Probing aerogels by multiple quantum filtered Xe-131 NMR spectroscopy

At the interface between solid surfaces and cavities filled with gaseous or liquid xenon, the nuclear magnetization of Xe-131 (S = 3/2) is subject to quadrupolar interactions which may lead to higher rank single-quantum coher ences that can be described by tensor elements T-2,T-+/-1 and T-3,T-+/-1. T his can be demonstrated by multiple-quantum filtered (MQF) NMR experiments. In gaseous xenon! on Pyrex surfaces, the primary source of such coherences was shown to be coherent evolution induced by a nonvanishing average quadr upolar coupling. In this contribution, MQF NMR is applied to aerogels fille d with liquid xenon to demonstrate the potential of this technique for mate rial sciences. Xenon in the liquid phase provides a sufficient spin density to obtain reasonable signal-to-noise ratios. Coherent evolution and relaxa tion both contribute to the creation of higher rank coherences depending on the presence or absence of water molecules on the surface. These two proce sses can be distinguished experimentally and provide complementary informat ion about the surface of the host material.