Fragmentation electron impact excitation

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. 

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... 

It is well known that the electron-impact ionization mass spectrum contains both the parent and fragment ions. The observed fragmentation pattern can be usefiil in identifying the parent molecule. This ion fragmentation also occurs with mass spectrometric detection of reaction products and can cause problems with identification of the products. This problem can be exacerbated in the mass spectrometric detection of reaction products because diese internally excited molecules can have very different fragmentation patterns than themial molecules. The parent molecules associated with the various fragment ions can usually be sorted out by comparison of the angular distributions of the detected ions [8]. 

Alternatively, the primary photofragments of Ib CO)] are probed by electron impact ionization of fragments formed on photolysis (33). Excitation into the same near UV band leads to generation of Mn(CO), Mi CCO) and M tCO). This experiment was also performed by excitation at 250 nm and 193 nm, of two higher electronic states of the molecule. 

Table I gives the mass fragments observed by electron impact ionization at each one of these three excitation energies. The results confirm the fact that both metal-metal and metal-CO cleavage occurs in the low energy band, while excitation into higher excited states enhances the metal-CO channel at the expense of the Mn-Mn bond homolysis.

Table I. Mass Fragments Observed by Electron Impact Ionization as a Function of Photolysis Excitation Energy...

Main reaction products from 308 nm photolysis of 92 naphthalene identified by mass spectrometry Emission cross-sections of excited fragments from the 493 electron impact dissociation of SiH4, SiD, Si2H, and GeH4... 

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