Collision Spectroscopy. II. Inelastic Scattering ofHe+by Ne

Abstract

The inelastic scattering of He 4 " by Ne has been studied as a function of angle 6 and energy loss AE 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 2/> 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 outside a threshold obeying the rule T C = (E8) c & 1035 eVdeg, 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 r Qi with the property that the product of the wave number k and the angular spacing A0 between peaks is substantially constant, i.e.,Ab-2ir/kA0 = 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 r c s 1.9 a.u. and at an energy of V c = 13.5 eV. The behavior of the reduced cross section p= 6 sm6(j(6 t 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 x 10 6 cm/sec. From this, the transition matrix element at the crossing is deduced by Landau-Zener theory to be roughly #i2^r<P ~ * eV * ^X higher energies the maximum height of the first Stueckelberg peak shows a general falloff as PQE=3.2 x 10" 6 cm eV, as well as a slow oscillation whose peaks are evenly spaced in reciprocal velocity with a characteristic spacing constant t;* = 8.8 x 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 Ne and He(ls2s), 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 1-^5000 eVdeg, arising from a crossing at r x & 1.23 a.u. with a second state dissociating to the same products. Its transition matrix element is shown to be roughly ^13 (^) ~3 eV. Approximate potential curves are deduced for three of the low-lying 2 2 states of HeNe + and compared with calculations by Michels. It is shown that the available crossings 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.

Photon ? ion angular correlations for He+ + He and He+ + Ne collisions

“…Differential cross sections for the sum of the channels He + +Ne 1P1, 3P0.1, 2 have been measured by Coffey et al [8] and by Baudon et al [34]. Curve crossing radii estimated from these experimental results turned out to be in good agreement with the results of a quantum chemical calculation by Sidis and Lefebvre-Brion [36].…”

“…All these parameters A~M, are a sum of ~P and 3p contributions, AMM, = 1AMM, + 3AMM,. (12) When these equations are written down for the cases with and without magnetic field, and (8) is used for sured by us, but except for the worse angular resolution, it agrees with other results [8]. The curve was measured for ECM=50eV, but is similar for other energies.…”

“…The slightly oscillating structure in Fig. 9 is the remainder of the actual structure; the maximum to minimum ratio of the actual oscillations [8] is about 2.…”

Partial differential cross sections for inelastic He++Ne collisions at low energy

“…Coffey et al [1] measured differential cross sections for the Ne(2p53s) excitation. They observed the typical Stueckelberg oscillations, and they used their data for a discussion of the scaling laws for inelastic collisions of this type.…”

“…The angular resolution of the experimental data is sufficient to resolve all structures completely. For the channel in (1), which actually summarizes the Ne states 1P o and 3po,l,z, we measured the angular variation of two more quantities: (1) the triplet fraction, and (2), for the states 1P 1 and 3P1, the fraction for which the Ne wave function is aligned at right angles to the scattering plane. The last quantity is of great interest, because it is related to the probability of finding the He ÷ electronic spin inverted after the collision [13].…”