Ion pair production and chemi-ionisation in collisions of He*(21,3S) with Li

A merging-beams technique has been used to study collisions of He*(21,3S) with Li. The He* represents a composite of 13% He(21S) and 87% He(23S). Absolute and relative cross sections, Q, have been measured in a range of relative kinetic energy, W, from 150 to 1500 eV for the ion pair production (IPP) of He++Li-, and from 0.01 to 500 eV for chemi-ionisation (CI) (i.e. Penning ionisation and associative ionisation). The relatively small (<AA2) and monotonically increasing Q with W for IPP is qualitatively explained on the basis of poor coupling in at least the first step of a ladder-climbing process between the incoming covalent and outgoing ionic potentials. Attempts to observe signals for IPP of He-+Li+ were obscured by noise. Information obtained for CI shows that: the Penning ionisation reaction is directed with most of the Li+ scattered in the incident Li direction, the He*-Li system is attractive with a measured well depth of 0.73 eV and the value of Q for total ionisation varies as W-0.34. Similarities to the He*-He* system are also given.

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