Collision Spectroscopy. II. Inelastic Scattering of He + by Ne

The inelastic scattering of ${\mathrm{He}}^{+}$ by Ne has been studied as a function of angle $\ensuremath{\theta}$ and energy loss $\ensuremath{\Delta}E$ over the range of barycentric energies $E$ from 22 to 500 eV. Many levels of Ne are excited, including autoionizing ones, and excited ${\mathrm{He}}^{+}$ is also seen. Excitation of the $2{p}^{5}3s$ configuration of Ne at about 16.8 eV could be resolved uniquely, and was studied in detail. The process does not occur in forward scattering, but only outsides a threshold obeying the rule ${\ensuremath{\tau}}_{c}={(E\ensuremath{\theta})}_{c}\ensuremath{\cong}1035$ eV deg, typical of a curve-crossing interaction. It shows pronounced Stueckelberg interference oscillations, arising from the existence of two semiclassical trajectories inside the crossing point at ${\mathcal{r}}_{c}$, with the property that the product of the wave number $k$ and the angular spacing $\ensuremath{\Delta}\ensuremath{\theta}$ between peaks is substantially constant, i.e., $\ensuremath{\Delta}b=\frac{2\ensuremath{\pi}}{k\ensuremath{\Delta}\ensuremath{\theta}}=0.44$ a. u. These and other properties suffice to show that the upper state is attractive and to identify the transition with the outermost crossing that perturbs the elastic scattering of ${\mathrm{He}}^{+}$ by Ne, which was earlier shown to be located at ${\mathcal{r}}_{c}\ensuremath{\cong}1.9$ a.u. and at an energy of ${V}_{c}\ensuremath{\cong}13.5$ eV. The behavior of the reduced cross section $\ensuremath{\rho}=\ensuremath{\theta}sin\ensuremath{\theta}\ensuremath{\sigma}(\ensuremath{\theta}, E)$ at the first Stueckelberg peak shows that the transition probability peaks at an energy of about 25 eV, i.e., when the velocity at the crossing point is about 2.6 \ifmmode\times\else\texttimes\fi{} ${10}^{6}$ cm/sec. From this, the transition matrix element at the crossing is deduced by Landau-Zener theory to be roughly ${H}_{12}({\mathcal{r}}_{c})\ensuremath{\lesssim}1$ eV. At higher energies the maximum height of the first Stueckelberg peak shows a general falloff as ${\ensuremath{\rho}}_{0}E=3.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{2}$ eV, as well as a slow oscillation whose peaks are evenly spaced in reciprocal velocity with a characteristic spacing constant ${v}_{*}=8.8\ifmmode\times\else\texttimes\fi{}{10}^{6}$ cm/sec. This slow oscillation is also seen as an amplitude modulation of the whole pattern of Stueckelberg oscillations, and we tentatively ascribe it to a second mode of dissociation of the excited state produced in the crossing, yielding ${\mathrm{Ne}}^{+}$ and $\mathrm{He}(1s2s)$, which are not detected in our experiments; the probability of producing one or the other set of products presumably depends on the time spent in passing through an interaction region, and thus on the velocity. Above 250 eV another excitation process appears with a threshold at ${\ensuremath{\tau}}_{x}\ensuremath{\cong}5000$ eV deg, arising from a crossing at ${\mathcal{r}}_{x}\ensuremath{\cong}1.23$ a.u. with a second state dissociating to the same products. Its transition matrix element is shown to be roughly ${H}_{13}({\mathcal{r}}_{x})\ensuremath{\lesssim}3$ eV. Approximate potential curves are deduced for three of the low-lying $^{2}\ensuremath{\Sigma}$ states of He${\mathrm{Ne}}^{+}$ and compared with calculations by Michels. It is shown that the available crossing may explain the measured total charge-transfer cross sections measured by Stedeford and Hasted at energies below 1 or 2 keV, but not the magnitudes near the maximum near 10 keV.