Subfemtosecond dynamics of structural protons in silica xerogels

Deep inelastic neutron scattering measurements on silica xerogels with different geometries (24 and 82 A pore size diameter) were performed on the VESUVIO spectrometer at the ISIS spallation neutron source, in order to investigate the modification of the ultrashort-time (10(-15)-10(-17) s) dynamics of the protons belonging to the silanol groups, when precursors are such as to modify the structural morphology. This paper is also intended to present a complementary experimental technique for the characterization of the microscopic dynamical properties of these materials. The mean kinetic energy < E-k > and the momentum distribution n(p) of the protons of the structural silanol groups in the two different systems are derived and compared at the same thermodynamic conditions. The results show that < E-k > is different for the two systems, reflecting the differences in the pore morphology. Indeed, < E-k > is higher for the system with an average pore diameter of 24 A, as compared to that of 82 A.