Low-Energy Collisions of Alkali-Metal Anions
H e a s u r e n e n t a of the total cross flection for electron detachment, cr.(E), are presented for low-energy (E < 300 eV) collisions of N a ' , K ' , and Cs' with atomic and molecular targets. For ma n y of the atomic (rare-gas) targets, the energy d e p e n d e n c e of p^(E) is striking: v i r ­ tually no detachment Is observed until relatively high collision energies (50 eV In the c e n t e r -o £-mass frame) ate reached, In c o n t r a d i s t i n c t i o n to what has been observed for similar collisions Involving H". The onsets for alkali a n i o n detachment are observed to be approximately equal to the thresholds for e x c i t a t i o n observed in c o l l i ­ sions of neu tral alkali atoms with these same targets. The similarity between the dynamics of the neutral system and that of the negative ion system, together with the o b s e r ­ vation (at greater energies) of d e t a c h m e n t accompanied by excitation of the alkali parent, suggests that electron detachment may be m e d i a t e d by a t w o -e 1ectron process In some cases, A simple curve crossing m e c h a n i s m adequately reproduces the observed o a (E) for several of these raregas targe t s „ M e a s u r e m e n t s of b o t h «jb (E) and the cross section for charge t ransfer o l (E) ha v e also b e e n c o m p l e t e d for H 2 , Dz , Nj , 0a , CO, C O j , S02 , N j O , CH, , and S F e targets. Electron detachment Is the d o m i n a n t process for all of these t a r ­ gets except 0 j T S02 , a n d SF a , with thresholds on the order of a few eV, Structure in o,(E) for the C02 target has been a t t r i b u t e d to charge transfer to a metastable state of C02 ' (2 Aj ) . Similarly, In the case of KjO, both t>9 ( E) and ^ ( E ) exhibit b e h a v i o r which suggests that charge transfer to a temporary negative ion state of that molecule la Involved in the colllslcnal dynamics. Charge transfer is also o b served to be an Important exit channel for the o t h e r molecules, with par t i c u l a r l y large cross sections (> 100 A 2 ) for the 02 , S O a , and SF6 targets. low-e ne rgy collisions o f a l k a l i-me t a l anions CHAPTER