P_0$ as a function of the incident ion energy $T$ and scattering angle $\theta$; in both cases $P_0$ is found to oscillate rapidly as a function of $\theta$, with some dependence upon $T$ also. The periodicity of $P_0$ as a function of $T$ and $\theta$ is consistent with a description of the collision wherein the electronic state of the ion-atom system changes adiabatically and is a superposition of symmetric and anti-symmetric energy eigenstates of the diatomic molecular ion. The data are discussed in terms of this description, yielding empirical values of an integral $I$, defined as the energy difference $\Delta E$ between these two eigenstates, integrated over the collision path of relative motion. For the range of collisions studied, $I$ varied from about 6 to 26 eV-Å in the case of ${\mathrm{Ar}}^{+}$ on Ar, and from about 5 to 25 eV-Å in the case of ${\mathrm{Ne}}^{+}$ on Ne. An approximate functional dependence of $I$ upon internuclear separation $R$ is also obtained, although there is some evidence that $\Delta E$ may also depend upon the relative collision speed $v$ in the case of the ${\mathrm{Ar}}^{+}$-on-Ar collisions. It is proposed that the $\Delta E$ measured for the gentler collisions which produce no excitation of either atomic system is the energy difference between the ${\Pi }_g$ and ${\Pi }_u$ states of the quasimolecule.