COLLISION SPECTROSCOPY .2. INELASTIC SCATTERING OF HE+ BY NE

The inelastic scattering of He+ by Ne has been studied as a function of angle θ and energy loss ΔE over the range of barycentric energies E from 22 to 500 eV. Many levels of Ne are excited, including autoionizing ones, and excited He+ is also seen. Excitation of the 2p53s 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 τc=(Eθ)c≅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 rc, with the property that the product of the wave number k and the angular spacing Δθ between peaks is substantially constant, i.e., Δb=2πkΔθ=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 He+ by Ne, which was earlier shown to be located at rc≅1.9 a.u. and at an energy of Vc≅13.5 eV. The behavior of the reduced cross section ρ=θsinθσ(θ,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 × 106 cm/sec. From this, the transition matrix element at the crossing is deduced by Landau-Zener theory to be roughly H12(rc)1 eV. At higher energies the maximum height of the first Stueckelberg peak shows a general falloff as ρ0E=3.2×10-16 cm2 eV, as well as a slow oscillation whose peaks are evenly spaced in reciprocal velocity with a characteristic spacing constant v*=8.8×106 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 Ne+ and 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 τx≅5000 eV deg, arising from a crossing at rx≅1.23 a.u. with a second state dissociating to the same products. Its transition matrix element is shown to be roughly H13(rx)3 eV. Approximate potential curves are deduced for three of the low-lying 2 states of HeNe+ 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. © 1969 The American Physical Society.