Electronic excitation cross section

There are no experimental data of excitation cross sections for proton and alpha particle impact for water. The proton cross sections were obtained by scaling of the electron excitation cross sections for high-energy protons >500 keV [201]. For the lower-energy regions, the semiempirical model developed by Miller and Green [202] was adopted, which is based on the electron impact excitation. They assumed an analytical function for each excited level of the form... 

A considerable amount is known about details of the primary events in electronic excitation. Electronic excitation cross sections are dependent on the charge and velocity of the incident primary ion. Both of these parameters have been confirmed to be the important variables in the emission of secondary molecular ions under bombardment by ions in the 50-100 MeV regime (1 MeV/u) (5-6). The excitation promotes electrons within atoms and molecules to very high energies resulting in ionization and emission of secondary electrons (up to 50 per incident ion) (7). The excitation of electrons in core... 

Noxon [159] observed the decay of 6300-A radiation and the rise of 7618-A radiation, thereby measuring the rate constant k = 9 x 10-11 cm3/sec. This reaction is probably important in aurorae [160]. Electronically excited oxygen may also be produced efficiently by the impact of relatively low-energy electrons. Cartwright et al. [123] have measured (a1 A ), (b 1E+), (c1 ), A 3Z+), and (B 3E ) electron excitation cross sections. They find that excitation of the (c u) and of the (B 32 ) states yields dissociation into two O(3P) atoms and Of3/5) + Of1/)), respectively, but produces little direct radiation. The cross sections for excitation of (a XA0) and (b ) are shown in Figure 6.8. 

Ne LIF used to measure electron excitation cross-sections for Ne 44... 

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.

Electronic excitation The two electronic excitation cross sections were determined by analysis of swarm data. 

Classical calculations have been performed by Olson for double ionization of helium by protons and antiprotons, as has already been discussed in chapter S. Calculations of correlated two-electron excitation cross sections have been reviewed by Giese [7.3]. 

The power dissipated at two different frequencies has been calculated for all reactions and compared with the energy loss to the walls. It is shown that at 65 MHz the fraction of power lost to the boundary decreases by a large amount compared to the situation at 13.56 MHz [224]. In contrast, the power dissipated by electron impact collision increases from nearly 47% to more than 71%, of which vibrational excitation increases by a factor of 2, dissociation increases by 45%, and ionization stays approximately the same, in agreement with the product of the ionization probability per electron, the electron density, and the ion flux, as shown before. The vibrational excitation energy thresholds (0.11 and 0.27 eV) are much smaller than the dissociation (8.3 eV) and ionization (13 eV) ones, and the vibrational excitation cross sections are large too. The reaction rate of processes with a low energy threshold therefore increases more than those with a high threshold. 

The quantity L(0) = In I, where I is the mean excitation potential introduced by Bethe, which controls the stopping of fast particles (see Sect. 2.3.4) L(2) = In K, where K is the average excitation energy, which also enters into the expression for Lamb shift (Bethe, 1947). Various oscillator sum rules have been verified for He and other rare gases to a high degree of accuracy. Their validity is now believed to such an extent that doubtful measurements of photoabsorption and electron-impact cross sections are sometimes altered or corrected so as to satisfy these. 

The first (and still the foremost) quantum theory of stopping, attributed to Bethe [19,20], considers the observables energy and momentum transfers as fundamental in the interaction of fast charged particles with atomic electrons. Taking the simplest case of a heavy, fast, yet nonrelativistic incident projectile, the excitation cross-section is developed in the first Born approximation that is, the incident particle is represented as a plane wave and the scattered particle as a slightly perturbed wave. Representing the Coulombic interaction as a Fourier integral over momentum transfer, Bethe derives the differential Born cross-section for excitation to the nth quantum state of the atom as follows. 

For low energy electron region, namely, below 200 x Z eV, this equation will not be valid because of the limitation of Born approximation in this energy region, the calculations will be made using electron-impact ionization and excitation cross sections for gaseous targets [7]. 

The cross sections for ESD processes on most surfaces are usually much smaller than cross sections for comparable gas phase processes involving electron-induced dissociation and dissociative ionization . This may be a consequence of the fact that many fragments remain adsorbed on the surface and/or that non-radiative processes such as those described in Sect. 2.1.1 cause the molecule to de-excite before it dissociates. For 100 eV electrons, typical cross sections for gas-phase dissociation are 10 cm (see Ref. 150). For most adsorbates, cross sections lie in range of 10 to 10 cm. A few examples of higher cross sections for adsorbed layers are known, and many examples of smaller cross sections exist. 

A similar conclusion can be drawn from the interaction of metastable nitrogen molecules (the state) with these same surfaces. The relative excitation cross section (the excitation function) for this state is shown in Fig. 36 (see ref. ). Direct excitation by electron impact has a threshold at approximately 6eV and has a maximum at slightly higher energies. De-excitation from the etc. 

Absolute electron-impact cross sections have been measured by Chamberlain, et al.111 for excitation of the 2 P and 2 5 states of helium at a scattering angle of 5° in the energy range 50-400 eV (these conditions are far from the optical limit). Measurements of this type are extremely difficult but with care may approach an accuracy of 5 to 10%. Such determinations are useful for normalizing relative measurements. 

Using these assumptions they could then fit their data to determine excitation cross sections, o(n, Wo), for the excitation of a Rydberg state of principal quantum number n by an electron of energy W0. They found the values of o (W0) given in... 

Table 3.1. Electron impact cross sections for the excitation of rare gas Rydberg states for the electron energy at the peak of the cross section and 100 eV.a...

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