Temporary negative ion states

Campbell and Farago [89, 90]. They reported 0.5% asymmetry in the scattering of 5 eV polarized electrons from camphor. Calculations stimulated by these results pointed out the importance of shape resonances [91] and temporary negative-ion states [92] in the scattering process. 

Electrons may interact with molecules and cause them to dissociate. These processes are collectively termed EID. EID proceeds via two main pathways. The first arises by the attachment of an electron, where it resides in one of the lowest unoccupied ground state molecular orbitals and forms a negative ion. This state is usually short lived. In many molecules, these temporary negative ion states are dissociative, and produce a neutral and negatively charged fragment species. 

Before discussing possible mechanisms that could explain these results, it is worthwhile comparing the time constants to results from previous electron-induced surface chemical reactions. The extracted time constant, t, equals 1/ct/, where/is the X-ray flux density and % is the photolysis cross section. Using approximate values of the beam area (3 x 10 4 cm2) and X-ray flux (3.5 x 10n ph/s), yields a flux density of 1.7 x 1015 ph/s cm2 and a a of 6 x 10-19 cm2. Our results show that it is the secondary electrons that are inducing the chemical changes. Therefore, it is more applicable to use the secondary electron flux to compute the cross section. An upper bound to this is given by the TEY flux density. This is determined from the measured sample current of 3.8 nA or 2.4 x 1010 e/s, which results in a cross section of 9 x 10 18 cm2 (9 Mb) This value compares fairly well with reported dissociative electron impact cross sections for CO production from condensed films of acetone (9.6 Mb) [124] or methanol (4.2 Mb) [125] via a DEA mechanism. In the present case a DEA mechanism, in which a temporary negative ion state is formed,... 

Stephen TM, Shi X, Burrow PD (1988) Temporary negative-ion states of chiral molecules camphor and 3-methylcyclopentanone. J Phys B 21 L169... 

Temporary Negative Ion States in Hydrocarbons and Their Derivatives... 

Beginning in 1965, a number of groups initiated studies of temporary anion formation in hydrocarbons (5-8), with extensive contributions from the groups of Compton (9) and Christophorou (10) at Oak Ridge. Except for the work of Hasted and coworkers (7), these studies relied primarily on the trapped-electron (11) or SFg scavenger methods (12) for the detection of temporary negative ion states. 

In this review, we have shown that electron transmission spectroscopy provides a useful and relatively simple way to survey the temporary negative ion states of complex molecules. The utility of the technique has been further enhanced by a number of recent developments which we briefly mention here. 

The existence of temporary negative-ion states of CO2 at low energies led Phelps " to suggest that CO2" in a bent configuration might well be stable. Experimental evidence has recently been obtained for the production of CO2 by the reaction... 

Localized electrons can also be considered as temporary negative ion states. The electron can leave its trap either by absorption of phonons (thermal activation) or by absorption of photons (photoassisted diffusion). The latter process requires an energy of the order of 1 eV, as can be deduced from the optical absorption spectrum of localized electrons in n-hexane. The process of photoassisted diffusion has been demonstrated experimentally by Balal et al. (Balakin et al., 1981 Balakin and Yakovlev, 1979). Electrons were produced by photoionization of anthracene in n-hexane or 2,2,4-trimethylpentane by a laser pulse of 347 nm. Subsequent illumination of the solution with a pulse of 694 nm led to a temporary increase in the photocurrent due to the liberation of trapped electrons. The process responsible for this increase can be envisaged as follows ... 

The TT ( A ) States of the Temporary Negative Ions of Cyclopropene and Cyclobutene. 

The formation of a negative ion state by Feschbach resonance (temporary binding of an electron to PH3 in a Rydberg excited state) was discussed [3]. 

Two energy terms applicable to thermal electron reactions referenced to the ground state of the neutral are the electron affinity Ea and the vertical electron affinity VEa. Ea is the difference in energy between the most stable state of the neutral and a specific state of a negative ion. VEa is the difference in energy between the anion and neutral species in the geometry of the neutral. The anion can be temporary and relax to either the ground state, an excited valence-state anion, or dissociate. 

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