Electron impact excitation of ions from

EIEIO Electron Impact Excitation of Ions from Organics uses... 

Cody, R. B. and Freiser, B. S. Electron impact excitation of ions from organics an alternative to collision induced dissociation. Anal. Chem. 51 547-551, 1979. 

BH Wang, FW McLafferty. Electron impact excitation of ions from larger organic molecules. Org Mass Spectrom 25 554—556, 1990. 

This method has been employed in many areas of atomic physics. Imhcff and Read have used electron-photon coincidences to measure helium lifetimes thus ensuring the complete absence of cascade processes from affecting the measurement. Pochat et al have measured differential cross-sections for electron impact excitation of n = 4 and 5 states of helium using the decay photons of appropriate wavelengths to uniquely specify the coincident scattered electrons. In addition to several other similar examples, it has also been employed for particles other than photons and electrons e.g. between two electrons as in the (e, 2e) experiments and between ions and photons in ion-atom collision experiments. 

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. 

Other means of manipulating ions trapped in the FTMS cell include photodissociation (70-74), surface induced dissociation (75) and electron impact excitation ("EIEIO")(76) reactions. These processes can also be used to obtain structural information, such as isomeric differentiation. In some cases, the information obtained from these processes gives insight into structure beyond that obtained from collision induced dissociation reactions (74). These and other processes can be used in conjunction with FTMS to study gas phase properties of ions, such as gas phase acidities and basicities, electron affinities, bond energies, reactivities, and spectroscopic parameters. Recent reviews (4, 77) have covered many examples of the application of FTMS and ICR, in general, to these types of processes. These processes can also be used to obtain structural information, such as isomeric differentiation. 

Autoionization spectra resulting from specific resonances can be obtained by electron-electron coincidence measurements (Haak et al. 1984 Ungier and Thomas 1983, 1984, 1985). To associate a fr.rgmentation pattern with a particular core hole excited state and a particular autoionization or Auger decay channel, a double-coincidence experiment must be done using electron impact excitation. The energy of the scattered electron must be determined, the energy of the emitted electron must be detennined, and the ions produced in coincidence with these two events must be determined. The difficulties inherent in these kinds of experiments have been aptly summarized by Hitchcock (1989), If you can do it by photons, don t waste your time with electron-coincidence techniques. ... 

The number of ions that are formed in the different metastable states depends, of course, on the conditions in the ion source. Comparison of a few results from different labora )ries indicates that if the ions are produced in an ordinary ion source (electron impact with l(X)-eV electrons), the abundances of the metastable states compared with the ground state do not vary much. Therefore, in Table I, the relative abundances are given for production of ions from the indicated compounds. These values must be considered as very uncertain, as indirect methods involving several assumptions have been used to determine the values. The inclusion of these values in Table I implies an attempt to fulfil the need for information on excited states that has been expressed repeatedly during recent years. ... 

One possible reason for the higher Tr of INS is that the predominant population process of B 2Zu+ of N2+ ion is the electron impact excitation from the ground state X Zg of N2 ion, not from any state of neutral molecules. Then, the rotational energy distribution of B Zu of N2 should be close to that of the ground state X 2Zg+ of N2+ ion. It is also noteworthy that Tv of INS is also about twice as high as that of 2PS as in Fig. 11(b). Further discussion is necessary to conclude the reason for the higher rotational temperature of INS. 

On the other hand, the formation of ethylene was ascribed mainly to the unimolecular decomposition of a neutral excited propane molecule. These interpretations were later confirmed (4) by examining the effect of an applied electrical field on the neutral products in the radiolysis of propane. The yields of those products which were originally ascribed to ion-molecule reactions remained unchanged when the field strength was increased in the saturation current region while the yields of hydrocarbon products, which were ascribed to the decomposition of neutral excited propane molecules, increased several fold because of increased excitation by electron impact. In various recent radiolysis 14,17,18,34) and photoionization studies 26) of hydrocarbons, the origins of products from ion-molecule reactions or neutral excited molecule decompositions have been determined using the applied field technique. However, because of recent advances in vacuum ultraviolet photolysis and ion-molecule reaction kinetics, the technique used in the above studies has become somewhat superfluous. 

FIG. 15. Electron impact reaction rales as a function of the average electron energy. A I 1 mixture of SiHa and H2 was used, at a total pressure of 83 Pa. (a) Reaction rates for SiHa, (b) reaction rates for Si2H6 (dotted lines) and H2 (solid lines). Abbreviations are ion. ionization dis, dissociation vib, vibrational excitation att, attachment. See Table II for details and references. (Adapted from G. J. Nienhuis, Ph.D. Thesis. Universileit Utrecht. Utrecht, the Netherlands. 1998.)... 

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. 

---