EFFECT OF SURFACE-DEFECTS ON CHARGE-EXCHANGE OF LOW-ENERGY DEUTERIUM IONS SCATTERED FROM SRCL2 AND BAF2

Capture and loss of valence electrons during scattering of low-energy (10-1000 eV) deuteron has been investigated at polycrystalline SrCl2 a nd BaF2 surfaces with and without surface defects. The electronic tran sition is found to be a local process on the stoicheiometric surfaces; the primary D+ ion survives neutralization only when scattered from h ighly ionized target species and the D- ions are also formed during a close atomic encounter with individual target ions. However, the local character of D- formation disappears at the damaged (or metallic) sur faces due to electron-beam irradiation. These findings clearly show th at there are at least two competing channels for D- formation; the non local resonance tunneling process and the local molecular-orbital proc ess. The nonlocal process is suppressed at the ionic-compound surfaces due to the existence of the large band gap and the valence electrons are captured mainly via a transient chemisorptive bond. The D- ion is formed preferentially on the cationic site rather than on the anionic site. This is because amphoteric deuterium, being coordinated negative ly (positively) on the cationic site (the anionic site), is scattered without losing the memory of such transient chemisorption states. Thes e findings give unique insight not only into femtosecond dynamics of s urface chemistry but also into surface electronic properties of insula ting ionic compounds.