Appearance potential photon impact

Appearance potential - The lowest energy which must be imparted to the parent molecule to cause it to produce a particular specified parent ion. This energy, usually stated in eV, may be imparted by electron impact, photon impact, or in other ways. More properly called appearance energy. [3]... 

The usual methods for study of mass spectra of gases and appearance potentials involve ionization of the molecules by electron impact or photon impact. These methods are of great importance both for analytical purposes and for other applications, e.g., radiation chemistry. 

A second illustration concerns the appearance potentials observed by use of electron or photon impact. Then, the process at the appearance potential (AP) may be... 

The study of appearance potentials by use of electron or photon impact can therefore give valuable information about the dissociation limit, but the reaction mechanism itself may be unknown. On the other hand, in the determination of the appearance potential by use of charge exchange, the mechanism may be unambiguous, but the value of the appearance potential may have little connection with the dissociation limit of the process. 

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