Delayed-Coincidence Study ofO++ Ar Collisions at 50-200 keV

Delayed-coincidence techniques have provided measurements of the distributions in values of $Q$, the energy transferred to inelastic processes, during ${\mathrm{O}}^{+}$ + Ar collisions at 50-200 keV. As a function of decreasing ${\mathcal{r}}_0$, the distance of closest approach of the colliding particles, the average values $\overline{Q}$ of the $Q$ distributions increase rapidly. At 50 keV, for example, $\overline{Q}$ increases from 142 eV at ${\mathcal{r}}_0$ of 0.26 Å to 707 eV at 0.07 Å. For fixed ${\mathcal{r}}_0$ the $\overline{Q}$ values increase with bombarding energy and lie 150-200 eV higher at 200 keV. The $Q$ distributions for ${\mathcal{r}}_0$ between 0.11 and 0.20 Å consist of two overlapping symmetrical distributions. Although one of these two distributions lies as much as 215 eV above the other, measurements of the average charges of the oxygen ions after their collisions show that none of the extra energy goes into ionization of the beam particles. These distributions occur over the range of ${\mathcal{r}}_0$ where penetration of the argon $L$ electron shell is expected; differential cross sections for scattering of the oxygen ions also show structure in this region. The qualitative behavior of the $Q$ distributions agrees generally with theoretical expectations. However, the energy differences between the centers of the two overlapping distributions do not agree with simple theoretical predictions, which also do not forecast the observed variation of these differences with ${\mathcal{r}}_0$.