In-situ NMR Measurements of Vapor Deposited Ice

TL;DR

This study uses in-situ NMR to analyze vapor deposited ice, revealing complex relaxation behaviors dependent on growth conditions and suggesting microporous structures influence relaxation dynamics.

Contribution

It provides new insights into the relaxation patterns of vapor deposited ice and links microporous structures to fast relaxation components.

Findings

Relaxation times span over 6 orders of magnitude.

Growth temperature affects relaxation pattern (multi-timescale vs. single exponential).

Microporous structures are stable up to desorption temperatures.

Abstract

In-situ NMR spin-lattice relaxation measurements were performed on several vapor deposited ices. The measurements, which span more than 6 orders of magnitude in relaxation times, show a complex spin-lattice relaxation pattern that is strongly dependent on the growth conditions of the sample. The relaxation patterns change from multi-timescale relaxation for samples grown at temperatures below the amorphous-crystalline transition temperature to single exponential recovery for samples grown above the transition temperature. The slow-relaxation contribution seen in cold-grown samples exhibits a temperature dependence, and becomes even slower after the sample is annealed at 200K. The fast-relaxation contribution seen in these samples, does not seem to change or disappear even when heating to temperatures where the sample is evaporated. The possibility that the fast relaxation component is linked to the microporous structures in amorphous ice samples is further examined using an environmental electron scanning microscope. The images reveal complex meso-scale microporous structures which maintain their morphology up to their desorption temperatures. These findings, support the possibility that water molecules at pore surfaces might be responsible for the fast-relaxation contribution. Furthermore, the results of this study indicate that the pore-collapse dynamics observed in the past in amorphous ices using other experimental techniques, might be effectively inhibited in samples which are grown by relatively fast vapor deposition.