Electron impact detachment

Finally, if the molecular ion is negatively charged, electron impact detachment (without or with dissociation) occurs if the electron energy is larger than the electron affinity ... 

The present article will be restricted to discussions of dissociative recombination and resonant ion-pair formation (process 3). Dissociative excitation, dissociative ionization, and electron impact detachment of negative ions have been reviewed recently.The field of merged-beams experiments in atomic and molecular physics has recently been reviewed, and the reader is referred to this excellent article for a detailed discussion of the merged-beams technique. 

Destruction of Negative Ions Associative Detachment, Electron Impact Detachment, and Detachment in Collisions with Excited Particles... 

Another detachment mechanism, electron impact detachment, is important for high degrees of ionization and canbe described as... 

L.H. Andersen, D. Mathur, H.T. Schmidt, L. Vejby-Christensen, Electron-impact detachment of D Near-threshold behavior and the nonexistence of resonances, Phys. Rev. Lett. 74 (1995) 892. 

Processes (19.61)-(19.63) are similar to electron-impact processes (19.33), (19.34) and (19.25), except that in the present case the intermediate (H-7) resonant state is formed not by capture of a free electron but rather by capture of loosely bound electron from 11. There may be also other mechanisms for electron detachment processes (19.61) and (19.63) and for the dissociation process (19.62). 

Thermal electron attachment to SF6 has been studied in the ECD and NIMS at atmospheric pressure and in a chemical ionization mass spectrometer at lower pressures. From these data rate constants for thermal electron attachment and detachment to SF6, and the Qan values and electron affinities of SF6 and SF5 have been determined. The Morse potential energy curves for multiple negative-ion states were calculated using electron impact data and electron affinities. 

This process is somewhat similar to direct ioiuzation by electron impact (Thomson mechanism, see Section 2.1.5). For electron energies of abont 10 e ( the cross section of the detachment process canbe fairly high, about 10 cm. The cross-sectional dependence on incident electron velocity We (Inocnti, Kim, Platzman, 1967) canbe illnstrated by electron detachment from a hydrogen ion e + H H + 2e ... 

Electron detachment (5-126) together with dissociative attachment (5-125) creates a chain reaction of water decomposition. One electron is able to participate in the H2O dissociation process many times in this case, which makes the whole kinetic mechanism energy effective. Cross sections of the electron detachment by electron impact (5-126) are shown in Fig. 5-58 a typical value of rate coefficient for the detachment process is very high (kd = 10 cm /s Smirnov Chibisov, 1965 Tisone Branscome, 1968 Inocuti et al., 1967). The chain mechanism, (5-125) and (5-126), is initiated by ionization of H2O molecules. The chain termination is related to fast ion-ion recombination,... 

Figure 5-58. Cross sections of electron detachment from H negative ions by electron impact as a function of electron energy.

D.S. Walton, B. Peart, K. Dolder, Structure observed during detailed measurements of detachment from by electron impact, J. Phys. B 3 (1970) L148. 

T. Tanabe, 1.1. Katayama, H. Kamegaya, K. Chida, T. Watanabe, Y. Arakaki, et al.. Search for resonances in the detachment of by electron impact with a high-resolution... 

So far, most of this discussion applies to the generation and study of radical cations. How about radical anions These species are more difficult to generate in isolated form in cryogenic matrices because in solvents that do not significantly stabilize charged species (such as noble gases), isolated radical anions are prone to suffer detachment of the loosely bound extra electron, either spontanously or under the impact of low-energy radiation. This situation contrasts with that encountered in solvents such as methyltetrahydrofuran, which can be routinely... 

Ions can also be produced by ionizing radiation i.e. by the impact of particles or photons with sufficient energy to break up molecules or detach electrons from atoms A — A + e". Negative ions can be formed by capture of electrons by atoms or molecules A + e" —> A". 

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