Quasi-free electrons, definition

It is absolutely vital for our discussion to have a good definition of a localized electronic state as opposed to one which is delocalized. The later will also be referred to as a quasi-free electron. Our primary definition will be that the localized electron is characterized by a wave function which decays exponentially with distance the same type of definition used for simple atomic wave functions. The quasi-free electron... 

In the gaseous phase, an electron ejected from a molecule becomes free, and so for each filled electron level we have only one ionization potential. However, in the condensed phase an ejected electron can be in three different states free, quasi-free, and solvated. So the definition of the ionization potential becomes ambiguous. 

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