Molecular-dynamics simulation of positive-ion and neutral halogen desorption following NaK-shell Auger cascades in the NaF crystal

A Na K-shell Auger cascade in NaF can produce either four to six neighboring valence holes, or two to four neighboring valence holes and a ${\mathrm{F}}^{+}$ ion. If the holes remain localized as neutral atoms where they are created, they will assist the ${\mathrm{F}}^{+}$ ion to desorb, or assist in the desorption of ${\mathrm{Na}}^{+}$ ions. The neutral atoms themselves may also desorb. We use molecular-dynamics calculations to investigate whether the ${\mathrm{F}}^{+}$ ion and neutral F atoms produced by the Na K-shell Auger cascade in NaF can in fact lead to the desorption of positive ions and/or neutral F atoms. Simple fits to the Gordon-Kim potentials are used for the ionic interactions. Ab initio two-body potentials are developed to describe the interaction between a neutral F atom just outside of the crystal surface and a Na or F ion in the surface plane. The potentials exhibit a repulsive inner wall similar to that found for the diatomic molecules, and a shallow well. The integration time required for a satisfactory simulation of positive-ion desorption by the Knotek-Feibelman mechanism is shown to be 1–2 ps. Using 2-ps simulations, it is shown that only when the ${\mathrm{F}}^{+}$ ion is accompanied by four neighboring F atoms from the same Auger cascade does positive-ion desorption occur from the perfect (100) surface. Analogous results are obtained in simulations designed to relate to the imperfect (100) surface. It is concluded that the neutral atoms assist positive ions to desorb only in configurations which appear to be too rare to be of experimental significance. It is shown that crystal temperature, near room temperature, is not a factor in determining whether positive-ion desorption occurs by the Knotek-Feibelman mechanism. This work extends that of Walkup and Avouris on NaF to the case where the Auger-produced ${\mathrm{F}}^{+}$ ion is accompanied by neutral F atoms.