Theoretical studies of atomic emission and defect formation by electronic excitation at the (100) surface of NaCl

The F and H centers and self-trapped excitons near the (100) surface of NaCl have been simulated using the embedded-cluster Hartree-Fock calculation technique. An F center in the ground state in the top surface layer is shown to be stable. The activation energy of 1.94 eV is needed to emit a neighboring Na atom, while a Na atom can be emitted spontaneously when an F center is excited to the lowest excited state. The H center in the first two surface layers is found to be decomposed into a ${\mathrm{Cl}}^{\mathrm{-}}$ ion on the lattice site and a ${\mathrm{Cl}}^0$ atom physisorbed on an anion with a binding energy of 0.14 eV. The H center located below the third layer is stable. The self-trapped exciton in the top and second layers is found to be unstable and decomposes into an F center and a ${\mathrm{Cl}}^0$ atom in the vacuum, while that below the third layer is relaxed similarly to those in the bulk. Existing experimental results on photoinduced emission of alkali and halogen atoms are discussed based on the results of calculations.