软X射线同步辐射光激励下表面吸附物质的选键解离与解吸

Bond-specific desorption from adsorbates by soft X-ray resonant photoexcitation

用同步辐射软Ｘ射线作为光源，可实现分子中某些原子芯能级的共振激发。而芯能级电子被共振激发到非占据分子轨道后，其去激发过程通常以旁观者俄歇过程为主。由于在旁观者俄歇过程中被激发的电子在随后的去激发过程中并不参与，而是保留在具有反键特性的非占据轨道中，这样的分子激发态一般有较陡的排斥势能面，所以它在紧随其后的离解与解吸过程中起着重要的作用。由于反键轨道一般定域在特定的化学键上，所以这一共振激发往往可引起分子中某些特定化学键的断裂。这就象一把“化学剪刀”一样，它可将吸附在基底表面吸附物质的某些碎片裁剪掉。通过芯能级共振激励离子解吸的实例，对其中的某些过程与机理进行探讨。

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.