214 

 bonds. This can result if the excited MX species makes the transition to an 



attractive state (forming both MX~' and M+X~'), or makes the transition to a 



repulsive state (forming M+X only). The fragment retaining the negative charge 



in these equations is the fragment that has the greater EA. Dissociative electron 



capture is typically observed for electron energies of 2-15 eV. The extent of 



fragmentation for dissociative capture and ion-pair production (below) is dependent 



upon the internal energy of the ion and thus the incident electron energy. The 



internal energy can also be influenced by temperature; increased source temperature 



has been shown to produce an approximately linear increase in fragmentation [72,74]. 



Ion-pair production (Eq 5-12) occurs for electron energies greater than 15 eV. 

 In this case, the intermediate is more often an excited molecule rather than an MX 

 species [72]. The primary electron (Cp") functions to excite MX to (MX*) which then 

 dissociates to form the ion pair. 



In the absence of reagent gas, a conventional 70 eV electron beam yields 

 approximately 1000 times more positive ions than negative ions [76]. Additionally, 

 the negative ions are typically only low mass fragment ions or "molecular debris." 

 [72]. Normally, the sensitivity with respect to detection of informative fragments of 

 negative ions under these conditions is poor. In cases where an MX~' is produced 

 of sufficient intensity to be observed, it can be explained by the production of 

 secondary thermal electrons. That is, the species captures the primary electron, 

 undergoes autodetachment as well as releasing a low energy secondary electron of 

 near-thermal energy to form and M"^' ion. The secondary near- thermal electron can 



