Electron autodetachment

FIGURE 16.2 Schematic potential energy curves showing electron attachment, electron autodetachment, and DEA for a TMA. (Surf. Sci. Rep., 65, Arumainayagam, C.R., Lee, H.L., Nelson, R.B., Haines D.R., and Gunawardane, R.P., Low-energy electron-induced reactions in condensed matter, 1-44, Copyright 2010, with permission from Elsevier.)... 

While a positive EA is a necessary criterion for production of the radical anion [M] , the observation of a [M] anions in NlCl spectra also depends on the lifetime of these radical anions in the NlCl source. In other words, molecules with positive electron affinities may not be observed because they lose an electron (autodetachment) due to collisions with the reagent gas. Autodetachment of the radical anion is important for small molecule and organic molecules with small positive (<50 kJ/mol) EAs (75,110,111). Also, once a sample molecule has captured an electron, it may follow a dissociative electron attachment reaction pathway to produce [M—X] anions (Reaction 7.35) or may produce ion pair [X] anions (Reaction 7.36) (69,103). 

In the gas phase, CO2 is metastable against electron autodetachment (1.2). The existence of... 

As the IE of a molecule is governed by the atom of lowest IE within that neutral (Chap. 2.2.2), the EA of a molecule is basically determined by the atom of highest electronegativity. This is why the presence of halogens, in particular F and Cl, and nitro groups make analytes become attractive candidates for EC (Table 7.3). [78] If EC occurs with a neutral of negative EA, the electron-molecule complex will have a short lifetime autodetachment), but in case of positive EA a negative molecular ion can persist. 

The branching ratio between autodetachment and electron transfer governs the yield of strand breaks. In Figure 13, we show a qualitative depiction of the energy surfaces involved in this class of electron-transfer processes. 

The excited state lies 2.07 eV above the ground state thus there is sufficient energy available in all solvents to reach this state. The kinetic analysis indicated that the attachment rate is fast in all solvents. In TMS, neopentane, and 2,2,4,4-tetramethylpen-tane, the energy levels of the electron are low enough that the reverse reaction, autodetachment from the excited anion, occurs with small activation energy. 

Collisions of some halogen bearing molecules, such as SF6, with Rydberg atoms result in attachment of the Rydberg electron to the molecule to form a negative molecular ion. Simple attachment, dissociative attachment, and attachment followed by autodetachment have all been observed. Collisions with attaching molecules are an excellent example of a process dominated by the electron... 

Fig. 11.18 Arrival time spectra of the products of the collisional ionization of Xe 26f high Rydberg atoms by CH3I, and C6F6. As shown, collisions with CH3 lead only to r. Collisions with C F14 lead to both C7F14 and e, as shown by the large signal at early times due to electrons. C6F6 leads to the production of a long lived autodetaching state of QF6 which produces a nearly continuous electron signal at early times (from ref. 79).

The associative resonance capture is favoured for molecules with several electronegative atoms or with possibilities to stabilize ions by resonance. The energy to remove an electron from the molecular anion by autodetachment is generally very low. Consequently, any excess of energy from the negative molecular ion as it is formed must be removed by collision. Thus, in Cl conditions, the reagent gas serves not only for producing thermal electrons but also as a source of molecules for collisions to stabilize the formed ions. 

Capture may take place to bound states of the negative ion that undergo radiationless transitions to repulsive states of the negative ion resulting in dissociative electron capture. These radiationless intramolecular transitions (Auger transitions) are a result of overlap of the discrete states with a continuum of states of AB-. These types of processes are illustrated for diatomic molecules in Figure 2.2b. Electrons are first captured into the bound state represented by curve 1, and before autodetachment can take place an intramolecular radiationless transition occurs to curve 2 resulting in dissociation into A and B-. 

Figure 2.2d shows the simplest type of nondissociative electron capture into discrete states of AB- that will occur between energies E1 and Ei, resulting in a vibrationally excited AB- molecular ion. If the capture process remains an isolated event, the electron will be ejected by autodetachment (Auger process) within a time comparable with a vibration time. 

A second major difference between ETS and PES results from the unique properties of temporary anion states. The temporary anion states associated with the occupati YJ of un lled orbitals of most hydrocarbons have lifetimes of 10 - 10 s. Since the FVfHM resolution attainable in ETS is 0.02 - 0.05 eV, this means that the line widths due to autodetachment are generally greater than the experimental resolution. It is precisely these short lifetimes which make the study of these anions in the gas phase exceedingly difficult by optical spectroscopy and which makes electron scattering techniques ideally suited. 

Figure 2. Negative ion autodetachment lifetime (9) and negative ion current (measured without the retarding potential difference method) (O) for Ct(CN)T as a function of electron energy (VI)...

In their detailed theoretical treatment, Hol0ien and Midtdal found that the only stable electronic configuration of the He ion is ls 2s 2p P and that this state is metastable toward radiative decay and is not subject to autodetachment. From the energy calculations they found an electron affinity for Is 2s S He relative to this state of He of at least 0.075 eV, although a later correction of a calculational error causes this value to become negative The original value was quite close to the experimental photodetachment measurements of Brehm, Gusinov and Hall ° ... 

In summary, the He ion has been produced, studied and employed experimentally. Theoretical calculations indicate that this ion is the ls 2s 2p Ps/2 state, a state metastable with respect to both radiative decay and autodetachment. The results of theoretical calculations and experimental measurements are in reasonable agreement that the electron affinity of Is 2s He to form He is 0.08 0.02 eV and that the lifetime of this metastable He ion is about 0.45 ms. Resonant states of He are also known and have been characterized ... 

Holoien and Geltman also have found that the ls 2p state of He is bound with respect to the Is 2p P° state of He the electron affinity of this state is calculated to be > 0.20 eV. This state is metastable against autodetachment via the Coulomb interaction. Holoien and Geltman indicate its lifetime against autodetachment to be of the same order as that for the P state and that it may radiate to the metastable P° state. The extent of contributions of the P° and P metastable states to the observed He ion beams is presently unknown. Buchel nikova Smir-nov and Masseyhave summarized much of the present knowledge of atomic negative ions. 

Several conditions must be fulfilled for an anionic electronic state to exist (i) it should possess positive electron affinity with respect to its parent neutral state (ii) it should exhibit slow depletion by spin-forbidden autodetachment for at least one fine-structure component and by radiative depletion and (iii) its wave-function should undergo weak interaction with the electron continuum wave. Such stable and metastable states have been identified for several negatively charged atoms and molecules, in both ground and electronically excited states. Long-lived electronically excited molecular systems, where the anionic ground state is not bound, do exist and have been observed experimentally. For a detailed presentation of the examples already known is referred in Refs. [1-3]. 

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