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.