Differential Cross Section for Excitation of the2S3and Higher States of He by 600-eVHe

Abstract

Measurements of the differential cross section for excitation of the 2 3 S state of He by 600-eV He 4 " ions are reported. Pronounced regular oscillations are observed in the angular dependence of the cross section. Similar behavior is observed qualitatively in the excitation of higher states.Ion-atom elastic differential scattering in the rare gases, especially He + + He, has revealed a wealth of interesting structure including interference effects due to two types of symmetry, electronic 1 and nuclear, 2 and a perturbation 3 due to the crossing of two molecular states of symmetry 2 Sp. + . Inelastic differential scattering is expected to be equally interesting, and experiments with energy resolution insufficient to separate individual excited states have nevertheless shown considerable structure in the inelastic-scattering patterns obtained from several rare-gas ion-atom pairs. 4 It is of considerable interest to attain sufficient resolution to isolate individual states, especially because the curve crossings responsible for the perturbations seen in elastic scattering are expected also to provide a mechanism for specific inelastic transitions at low energies, and these events should be strongly angle dependent" In particular, if the leading elastic perturbation in He + + He has been diagnosed correctly, the same crossing should lead to the excitation of He in the state 2 3 S (and perhaps also 2 l S). We report here the initial results of a study of the differential inelastic scattering in the system He + + He with sufficient energy resolution to isolate the 2 3 S level of He, and also to separate partially some of the higher states.The experimental apparatus is basically the same as that used in earlier elastic-scattering measurements. 5 The small-angle angular resolution was improved by limiting the slits of the scattered-beam collimating apertures to 0.05 cmx0.1 cm. The energy resolution of the 127° cylindrical electrostatic analyzer was improved by retarding the ions to 90 eV or less before analysis. Using the analyzer in this manner to observe the energy spread in the incident ion beam, the spread was minimized by adjusting variables of the ion source. Although the analyzer resolution can be increased by lowering the analyzing energy, the signal-to-noise ratio decreases at the same time. We found empirically that the optimum operating condition was realized when the resolution was about equal to the beam energy spread, which ranged from 0.8 to 1.0 eV full width at half-maximum.To establish the energy scale accurately in the region of energy of the inelastically scattered ions, the elastic ions were retarded by an additional, accurately measured voltage. The elastic and inelastic ions were thereby analyzed at nearly the same kinetic energy, and the energy difference was measured to ±0.1 eV by a superposition of the two profiles on an X-Y plot. The difference between the elastic ion energy E e and inelastic ion energy E{ is given by __ _ Ecos 2 9[, (. 2AE Y /2 1 AE m for AE«E and small 9,where AE is the exci...

Time-of-Flight Analysis of Neutral Products from Inelastic Ion-Molecule Collisions

“…Utilizing K and R computed from the ramp voltage calibration, the effective mass of the inelastically scattered He o was determined from Eq. (11). From the known 4.0026 mass of He, the velocity or energy shift of the inelastic from elastic peaks has been determined to be 21±4 eV.…”

“…Since there is a correspondence between the gate driving voltage and each point in the mass scale, the above Eqs. (11) was solved to yield K, which is 0.000886 for 800 V ions in our instrument. Utilizing K and R computed from the ramp voltage calibration, the effective mass of the inelastically scattered He o was determined from Eq.…”

Vibrational Excitation in Collisions Involving Oxygen Ion Beams

“…(2) reduces to that given by Lorents, Alberth, and Hesterman. 25 .1E is calculated from the equation (4) where v and j are vibrational and rotational quantum numbers of the diatomic species,26 I is its moment of inertia, and primed quantum numbers are those for the final states. In the case of interactions of the type 02+X 27rg(v, j) + Ar--702+X 27rg(V', j') +Ar, (5) excitation of the target atom is not possible below 20.7-eV LAB energy, the threshold for excitation of the first electronically excited Ar state.…”

Inelastic collisions

“…They find that the perturbations are due to ' crossing ' of the Z, potential energy curve for He,+ with a curve which at infinite separation leads to He+ and an excited He atom. The corresponding inelastic scattering, involving excitation of the 2%' state has been observed by Lorents et al (1966) and by Coffey et al (1969). Knowing the interaction VB from analysis of charge transfer data (see 11, Section 3) the crossing point separation R, could be estimated from the threshold value T~ of the reduced scattering angle for onset of the perturbahon.…”