Ion internal energy

The physicochemical aspects of the ionization process in general, ion internal energy, and the principles determining the reaction pathways of excited ions have already been addressed (Chap. 2). After a brief repetition of some of these issues we will go more deeply into detail from the analytical point of view. Next, we will discuss technical and practical aspects concerning the construction of El ion sources and sample introduction systems. Finally, this chapter directly leads over to the interpretation of El mass spectra (Chap. 6). 

All such reactions are characterized by a lowering of the cross section as the internal energy of the adduct is increased. The lifetime of the benzene dimer ion, as well as those of similar adduct ions, is very sensitive to its internal energy since it is very loosely bound (it has a binding energy of 8 kcal/mole). Increasing the product-ion internal energy by vibrational excitation of the reactant readily promotes the dissociation back into reactants. 

The result of another type of PEPICO experiment is the breakdown diagram, which is a plot of the fractional abundance of the ions formed by the dissociation of energy-selected parent ions as a function of the photon energy. It is also represented as a plot of branching ratios to the various dissociation products as a function of the precursor ion internal energy. The breakdown diagrams are obtained directly from a series of PEPICO mass spectra at several photon energies. 

The collisions takes place with helium in the ion trap, and with nitrogen of argon in the triple quadrapole instrument. In a collision with a heavier neutral target effectively more translation energy can be converted in ion internal energy. 

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