  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.
