Generation of Negative Ions

Electron capture by HCl and HBr via (HX)2 dimers has also been observed. 

Negative atomic ions have also been detected in oxygen, nitrogen oxide, carbon monoxide, water vapour, HgS, HgSe, N2O, etc. The process e + HgO - HO + H has been observed in water vapour along with e + H2O Hg + 0 . Hydroxyl ions have not been detected in these reactions. This is ascribed to the high excitation energy of the molecular H20 state which dissociates to H and HO [251]. 

The relevant cross section or probability of the process P was calculated from the measured yield of atomic ions and was found to depend both on the nature of the dissociating molecule and the electron energy. 

In contrast to this, the formation of a halogen ion X in iodine, hydrogen halides, and also in CCI4, CFgl, CCI2F2 appears to be possible at very low electron energy. For instance, the cross section of process e 4- HX - X -j- H shows a maximum close to 0.8 (HCl), 

2 (HBr and DBr) and 0.05 eV (HI). In every case, the process starts at an electron energy almost completely coinciding with the value Dhx Ex dissociation heat, 

---

Critical Phenomena in Dusty Plasma Kinetics Nucleation of Nanoparticles, Winchester Mechanism, and Growth of First Generation of Negative Ion Clusters... 

Besides dissociative electron capture, i.e. generation of negative ions as a result of the molecule dissociation by interaction with an electron, electron capture without dissociation of the molecule has also been observed. For instance, molecular 0 ions upon capture of thermal electrons by O2 molecules have been detected [42]. Comparison of the calculated cross section 0 = 1.2 10" cm (P = 2 10" ) with that calculated for radiative electron capture [294] shows that 0 forms according to... 

Wenthold and co-workers have used this approach to generate the negative ion of 5-dehydro-m-xylylene (Eq. 5.19). ° Using bracketing experiments to determine the gas-phase acidity at the 5-position of m-xylylene, and the kinetic method to determine the triradical electron affinity, the BDE at the 5-position of m-xylylene was measured (Eq. 5.20). 

Stemmier, E.A. Flites, R.A. The Fragmentation of Negative Ions Generated by EC Negative Ion Mass Spectrometry a Review With New Data. Biomed. Environ. Mass Spectrom. 1988, 17, 311-328. 

The oxidation potential decreases in the order Si—Si Si—Ge>Ge—Ge>Si—Sn> Ge—Sn >Sn—Sn in accord with the ionization potential (7P) of the corresponding dimetal. Anodic generation of silicenium ions from disilanes was also reported. The reduction potentials of silicenium ions were determined by cyclic voltammetry of neutral precursor disilanes49. The reduction potential shifted to the negative direction as the center element changed from C to Ge as shown in equation 44. 

If the electrode in the vacuum is isolated from the ESI counter electrode, then the voltages in the atmospheric region can be reversed essentially and the CEC column exit can be set to ground potential with the ESI counter electrode at a sufficient negative potential for the generation of positive ions. This can be achieved by replac-... 

Anion photoelectron spectroscopy is conducted by crossing a mass-selected beam of negative ions with a fixed-frequency photon beam and energy-analyzing the resultant photodetached electrons (Figure 21-8). There are three main regions of such an apparatus the source that generates the anions to be studied, the mass... 

An alternative procednre for the prodnction of negative ions is electron-captnre negative ionization (ECNl). It is as a highly selective ionization method, as only a limited number of analytes are prone to efficient electron capture, e.g., fluorinated compounds or derivatives. It takes place by captnre by the analyte molecules of thermal electrons, and resnlts in the generation of radical anions. The process must be performed in a medinm-pressnre ion sonrce in order to slow down the electrons and to remove excess energy from the radical anion formed upon electron attachment. The formation of negative ions by electron capture can occur by two mechanisms ... 

Gas-phase ionisation by electron impact (and by other means, see later) generates many more positive ions than negative ions and conventional EIMS measurements therefore concentrate on the positive ions. Newer mass spectrometers offer the option of negative-ion EIMS, which can have some advantages such as cleaner spectra (less background - fewer peaks near the baseline) and intense [M- 1] peaks. 

---