Coulomb explosion model

Coulomb explosion model A model for describing sputtering... 

According to the ion-evaporation model, the droplets become smaller until a point is reached at which the surface charge is sufficiently high for direct ion evaporation into the gas phase to occur. In the case of the charge-residue model, repeated Coulombic explosions take place until droplets are formed that contain a single ion. Evaporation of the solvent continues until an ion is formed in the vapour phase. 

The new model has also been applied to the calculation of thermally averaged probability density functions for the out-of-plane inversion motion of the CH and H3O ions [9]. Such probability densities can be obtained experimentally by means of Coulomb Explosion Imaging (CEI) techniques (see, for example, Refs. [10,11]), and the results in Ref. [9] will be useful in the interpretation of the resulting images, just as analogous calculations of the bending probability distribution for the CHj ion were instrumental in the interpretation of its CEI images (see Refs. [9,12] and references therein). 

The alternative mechanism, the ion-evaporation model, was proposed initially by Iribarne and Thomson13 (Fig. 4) and involves desolvation of the droplets, producing an increase in charge density over the droplet surface that causes coulombic explosion and eventually leads to ejection of individual ions. 

For the diatomic molecules that were studied—nitrogen, oxygen, nitric oxide, and carbon monoxide—the concept of a Coulomb explosion appears to be relevant. The yield of atomic ions is high, 93% to 97%, and the ion kinetic energies of around 7 eV for +1 ions and about twice this value for -1-2 ions are consistent with the Coulomb repulsion model. For the polyatomic molecules the situation is different. The yield of atomic ions drops to 85% for carbon dioxide and to 74% for carbo i tetrafluoride. For excitation of a core to bound state resonance in nitrous oxide, involving the terminal nitrogen atom, the yield of atomie ions is only 63% (Murakami et al. 1986). These molecules do not simply explode following excitation of a core electron. 

With these microdroplets, the solvent evaporation process will continue. Another Coulomb explosion may follow, leading to further reduction of the droplet size. The microdroplets play an important role in the analyte ionization. However, there are two theoretical models describing the actual ionization event, leading to gas-phase ions amenable to MS. In both models, the concept of preformed ions in solution plays an important role. 

In the mid-1960s. Dole and co-workers were the first to combine electrospray nebulization with MS. Their hypothesis was that the nebulization of a dilute protein solution would lead to gas-phase protein ions as a result of continued solvent evaporation and Coulomb explosion. This would in the end lead to a very small droplet containing only one charged protein molecule. This small droplet can be considered as a solvated protein ion. Desolvation of this ion by solvent evaporation results in a gas-phase protein ion. This model is nowadays called the charge-residue model. 

The CTMC calculations give a physical insight into possible mechanisms for the reason that antiprotons are much more efficient at ionizing helium than protons. A Coulomb explosion is seen to occur when the antiprotons pass between the electrons and the nucleus. For distant collisions the antiproton is seen to knock the nearest electron towards the further one, but protons pull the nearest electron away from the far one. Vegh has also proposed a similar, if more complicated, model depending on the radial coupling between the electrons [7.17]. 

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