Electron cross section

Figure 6 A Platzman plot of the ratio of the secondary electron cross sections for ionization of N2 by electrons to the comparable Rutherford cross sections. Q is the energy loss, Q = W+B, and is equivalent to the parameter E used in the text. (From Ref. 44.)...

In this section, we shall review various factors that must be considered in accurate evaluation of center parameters from the raw data. First, the extent of the (already mentioned) lattice relaxation must be considered (see below). Second, there can be additional complications, such as excited states or a held dependence of the cross section. In any case, one tries to separate out such complications and thus obtain an electronic cross section. This latter can then be compared to appropriate theory (Section 12). 

The next important aspect to be considered is the electron-phonon interaction (lattice relaxation). Here, the effect of momentum conserving phonons, or promoting modes, can in principle be included in the electronic cross section this is discussed, for instance, by Monemar and Samuelson (1976) and Stoneham (1977). However, the configuration coordinate (CC) phonons (or accepting modes) are treated separately. The effect of these CC modes is usually expressed by the Franck-Condon factor dF c, where this factor is the same as the defined in our Fig. 16. Thus assuming a single mode,... 

Fig. 19. Predicted dependence of the photoionization spectral dependence on the Franck-Condon factor [dF c—see Eq. (53)]. The parameter values are appropriate for the electron cross section ( ) for in GaP. The level depth is E, = 0.9 eV, the band gap is Et = 2.2 eV, the average optical gap (the Penn gap) is Ep = 5.8 eV, and the temperature is 400°K. [After Jaros (1977, Fig. 5e).]...

There still remains one part of the electronic cross section to be discussed the density of states. It has been increasingly realized lately that the density of states is by now relatively well known for most semiconductors. It can thus be incorporated properly in the cross section, although possibly only by a numerical analysis. As can be seen from Tables III and IV, most recent papers that treat a specific center do indeed either include the proper density of states or show that a parabolic density is appropriate in the range of their analysis. That this density of states can be very important in the shape of the cross section has also been recently emphasized (Nazareno and Amato, 1982). 

If the orbitals belonging to different subshells are assumed to have the same one-electron cross sections, the integrated ionization cross section of a particular subshell is simply proportional to the occupancy of that orbital in the subshell in the molecule. 

Cross sections for neon and argon have also been presented by Coleman et al. (1982) and Mori and Sueoka (1994), though here there are no theoretical data for comparison. The positron and electron cross sections (the latter from the work of de Heer, Jansen and van der Kaay, 1979) are of very similar magnitude, despite the fact that triplet states cannot be excited by positron impact. 

The last term in (13) is the average one-electron cross-section defined in Eq. (7) and thus we obtain a formula for the coefficients required in Eq. (4) ... 

The following tables have been compiled using the formulae given in Sections 3 and 4. The numbers given are in fact the Z coefficients defined in Eq. (4), and are thus the intensities expressed in units of the one-electron cross-section for the orbital which is ionised. Cases where repeated states can arise, as discussed above, are marked with an asterix ( ) in these cases the total intensity given in the table may be distributed among two or more states. 

Beta particle—One type of radioactive decay particle emitted from radioactive atomic nuclei. A beta particle is the same thing as an electron. Cross section—measure of the probability that a subatomic particle will interact with matter. 

Uehara S., Nikjoo H., Goodhead D.T., Comparison and Assessment of Electron Cross Sections for Monte Carlo Track Structure Codes, Radiat. Res., 1999,152,202-213. 

Photons are scattered by the electrons around an atom heavier atoms, with more electrons, scatter much more strongly than light atoms. To emphasise this trend some X-ray scattering cross sections (based on the Thomson free electron cross section, oj == 0.7 bam) are given below. These can be compared with the neutron cross sections given in Appendix 1. 

Figure 3. (a) IR spectra obtained from two diamond/p-SiC nanocomposite films deposited on W substrates by using different TMS flow rates. The transverse optical phonon band around 800 cm corresponds to the presence of p-SiC. (b) Backscattered electron cross-sectional micrograph of a gradient natured diamond/p-SiC nanocomposite film deposited on BEN pre-treated (100) Si substrate. The bright spots indicate p-SiC phase. 

---