Chemi-ionization in collisions of metastable helium with metastable helium

Abstract

Absolute and relative cross sections were obtained for the Penning-ionization (PI) reaction He*+He*→He+He++e and the associative-ionization (AI) reaction He*+He*→He+2+e. The studies were made using a merging-beams technique for a relative kinetic energy W of the rectants from nominally 0.01 to 10 eV. The He* represents a composite of about 13% He(2 1S) and 87% He(2 3S). The He*–He* system is found to be attractive with a well depth of approximately 0.5 eV. The PI reaction is directed with most of the Penning ions scattered in the direction of the parent He*. The fraction of AI to total ionization (AI plus PI is only a few percent. Cross sections for total ionization vary as W−0.38 for W<0.1 eV. At W=0.033 eV our absolute total ionization cross section of 112×10−16 cm2 agrees, within mutual errors, with the values obtained from most other experiments and from theory.