Field ionization basic mechanisms

There are basically two kinds of experiments which can be used for studying the mechanisms of the field ionization process near a field ion emitter surface and also the field ion image formation process. They are the measurement of field ion current as functions of tip voltage, tip temperature, and other experimental parameters, and the measurement of the ion energy distribution. 

Virtually all non-trivial collision theories are based on the impact-parameter method and on the independent-electron model, where one active electron moves under the influence of the combined field of the nuclei and the remaining electrons frozen in their initial state. Most theories additionally rely on much more serious assumptions as, e.g., adiabatic or sudden electronic transitions, perturbative or even classical projectile/electron interactions. All these assumptions are circumvented in this work by solving the time-dependent Schrodinger equation numerically exact using the atomic-orbital coupled-channel (AO) method. This non-perturbative method provides full information of the basic single-electron mechanisms such as target excitation and ionization, electron capture and projectile excitation and ionization. Since the complex populations amplitudes are available for all important states as a function of time at any given impact parameter, practically all experimentally observable quantities may be computed. 

As discussed above, every laser exposure of a sample leads to the removal of a bulk volume - that is, many monolayers of matrix molecules of the sample. The term desorption is, therefore, somewhat ill-chosen for this process, and was so even for the field desorption for which it was originally coined. Ablation (removal of bulk material from surfaces) is the more specific term, and is used interchangeably with desorption throughout this chapter. The processes of material ablahon and the ionizahon of a minor fraction of the matrix and analyte molecules are, no doubt, intimately intertwined, and both take place on a micrometer geometric and a nanosecond time scale. It is experimentally very difficult - if not impossible - to sort out the complex contributions of the physical processes induced by the laser irradiation in all detail. Despite this complexity, it is of considerable merit to treat the ablation and ionization mechanisms separately. From such a discussion, some basic understanding can be derived, particularly, because the vast majority of the ablated material comes off neutral. 

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