Bombardment induced electron-capture processes at sodium halide surfaces

Abstract

Discrete features observed in the energy distribution of electrons emitted from ion-bombarded sodium halide surfaces can be attributed to a new type of collisional deexcitation mechanism. Such a mechanism involves sodium atoms in bombardment-excited autoionizing states that are the result of cascade collisions within the crystal lattice. This deexcitation process, in contrast to that for a metal, is not simply a consequence of the inner-shell lifetime of the initial collisionally excited sodium Na+* ion. Rather, the deexcitation consists of a sequence of lattice collisions during which the excited Na+* ion captures an electron to form the inner-shell-excited Na0* states responsible for the observed transitions. The formation of such autoionizing Na0* states is described within the framework of a new model in which excitation processes and localized collisional electron-transfer mechanisms are taken into account. These localized electron-transfer processes make possible new channels for electronic deexcitation, chemical dissociation, and defect production; they are critical for understanding inelastic ion-surface collisions in solids.

Charge Supported Growth and Superplasticity of Sodium Nanostructures

“…According to the equation, the energy transfer is inversely proportional to the mass of the target element, which indicates that sodium atom is more preferred to be energetic and knocked off. Following previous studies, ,− the e-beam irradiation of NaCl crystals produces various particles (Figure a), including positively charged (Na + , Na 2+ , and Cl + ), negatively charged (Cl – and NaCl – ), and neutral particles (Na 0 , Cl 0 , etc). Na + comprises the majority of positively charged ions, Cl – comprises the majority of negatively charged ions, and Na 0 comprises the majority of neutral particles. ,, Overall, the neutral Na 0 and positive Na + make up the largest part, and the negative Cl – make up a much smaller part. ,, The generation of Na species under e-beam irradiation supplies a source of sodium for the SNs’ growth.…”

“…Some sodium species (Na 0 , Na + , etc.) are absorbed on the NaCl surface and form a negatively charged layer − that can be the driving force supporting the sodium supply in SN growth (Figures a and b).…”

“…Seized growth is believed to be caused by the depletion of negative charges at the tip. From another aspect, given that Cl 0 , Cl – , and Cl + are also generated during the irradiation process, ,, the electrostatic absorption of chloride ions (especially Cl + and Cl 0 ) and the reaction of sodium with chloride are also possible (process III in Figure a). Owing to the relatively low yield, they react with a relatively low velocity.…”

“…Na + comprises the majority of positively charged ions, Cl − comprises the majority of negatively charged ions, and Na 0 comprises the majority of neutral particles. 19,21,23 Overall, the neutral Na 0 and positive Na + make up the largest part, and the negative Cl − make up a much smaller part. 19,21,22 The generation of Na species under ebeam irradiation supplies a source of sodium for the SNs' growth.…”

“…19,21,23 Overall, the neutral Na 0 and positive Na + make up the largest part, and the negative Cl − make up a much smaller part. 19,21,22 The generation of Na species under ebeam irradiation supplies a source of sodium for the SNs' growth.…”