Atomic and molecular ion desorption from solid surfaces via positron annihilation

Positrons, the antiparticles of electrons, have been used for basic research such as scattering with atoms and molecules and positronium studies. Studies of solids and crystal structure analysis of surfaces have also been conducted using positrons, and much knowledge has been obtained. In recent years, studies on desorption of surface atoms and molecules as ions induced by positron annihilation at the surface have also been conducted.

It has long been known that atoms on a solid surface are desorbed when the surface is irradiated by electrons with energies higher than the binding energy of the inner-shell electrons of the atoms constituting the surface. This phenomenon is explained as the inner-shell ionization caused by electron impact and the subsequent emission of Auger electrons, resulting in the atoms becoming positively divalent or more valence and desorbing due to Coulomb repulsion with the surrounding atoms [1]. The same phenomenon occurs in the case of low-energy positron bombardment [2]. However, there is no energy threshold in the case of positron bombardment, and the number of desorbed ions per incident particle is much higher than in the case of electron bombardment [3]. This phenomenon can be explained by the fact that the incident positrons are trapped on the surface as they diffuse back through the solid, and the atoms are desorbed by pair annihilation with the inner-shell electrons of the surface atoms.

Recently, it was discovered that F₂⁺ ions desorb when a slow positron beam is injected onto a LiF surface [4]. This is in contrast to the case of electron irradiation, where only monatomic ions such as H⁺, Li⁺, and F⁺ are desorbed. This desorption of molecular ions by slow positrons is interpreted as the formation of e⁺[F^-F^-] at the surface, followed by the emission of Auger electrons by pair annihilation with the inner-shell electrons of the positrons, resulting in desorption as F₂⁺. This technique may pave the way for the formation of new molecular ions that cannot be achieved by other methods.