Speaker: Andrew Cassidy, Center for Interstellar Catalysis, Department of Physics and Astronomy, Aarhus University, Denmark Title: Spontaneous electric fields in thin films of molecular solids
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1525-626 and Zoom
Speaker: Andrew Cassidy, Center for Interstellar Catalysis, Department of Physics and Astronomy, Aarhus University, Denmark
Title: Spontaneous electric fields in thin films of molecular solids
The condensation of dipolar, molecular species to form thin films, can lead to the spontaneous orientation of dipoles in the resulting solid. This phenomenon is long range and homogenous, producing polarized films of nanometer thicknesses that habour internal electric fields of up to 107 V/m. Simple molecular species have been used to demonstrate this phenomenon, including nitrous oxide, carbon monoxide, methyl formate and, most recently, ammonia. The spontaneous generation of the electric field upon growth has been termed the “Spontelectric Effect” and a mean field model has been developed to explain the temperature dependent variation of the strength of the electric fields produced.
Molecular films find technical applications as semi-conductors and pharmaceutical products, and represent the largest source of solid molecular ice in the Interstellar Medium. The spontelectric effect can have impact on all of these areas. In this seminar, I will introduce the experiments that led to the identification and characterisation of the spontelectric effect, namely the measurement of polarization charge on film surfaces and Stark effects in reflection absorption infrared spectroscopy measurements. I will then show how these internal electric fields can be used to understand the physics of Wannier-Mott excitons in ammonia ices and carbon monoxide ices, and introduce a potential role for polarized carbon monoxide ices in astrochemical-reactions.