Spontaneous polarization of thick solid ammonia films

Abstract

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.

Solid Ammonia Charging by Low-Energy Electrons

“…In this study, we will consider the layers’ thickness in ML units. The exposure of ammonia that is equivalent to 1 ML (calculated in Langmuir units, L, where 1 L = 10 –6 Torr s) was derived from the onset of the second monolayer desorption peak, detected from coverage-dependent ΔP-TPD measurements, in agreement with such spectra previously reported in the literature …”

“…The d­(ΔCPD)/d T spectra are then compared to the spectrum of a similar noncharged film (the red line with open circles). As has already been mentioned, neutral solid ammonia films reveal a net macroscopic polarization normal to the substrate plane when grown at temperatures in the range of 30–85 K . The ammonia films’ polarization responds to temperature increase (but not to temperature decrease, showing a “memory” effect of the film’s thermal history).…”

“…These voltages gradually and moderately decay with time (0.5 ± 0.1 V over the 1200 s time range). The 10% higher film’s voltage than the electron beam energy is apparently due to the beam energy distribution and the spontaneous development of film polarization during growth …”

“…Following the film growth, the sample was exposed to low-energy electrons, typically at kinetic energy of 4 ± 0.4 eV referenced to ground and measured at the exit from the commercial e-gun when irradiation of the bare ruthenium substrate is taking place. In addition, the adsorption of the molecules and film polarization in the direction normal to substrate plane (in this case parallel to the irradiation vector) may also affect the energy of the incoming electrons, whether they will be accelerated or retarded. The dissociative electron attachment (DEA) threshold in pure ammonia was reported to be above 5 eV and to have a maximum yield at 5.6 eV. , To confirm this threshold, no hydrogen desorption signal from the ammonia covered surface could be detected following the 4 eV electron bombardment (above the chamber’s background signal).…”

Sign flipping of spontaneous polarization in vapour-deposited films of small polar organic molecules

“…glass transition) is expected to cause an anomalous fluctuation in the voltage. 30 Results for most of our data are shown in Fig. SD1-SD3 (ESI †).…”