Probing aerogels by multiple quantum filtered ¹³¹Xe NMR spectroscopy

At the interface between solid surfaces and cavities filled with gaseous or liquid xenon, the nuclear magnetization of ¹³¹Xe (S = 3/2) is subject to quadrupolar interactions which may lead to higher rank single-quantum coherences that can be described by tensor elements T_2,±1 and T_3,±1. This 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 quadrupolar coupling. In this contribution, MQF NMR is applied to aerogels filled with liquid xenon to demonstrate the potential of this technique for material sciences. Xenon in the liquid phase provides a sufficient spin density to obtain reasonable signal-to-noise ratios. Coherent evolution and relaxation both contribute to the creation of higher rank coherences depending on the presence or absence of water molecules on the surface. These two processes can be distinguished experimentally and provide complementary information about the surface of the host material.