Electron-molecular ion interactions

MERGED-BEAMS STUDIES OE ELECTRON-MOLECULAR ION INTERACTIONS IN ION STORAGE RINGS... 

The principal advantages of using an ion storage ring for the study of electron-molecular ion interaction can be summarized in the following points. 

The merged-beams geometry in the electron cooler section gives the possibility to study electron-molecular ion interactions at meV collision energies. The spread of relative energies of electrons and ions can be made very small and depends essentially only on the quality of the electron beam. 

This chapter should be read in conjunction with Chapter 3, Electron Ionization. In electron ionization (El), a high vacuum (low pressure), typically 10 mbar, is maintained in the ion source so that any molecular ions (M +) formed initially from the interaction of an electron beam and molecules (M) do not collide with any other molecules before being expelled from the ion source into the mass spectrometer analyzer (see Chapters 24 through 27, which deal with ion optics). 

The ion source, across which an electron beam passes, is filled with methane, the reagent gas. There is a high vacuum around the ion source, so, to maintain a high pressure in the source itself, as many holes as possible must be blocked off or made small. Interaction of methane (CH4) with electrons (e ) gives methane molecular ions (CH4 "), as shown in Figure 1.2a. 

Thus two electrons exit the reaction zone, leaving a positively charged species (M ) called an ion (in this case, a molecular ion). Strictly, M" is a radical-cation. This electron/molecule interaction (or collision) was once called electron impact (also El), although no impact actually occurs. 

The formation of a simple El mass spectrum from a number (p) of molecules (M) interacting with electrons (ep. Peak 1 represents M , the molecular ion, the ion of greatest mass (abundance q). Peaks 2, 3 represent A+, B. two fragment ions (abundances r, s). Peak 2 is also the largest and, therefore, the base peak. 

Electron ionization occurs when an electron beam crosses an ion source (box) and interacts with sample molecules that have been vaporized into the source. Where the electrons and sample molecules interact, ions are formed, representing intact sample molecular ions and also fragments produced from them. These molecular and fragment ions compose the mass spectrum, which is a correlation of ion mass and its abundance. El spectra of tens of thousands of substances have been recorded and form the basis of spectral libraries, available either in book form or stored in computer memory banks. 

The interaction of electrons with molecules gives molecular ions, some of which can break down to give smaller fragment ions. The collection of molecular and fragment ions is separated by a mass analyzer to give a chart relating ion mass and abundance (a mass spectrum). 

More recently considered candidates are large molecular anions with delocalized anionic charge, which offer low lattice energies, relatively small ion-ion interaction, and hence sufficient solubility and relatively large conductivity. Delocalization of the charge is achieved by electron-with drawing substituents such as -F or - CF3. Furthermore, these anions show a good electrochemical stability to oxidation. In contrast to Lewis acid-based salts they are chemically more stable with various solvents and often also show excellent thermal stability. 

In the latter case the dopant is ionised, interacts with the solvent and, subsequently, solvent clusters interact with the analyte. Molecular and protonated molecular ions are observed, indicating that ionisation can occur via proton (toluene) and electron transfer (acetone). 

It has been estimated that the electron molecule interaction occurs in a very short time ( 10 16 s) and its product is the ejection of one electron from the molecule according to Equation. (2.1). The ions so formed constitute the molecular ion (M+ ). It is produced directly from the molecule but it is a positive ion. In fact, an electron has a negative charge and its removal from a neutral species causes the formation of a positively charged ion. In addition, molecules are neutral species with an even number of electrons that are paired two by two in the orbitals. When one of these is removed, there is a remaining odd number,... 

The obvious approach to answering this question is to remove an electron from this orbital and observe the effect on, for example, the metal-metal stretching frequency or metal-metal bond distance. Of course, removal of an electron from the delta bonding orbital creates a positive molecular ion for which determination of these properties may not be possible using normal techniques. In those cases where the ion is sufficiently stable that these properties can be measured, the meaning of the information may be clouded by changes in intermolecular interactions or other internal factors. 

Advances in Gas Phase Ion Chemistry is different from other ion chemistry series in that it focuses on reviews of the author s own work rather than give a generai review of the research area. This allows for presentation of some current work in a timely fashion which marks the unique nature of this series. Emphasis is placed on gas phase ion chemistry in its broadest sense to include ion neutral, ion electron, and ion-ion reactions. These reaction processes span the various disciplines of chemistry and include some of those in physics. Within this scope, both experimental and theoretical contributions are included which deal with a wide variety of areas ranging from fundamental interactions to applications in real media such as Interstellar gas clouds and pleismas used in the etching of semiconductors. The authors are scientists who are leaders in their fields and the series will therefore provide an up-to-date analysis of topics of current importance. This series is suitable for researchers and graduate students working in ion chemistry and related fields and will be an invaluable reference for years to come. The contributions to the series embody the wealth of molecular information that can be obtained by studying chemical reactions between ions, electrons and neutrals in the gas phase. 

The result is a radical cation, M", the molecular ion. Depending upon the magnitude of the interaction of the molecules with the high-energy electron, additional internal energy may be imparted into them. In some cases, this may be sufficient to cause fragmentation... 

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