Separating the measurement of rotation and translational diffusion in a cryo-condensed molecular glass Andrew Cassidy 1 , Signe Kyrkjebø 1 , Mads R. V. Jørgensen 2 , Artur Glavic 3 , Valeria Lauter 4 , Oksana Plekan 5 and David Field 1 1 Center for Interstellar Catalysis and Department of Physics and Astronomy, Aarhus University, Denmark, 2 Center for Materials Crystallography, iNano & Department of Chemistry, Aarhus University, Denmark & MAX IV Laboratory, Lund University, Sweden, 3 Laboratory for Neutron and Muon Instrumentation, Paul Scherrer Institut, Switzerland, 4 Neutron Scattering Division, Oak Ridge National Lab, Oak Ridge, USA, 5 Sincrotrone Trieste S.C.p.A., Italy Spontelectrics - spontaneously-electric - materials are nano-scale molecular solids that spontaneously self-orient their dipoles upon synthesis, and so generate electrically polarised films. These films can harbour electric fields up to 10 8 V/m, with the field arising from, and proportional to, the degree of dipole orientation. Hence, changes to field strength with time or with changes to temperature, offer a probe to measure the rotation of molecular dipoles. We combine these electric field measurements with neutron scattering data, with the later offering the means to measure translational molecular diffusion. This combination offers a new way to study diffusion in cryo-condensed molecular glasses, separating rotational diffusion from translational motion of molecules. Polarised Neutron reflectivity measurements were made at the Magnetism Reflectometer instrument at the Spallation Neutron Source at Oak Ridge National Laboratory. Molecular films where condensed onto a magnetically labelled substrate at cryo-temperatures by leaking gas from a specially designed high-vacuum gas line, directly onto the coldhead in the neutron beam-path. In this way, ultra-clean molecular films, tens of nanometers thick, were prepared in situ, prior to interrogation with a magnetically polarised neutron beam. This technique allowed for measurement of molecular layer thickness with a high precision of 0.5 nm, and measurement of molecular diffusion, between isotopically-labelled molecular layers, down to 10 -21 cm -2 s -1 ; two orders of magnitude increased sensitivity above that achieved in previous experiments. The result is a complete understanding of the translational and rotational diffusion barriers in a molecular glass synthesised from cis-methyl formate. These types of dynamics in cryo-molecular glasses are usually not accessible on laboratory timescales. References 1. Chem. Commun. , 2021, 57 , 6368-6371
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