Electron stimulated desorption of Cl− from adsorbed and condensed Cl2: Effects of environment and orientation

Abstract

Electron stimulated desorption (ESD) of Cl⁻ from condensed molecular chlorine in the energy range 0–15 eV is studied. Cl₂ is deposited in either multilayer amounts directly on a cryogenically cooled gold crystal or in sub-monolayer quantities on rare gas films (Xe, Kr) or ammonia ice films. Cl⁻ desorption from multilayer films shows an intense resonance peaking at 5.5 eV and a comparatively smaller feature at 3 eV in qualitative agreement with an earlier ESD experiment. The desorption signal is enhanced by about one order of magnitude when a 0.2 monolayer (ML) Cl₂ is adsorbed on a multilayer rare gas film. In this case, the desorption signal shows two clearly separated resonances peaking at 2.5 and 5.5 eV closely resembling dissociative electron attachment (DEA) from gas phase Cl₂. These resonances can be associated to the transitions Cl₂(¹Σ⁺_g) → Cl₂⁻(²Π_g) and Cl₂(¹Σ⁺_g) → Cl₂⁻(²Π_u), respectively, both final states representing core excited resonances. The shape of the resonance around 5.5 eV splits into different peaks when changing from grazing incidence of the electron beam to an impact angle of 45° with respect to the surface normal. On the basis of the pronounced angular dependence of the Cl⁻ intensity reported from gas phase DEA this observation is compatible with a situation in which the molecules are oriented along the surface normal. Compared to the noble gas films, ESD from sub-monolayer Cl₂ on top of a multilayer NH₃ film is suppressed while the overall shape of the yield function is approximately preserved. None of the present experiments show a Cl⁻ desorption signal below 2 eV while the charging behaviour of the film indicates that electron attachment is still operative in this energy domain. This suggests that the transient anion in its electronic ground state (Cl₂^−#(²Σ⁺_u)) is still formed by low energy electron attachment but is subjected to effective energy dissipation creating either stabilized ions (Cl₂⁻(²Σ⁺_u)) or fragment ions (Cl⁻ (²P)) with insufficient kinetic energy to leave the surface.