The resonant photoexcitation of some core - levels can be carried out by using energy - tunable synchrotron radiation soft X - ray. The core excited state decays mainly through spectator Auger transition after the electrons are resonantly photoexcited into unoccupied orbitals. Because the excited electron just stays in the unoccupied orbital generally with antibonding character in the spectator Auger processes

this kind of excited state of a molecule goes into a steep repulsive potential curve

so it is very important for specific ion desorption. Generally speaking

antibonding orbital is located on the given bond

so the resonant excitation can result in bond - specific rupture and ion desorption. This process will cut some special fragment from adsorbates just like a " chemical scissors". Some examples of photon - stimulated ion desorption by core - to - valence resonant excitation have been presented in this paper. The mechanisms in the processes were also explained.