Formed by electron impact

Since the observation made in study of the formation HeH+ indicated that this product was not formed by reaction of He + with H2, it had been assumed that the exothermic heat of reaction of He+ ions with H2 is probably deposited in the product HeH + as internal energy, decomposing the product into H+ and He. This idea was cited by Light (16) in his phase space theory of ion-molecule reactions to account for the failure to observe HeH+ from reactions with He+ ions. The experimental difficulty in the mass spectrometric investigation of this process is that H + formed by electron impact tends to obscure the ion-molecule-produced H+ so that a sensitive quantitative cross-section measurement is difficult. 

A Pt(s)[7(111) X (100)] crystal is positioned in front of a hole in one of the plates of the analyzer cell. Ions formed by electron impact are trapped in the analyzer cell and detected by FTMS. An exclmer laser, having a pulse width of 20 nsec, is used to desorb molecules from the Pt crystal. 

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. 

High etch rates and selectivity can be achieved by judicious selection of feed gases to a plasma reactor. The atomic and radical species formed by electron impact dissociation depend largely on feed gas composition, and the intrinsic etch rates measured in the absence of a plasma (i.e., downstream etching) provide a useful indicator of chemical selectivity in the presence of a plasma. For example, the ratio of (100) silicon (34) to thermal oxide (Si02) (37) etching by F atoms is 41 1 at room temperature. As etch rates generally follow an Arrhenius type dependence on substrate temperature. 

This type of mass spectrometer, which is not widely used, allows mass determination with a high precision. An ion cyclotron resonance spectrometer is basically an ion trap ions formed by electron impact, for example, are subjected to the orthogonal magnetic field B, which induces cyclotronic movement in the. rv plane (Fig. 16.8). The radius of the circular movement, which depends on kinetic energy, is given by equation (16.2). If the velocity v is small and the magnetic field B is intense, the radius of the trajectory will be small and the ions will be trapped in the ionisation... 

The apparatuses described thus far were all designed to detect the ionic products from ion-neutral interactions, whereas the TOF apparatus described by Moran and co-workers9911,146,147 has been employed to detect the neutral products from these processes. In the latter device the ions formed by electron impact are accelerated out of the source by an extraction pulse and are further accelerated by a series of grids. The primary beam is then focused by deflection and enters the drift region or flight tube, where mass separation of the different velocity groups occurs. 

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 neutralization-reionization mass spectrometric experiments on CH2Si+ formed by electron-impact dissociative ionization of ClCH2SiH3, Srinivas, Stilzle and Schwarz found evidence for the formation of a viable neutral molecule whose fragmentation pattern and collisional activation mass spectrum were in accord with a H2C=Si structure422. These authors suggested that their experiments supported electron-capture by CH2Si+" as a mechanism for the formation of H2C=Si in interstellar space. Various models have predicted that H2C=Si is one of the most abundant forms of silicon in dense interstellar clouds423. 

Studies on a range of saturated hydrocarbons indicate that the main decomposition reactions of electronically excited alkane molecules formed by electron impact are118,122-124... 

Photodissociation of larger ions (>200 daltons) formed by electron impact was also studied. These ions were produced by electron ionization of vapor sublimed from the standard heated solids insertion probe. Probe temperatures of 150-200 C were... 

A positively charged ion with an unpaired electron commonly formed by electron impact ionization, when the impinging electron knocks out an additional electron, (p. 542)... 

Using acetic anhydride as an example, Feldmann et al. examined the feasibility and properties of a new method for studying IRMPE/D dynamics which involves single-photon ionization to monitor the time evolution of parent molecules and neutral intermediates. They discussed in some detail alternative detection schemes, such as mass spectrometry with ions formed by electron impact or MPI/REMPI, but decided upon single-photon ionization because of its wide applicability, its sensitivity, and its selectivity. VUV ionization ensures that the perturbation of the system is minimal as well as the extent of ion fragmentation. They used frequency-tripled 3rd harmonic (355 nm) radiation from a Nd-YAG laser as a source of 118.4 nm light. 

LIF excitation spectra measured for Nj, COJ, C F, C FgH, Cf,F3Hj, and C4HJ formed by electron impact in a free jet expansion... 

That is, a fast ion, formed by electron impact and accelerated toward the plasma surface, excites the H ion to form H , and then the H ion reacts via a third body electron capture process to form the ion inferred from the experiments. Only the ions formed at the surface of the plasma could be accelerated through the same potential drop as the primary H ions. Further, Schnitzer and Anbar argue that since there is no significant change in the Ha ion current as the ion source pressure is varied, the formation of does not occur within the... 

At 30-70 eV, three primary ions are formed by electron impact [1071,1720] ... 

The gas-phase ion chemistry of COCIF has been studied by ion cyclotron mass spectrometry, and the relative abundance of the primary negative ions X , [Xj] and [XCO] (X = F or Cl) formed by electron impact was measured. The electron energy dependence of the dissociative electron attachment cross-sections for the negative ion formation was determined [1071,1073],... 

Somewhat akin to ion-molecule reactions is a process first identified by Hornbeck and Molnar (45) for forming the rare gas diatomic ions. Hornbeck and Molnar observed diatomic ions of all the rare gases and found their intensities to vary as the square of the pressure. The appearance potentials of the diatomics proved to be 0.7 to 1.5 e.v. below the ionization potentials of the corresponding atom, and hence the diatomic ions were not derived from atomic ions. As a consequence, they proposed that excited atoms formed by electron impact reacted upon collision with a neutral atom to form a molecular ion and eject an electron, thus ... 

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