The role of thermal history on spontaneous polarization and phase transitions of amorphous solid water films studied by contact potential difference measurements

Abstract

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.

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

“…The range of the thermal binding energies presented here (3.4–9.4 kcal/mol) is similar to the range reported for ASW films charged with low-energy electrons (4–8 kcal/mol) . Additionally, activation energies in the range of 2.7–8.6 kcal/mol were also observed for the depolarization of neutral ASW films ( T gr = 33–115 K) upon annealing . This suggests that the binding strength of charges to ASW films, whether positively charged protons or negatively charged electrons, is highly sensitive to and actually depends on the changes in film porosity that occur during annealing.…”

Solid Ammonia Charging by Low-Energy Electrons

“…Comparison between the spectra of charged and noncharged films can, therefore, assist in the interpretation of the former. A similar analysis was previously demonstrated in the case of ASW films. , …”

“…TP-ΔCPD measurements of charged films can serve as a tool for investigating morphological, structural, and dielectric modifications the films undergo during annealing, since the discharge accompanies these events. This results in magnification of features observed in noncharged films, which are correlated to such processes via their effect on the spontaneous development of polarization during adsorption , and the subsequent depolarization upon annealing.…”

“…A similar analysis was previously demonstrated in the case of ASW films. 43,63 The TP-ΔCPD profiles depict a sharp voltage drop of ∼8 V, from +4 to −4 V (see Figure S1b of the Supporting Information), in the temperature range of 45−120 K, representing the discharge of the film together with the ammonia multilayer desorption above 85 K. 57 The range from +4 to −4 V is the measured value of fully charged film (∼+4 V) and the ΔCPD of the film just following discharge but still having a surface covered by a monolayer of ammonia (∼−4 V). Above 120 K, the profiles follow that of the reference.…”

Comment on “Spontaneous Polarization of Cryo-Deposited Films for Five Normal Saturated Monohydroxy Alcohols, CnH2n+1OH, n = 1–5”

“…7 We mention that the "Aarhus" model described in ref 2 has proved remarkably successful in reproducing the deposition temperature dependence of spontaneous electric fields. This covers a variety of contexts involving surface potential measurements using electron beams, 2 Kelvin probes, 3,9 and vibrational and electronic Stark effects, 10−12 to name but a few references. c Calculated values, using parameters given in the text and obtained through inserting the observed value of g on the right hand side of eq 1 and checking for self-consistency.…”