The chemi-ionization of He-*(2 S-1,S-3) plus Ar,Kr,Xe for collision energies from 0.003 to 6 eV

Measurements of the collision energy dependence of the chemi-ionization cro ss sections of the model systems He*(2 S-1,S-3) + R, R = Ar,Kr,Xe, are pres ented for the relevant energy range 0.003-6 eV. Except for He*(2 S-1) + Xe, all systems show a pronounced minimum of ionization at thermal energies. I n the hyperthermal range, however, the two spin systems are in sharp contra st to each other: the triplet systems exhibit a broad saturating maximum of ionization, whereas the singlet systems reveal a shallow shaped second min imum (pronounced for Xe) which can be rationalized within the one-electron model potential calculations of Siska [J. Chem. Phys. 71, 3942 (1979)]. Usi ng an effective single-channel Schrodinger equation, one finds, for all sys tems, complex potentials depending on distance only which properly model th e measured features. A more profound picture appears when differential elas tic cross sections from experiments of other groups are included. In the tr iplet case, differential elastic and chemi-ionization data can be adequatel y described with one complex potential, i.e., the local approximation holds . This is not found for the singlet systems. The hyperthermal minima instea d give evidence that the optical singlet potentials are nonlocal. Various a spects of this striking spin dependence of the characteristics of He*(2 S-1 ,S-3) + R are discussed. In particular, the breakdown of the local approxim ation in the singlet case can be rigorously derived. (C) 1999 American Inst itute of Physics. [S0021-9606(99)00440-7].