Impact excitation

A review of the early experimental works can be found in references [56-58]. More recently, Chutjian recorded the electron-impact excitation spectrum of formaldehyde [59,60] and reported transition energies that are taken as reference values in many other works. So are the experimental values compiled by Robin [61]. 

Optical emission is a result of electron impact excitation or dissociation, or ion impact. As an example, the SiH radical is formed by electron impact on silane, which yields an excited or superexcited silane molecule (e + SiHa SiH -t-e ). The excess energy in SiH is released into the fragments SiH SiH -I-H2 + H. The excited SiH fragments spontaneously release their excess energy by emitting a photon at a wavelength around 414 nm. the bluish color of the silane discharge. In addition, the emission lines from Si. H, and H have also been observed at 288, 656, and 602 nm, respectively. 

A further technique exists for the determination of triplet energy levels. This technique, called electron impact spectroscopy, involves the use of inelastic scattering of low-energy electrons by collision with molecules. The inelastic collisions of the electrons with the molecules result in transfer of the electron energy to the molecule and the consequent excitation of the latter. Unlike electronic excitation by photons, excitation by electron impact is subject to no spin selection rule. Thus transitions that are spin and/or orbitally forbidden for photon excitation are totally allowed for electron impact excitation. 

Most likely excitation Impact excitation Recombination following Chemical degradation following... 

The operating principle of a TEEL display is as follows. The dissolved Mn atoms in the ZnS lattice are subjected to direct impact excitation by electrons that have been... 

Figure 3 Inelastic and elastic cross sections for electron impact excitation of the water molecule the data are from the review by Mark et al. [19]. The total interaction cross section ctt was determined from the sum of cross sections for all elastic and inelastic processes. Inelastic channels include the vibrational modes Cvi (the bending mode with threshold 0.198 eV), cTv2 (the sum of two stretching modes with thresholds 0.453 and 0.466 eV), and CvS (a lump sum of other vibrational excitation modes including higher hormonics and combinational modes with an assigned threshold of 1 eV). The electronic excitations and <7 2 have threshold energies of 7.5 and 13.3 eV. Ionization cross sections are those of Djuric et al. (O), and Bolarizadah and Rudd ( ). (From Ref 19.)...

Excited states can be formed by a variety of processes, of which the important ones are photolysis (light absorption), impact of electrons or heavy particles (radiolysis), and, especially in the condensed phase, ion neutralization. To these may be added processes such as energy transfer, dissociation from super-excited and ionized states, thermal processes, and chemical reaction. Following Brocklehurst [14], it is instructive to consider some of the direct processes giving excited states and their respective inverses. Thus luminescence is the inverse of light absorption, super-elastic collision is the inverse of charged particle impact excitation, and collisional deactivation is the inverse of the thermal process, etc. 

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... 

Similarly, an elaborate taxonomy of fragmentation reactions has been built up for the case of electron impact excitation,166 as studied by mass spectrometry. This taxonomy is accurately enough known to permit determination of the structures of complex organic molecules.166... 

For a molecule AB, we may compare the processes of photoabsorption and electron-impact excitation as follows ... 

Ion-impact excitation has been widely studied in the rare gases260 270 and for alkali metal ion-atom collisions.271 280 In many cases excitation functions have been measured (i.e., total cross sections as a function of initial relative translational energy), and in some instances the angular dependencies of the differential cross sections for inelastic scattering have been determined. The most striking feature of the results from these experiments is the oscillatory structure that is evident in many of the... 

Figure 37. Schematic illustration of dual-interaction model for ion-impact excitation resulting in oscillatory structure in total cross section see text.279...

Recent measurements utilizing the crossed-beam technique have been performed as follows.37 A metastable helium beam is formed by electron-impact excitation of a thermal helium beam effusing from a multichannel array. The optical quenching method12 described earlier is applied to obtain results for He(2 5 ) and He(23S) separately. The target gas beam is... 

In sum, electron impact excitation has the advantages of relative simplicity and generality, and it has the disadvantages of being inefficient and nonselective, with nearly all energetically possible states being produced. 

Fig. 3.5 Schematic diagram of the apparatus using electron impact excitation to the Xe metastable state followed by laser excitation to Rydberg states (from ref. 24).

A new R-matrix approach for calculating cross-sections and rate coefficients for electron-impact excitation of complex atoms and ions is reviewed in [307]. It is found that accurate electron scattering calculations involving complex targets, such as the astrophysically important low ionization stages of iron-peak elements, are possible within this method. 

Features in VEEL spectra derived from the impact mechanism became more prominent in off-specular directions where the dipole-induced contributions are radically attenuated by the operation of the MSSR. However, impact-excited intensity can contribute to on-specular spectra in the form of additional strengths... 

The type I spectrum of the di-cr species has been found at low temperatures on most investigated crystal planes of Fe (234, 235), Ni, Ru, and Pt (Tables V and VI). The intermediate type I spectrum, which we have related to the (7ia) species, occurs on Fe(lll), Rh(lll), Ru(0001), and Pd(100). In the Ru(0001) and Pd(100) cases, however, different spectra have been obtained in different laboratories (17), leading to some uncertainty as to whether these type I patterns may alternatively arise from superimposed component spectra from di-rx and n species (17), or to experimentally variable contributions from impact-excited features (Section IV.B). Such species could therefore profitably be reinvestigated, and the higher resolution of RAIRS would be particularly valuable in this respect. 

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