Probing Dynamics at Interfaces: Options for Neutron and X-ray Spectroscopy

Inelastic neutron and X-ray scattering experiments on surfaces and interfaces are a challenging topic in modern physics. Particular interest arises regarding surfaces and interfaces of soft matter and biological systems. We review both neutron and x-ray spectroscopic techniques with view at their applicability to these samples. We discuss the different methods, namely neutron triple-axis, backscattering and spin-echo spectroscopy as well as x-ray photon correlation spectroscopy, in the context of planar lipid membrane models as an example. By the combination of the different methods, a large range in momentum and energy transfer is accessible. Introduction An outstanding problem of modern condensed matter physics relates to the question how structure, thermodynamics, phase transitions and molecular motions change from the bulk values when the spatial dimensions are reduced. In recent years, a growing interest has arisen in studying dynamics at surfaces and interfaces in as large a range of time scales as possible. While most spectroscopic techniques, as e.g. nuclear magnetic resonance or dielectric spectroscopy, are limited to the center of the Brillouin zone at Q=0 and probe the macroscopic response, neutron and x-ray scattering experiments give the unique access to microscopic dynamics at length scales of e.g. intermolecular distances. To enlarge the Q-ω range to a maximum, several experimental techniques from neutron and x-ray scattering have to be combined. Figure 1 gives an overview of the accessible Q-omega range of different spectroscopic techniques. By combining neutron triple-axis or time-offlight, backscattering and spin-echo spectrometers, an energy range from about 50 meV (thermal triple-axis or time-of-flight) down to sub-μeV (spin-echo), corresponding to timescales from about 0.1 ps to 100 ns, is accessible. X-ray photon correlation spectroscopy (XPCS) even extends this range down in the neV range and beyond (detectable motions slower than about 50 ns). Neutron scattering gives access to length scales ranging from intermolecular distances of about 3 Å up to several hundred Å. Topics of interest are for instance the glass transition at the surface, the test of theoretical predictions derived from continuum mechanics, polymer surface dynamics or the dynamics of biological model systems such as planar lipid membranes. To solve these issues, several experimental challenges have to be met: The minuteness of the inelastic signals necessitates a sample preparation and experimental set-ups specially adapted for inelastic experiments. In this paper we focus on the application of different inelastic scattering techniques to the study of lipid membranes as prominent examples of low dimensional systems. The understanding of dynamics in these model membranes is of fundamental interest in biophysics. We mainly discuss energy resolved neutron techniques, but also include a short section on XPCS.