Charging Amorphous Solid Water Films by Ne⁺Ions Characterized by Contact Potential Difference Measurements

The study of amorphous solid water (ASW) films on solid substrates has been instrumental in understanding the structure and morphology of water ices. In addition, they have the potential to help researchers understand how complex molecules are formed in regions of the interstellar medium (ISM) where many surfaces are coated with ASW. We have studied ASW films charged by low-energy Ne+ ions under ultrahigh vacuum conditions by measuring the contact potential difference of the charged films with a Kelvin probe. The film becomes positively charged when impinging Ne+ ions oxidize the surface water molecules and subsequently scatter back to the vacuum as neutral Ne atoms. The charged ASW film follows plate capacitor physics, displaying a linear dependence of voltage on ASW layer thickness. Electrical fields of 2 (± 1) × 108 V/m are generated within the films. The level of charging, charge stability, and thermal binding energy of the charges to the ASW film are all very sensitively dependent on the film growth temperature and the temperature of the film during Ne+ ion impingement. We propose that these properties are affected by the film's porosity and the nature of the proton binding sites, which are dictated by the film growth temperature. The protons are trapped in undercoordinated water molecule defect sites and L-defect sites, with thermal binding energies ranging from 3.4 to 9.4 kcal/mol, as determined by differential contact potential difference (d(ΔCPD)/dT) measurements, obtained during sample annealing.