The charging of solid molecular films grown on grains is an important phenomenon observed in astrochemical processes that take place in interstellar space and is relevant in high altitude environmental physics and chemistry. In this work, we present the temperature dependence study of both the conductivity and the capacitance of amorphous solid water (ASW) films (hundreds of monolayers thick) deposited on a Ru(0001) substrate. These layers subsequently interact with low-energy electrons (5 eV) in the temperature range of 50−120 K under ultra high vacuum (UHV) conditions. The charging of the ASW films was measured via contact potential difference (CPD) detection utilizing an in situ Kelvin probe and found to be sensitive to the substrate temperature during film growth, to the substrate temperature during electron irradiation, and to the film thickness. Internal electric fields exceeding 10 8 V/m are developed. It is shown that solid water conducts and stores electrons with "memory" of the film's thermal history. Furthermore, we propose that trapped electrons discharge during substrate annealing in a process that is driven by the formation and propagation of cracks within the molecular layer, similar to the release of gas molecules embedded inside ASW films, at significantly lower temperatures than the onset of crystallization. Thermal binding energies of electrons to the ASW matrix are obtained from the discharge measurements, in the energy range of 0.26 ± 0.08 eV. These values are 1 order of magnitude smaller than those obtained via photoemission studies.

Monitoring thermal processes occurring in molecular films on surfaces can provide insights into physical events such as morphology changes and phase transitions. Here, we demonstrate that temperature-programmed contact potential difference (TP-∆CPD) measurements employed by a Kelvin probe under ultrahigh vacuum conditions and their temperature derivative can track films’ restructure and crystallization occurring in amorphous solid water (ASW) at temperatures well below the onset of film desorption. The effects of growth temperature and films’ thickness on the spontaneous polarization that develops within ASW films grown at 33 K–120 K on top of a Ru(0001) substrate are reported. Electric fields of ∼106 V/m are developed within the ASW films despite low average levels of molecular dipole alignment (<0.01%) normal to the substrate plane. Upon annealing, an irreversible morphology-dependent depolarization has been recorded, indicating that the ASW films keep a “memory” of their thermal history. We demonstrate that TP-∆CPD measurements can track the collapse of the porous structure at temperatures above the growth and the ASW-ice Ic and ASW-ice Ih transitions at 131 K and 157 K, respectively. These observations have interesting implications for physical and chemical processes that take place at the interstellar medium such as planetary formation and photon- and electron-induced synthesis of new molecules.

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) × 10 8 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.

Ammonia molecules have an important role in the radiation-induced chemistry that occurs on grains in the cold interstellar medium and leads to the formation of nitrogen containing molecules. Such grains and surfaces are primarily covered by water ices; however, these conditions allow the growth of solid ammonia films as well. Yet, solid ammonia know-how lags the vast volume of research that has been invested in the case of films of its “sibling” molecule water, which, in the porous amorphous phase, spontaneously form polar films and can cage coadsorbed molecules within their hydrogen-bonded matrix. Here, we report on the effect of growth temperature on the spontaneous polarization of solid ammonia films (leading to internal electric fields of ∼105 V/m) within the range of 30 K–85 K on top of a Ru(0001) substrate under ultra-high vacuum conditions. The effect of growth temperature on the films’ depolarization upon annealing was recorded as well. By demonstrating the ability of ammonia to cage coadsorbed molecules, as water does, we show that temperature-programmed contact potential difference measurements performed by a Kelvin probe and especially their temperature derivative can track film reorganization/reconstruction and crystallization at temperatures significantly lower than the film desorption.

UV photons and low-energy electrons play an important role in the evolution of various molecules in the interstellar medium (ISM). Here, we examined the product molecule formation as a result of irradiation of 193 nm photons and 6.4 eV electrons (same energy under identical laboratory conditions) on D 2 O|CH 4 + ND 3 |D 2 O sandwiched films deposited on Ru(0001) substrate at 25 K in ultrahigh vacuum as a model for processes in the ISM. Temperature-programmed desorption spectra performed following the irradiation revealed the signature of hydrazine and formamide product molecules. These molecules were, however, formed exclusively following the photons' irradiation. These results were compared with the products obtained from a D 2 O|CH 4 |D 2 O sample without ammonia, where deuterated formaldehyde was the dominant product, formed also by photons only. Our results indicate that the photon-induced activation of the cofrozen molecules within D 2 O occurs via direct (partial) dissociation of the host and embedded molecules, followed by sample annealing. The electron-induced activation occurs through a direct dissociative electron attachment mechanism.