Attachment cross section

The production of N02 by charge transfer from either SF6 or SF5 , previously observed by Curran (4), has been confirmed during this work. The attachment cross-section curves for SF6 and for SF5 closely overlapped in the low energy range, and it was impossible to determine whether SF6 or SF5 was the reacting ion. 

The rate constant for Reaction 3, however, is far from well established. For example, an experimental value of kz can be estimated from the measured attachment cross-section data of Buchel nikova (4) who observed a maximum cross-section ae, max = 5 X 10 18 cm.2 at 6.4 e.v. for electrons colliding with water molecules. (The cross-section falls below 1 X 10 18 cm.2 at energies lower than 5 e.v.) This suggests that... 

This expression can be generalized in the presence of an external field, and the ratio of the escape probability as a negative ion to that as an electron in the absence of a scavenger computed as a function of the external field. From such an analysis and taking L = 4 A, a typical intermolecular separation, Mozumder and Tachiya obtained electron attachment cross sections in NP as 4 x 10-16,5 X 10 17, and 1 X 1CF18 cm2, respectively, for SF6, CC14, and CS2 with -15% uncertainty. 

Fig. 11.17 Cross sections for Rydberg electron ( ) (ref. 78) and free electron (O) (ref. 80) attachment to C5F8, 1,1 1-C2C13F3, CC13F,SF6, and CCI4. The Rydberg electron attachment cross sections are calculated using Eq. (11.42) (from ref. 78).

Figure 12 reports electron impact cross sections for CF4 [47] and SF6 [48], In the SF6 data it should be noticed the very important attachment cross section for the formation of SF6 and SF5 with low energy electrons [49] which results from the comparable electron affinity of fluorine (3.40 eV) with respect to the dissociation energy of SF6 (bond strength F—SF5 = 3.95 eV), whereas the bond strength F—CF3 is 5.56 eV. 

It is interesting to note the structure in the total electron attachment cross section o (e) below M eV in Fig. 5, which indicates the existence of tSree NISs in this energy range. Since at thermal and epithermal energies the ions formed are long-lived and the measured attachment rates showed ( 8, ] ) no pressure... 

Figure 7. Total electron attachment rates as a function of mean electron energy, and swarm-unfolded electron attachment cross sections as a function of electron energy for CcFe (30) and CeFsCF, (19)...

Although the attachment cross sections reported here are total (i.e., for all negative ions produced), earlier work on... 

Table 9. Positions of electron attachment cross section maxima, values of o ( e) at these maxima, energy integrated cross sections, and thermal electron attachment rates for haloethanes...

The gas-phase ion chemistry of COCIF has been studied by ion cyclotron mass spectrometry, and the relative abundance of the primary negative ions X , [Xj] and [XCO] (X = F or Cl) formed by electron impact was measured. The electron energy dependence of the dissociative electron attachment cross-sections for the negative ion formation was determined [1071,1073],... 

The adsorbate substrate complex excitation mechanism predicts an action spectrum analagous to the absorption spectra of the complex. The direct mechanism predicts a photochemical action spectrum similar to that of the gas phase molecule. The final mechanism, dissociative electron attachment (DEA), suggests that the action spectrum should be referenced to the absorbance of the substrate, modified by the surface work function and electron attachment cross section of the adsorbate. The DEA mechanism appears to be of importance for many metal and semiconductor substrates, especially for the case of photochemistry induced by anomalously low energy radiation. 

If the photon energy is greater than the work function, O, of the adsorbate covered surface then free electrons will be generated. If the electrons, or a sub-population of them, are resonant with the electron attachment cross section of the adsorbate then reaction may occur on the ionic potential surface many molecules are unstable to electron attachment in the gas phase. In many cases the electron attachment cross section is peaked at low energies, so photoinduced electron reactions are... 

Experiments on attachment are performed at very low kinetic energy, since the resulting negative ions are fragile, which is why Rydberg electrons are very appropriate. The aim of the experiments is to measure the attachment cross section, or the attachment rate (which is essentially the product of the velocity and the cross section - see below) as a function of the incident energy. 

Figure 5 (Kurachi and Nakamura, 1990) presents a survey of electron collision cross sections of CF4. In addition to the momentum-transfer cross section q , it shows the vibrational-excitation cross sections q T, and q (for two different vibrational modes), the (total) electronic-excitation cross section q, the dissociation cross section q j , the electron-attachment cross section qg, and the (total) ionization cross section 9,. Each of the cross sections is a function of the electron kinetic energy and reflects the physics of the collision process, which is being clarified by theory. The cross sections designated as total can be discussed in greater detail in terms of different contributions, which are designated as partial cross sections.

A number of molecular species, such as NF3, HCl, HBr, HF, and N2O, exhibit strong dependence of attachment rate on temperature. This comes about because the dissociative attachment cross section increases with increasing vibrational quantum number (see Christophorou et al, 1994). This can be seen in the temperature dependence of the N2O dissociative attachment cross section shown in Fig. la, and its effect on the attachment rate coefficient can be seen in Fig. lb. The attachment cross section (Christophorou et al, 1971), where the products are NO + 0, is very temperature-dependent (Chantry, 1969), as shown in Fig. lb, which means that it is very sensitive to the degree of vibrational excitation. In a plasma, one does not need an elevated gas temperature to populate the molecular... 

Fig. 7. (a) Temperature dependence of effective dissociative attachment cross section in N2O (A) temperature dependence of dissociative attachment rate coefficient. 

Fig. 8. Correlation between the peak values of dissociative attachment cross sections and energies of the cross-section maximum.

There have been four quantitative measurements of the attachment cross section or the attachment rate coefficient in BCI3 since 1959 ... 

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