Surfaces and interfaces are of crucial importance for the understanding of properties and processes in matter. Although most characteristics of surfaces differ drastically from the ones in the bulk, rarely more than five atomic layers are of relevance to describe the fundamentals of topics such as heterogeneous catalysis, corrosion or microelectronics. Despites this intense interest there are only very few techniques available to probe local properties with high sensitivity on surfaces and interfaces.
The Apparatus for Surface Physics and Interfaces at Cern (ASPIC) achieves remarkable resolution of local electronic and magnetic properties and high surface sensitivity at the same time. Therefore it combines nuclear perturbed angular correlation (PAC) spectroscopy with state of the art sample preparation and manipulation hosted in a common ultra-high vacuum system. Samples can be prepared in-situ by molecular beam epitaxy (MBE), argon ion beam sputtering and heat treatment between 77K and 3000K. A low energy electron diffractometer (LEED) and an Auger spectrometer (AES) are in place for chemical and structural characterisation. A base pressure of 10-10 mbar ensures little surface contaminations during the time required for detailed measurements, whereas several layers of gas particles absorb on a surface and change its properties considerably within seconds e.g. in ISOLDE’s beamline vacuum of 10-6 mbar.
Once a sample is prepared, suitable radioactive probe nuclei are collected from ISOLDE through a dedicated UHV beamline and transferred to the sample’s surface by a technique called soft-landing, where the ions are deposited with thermal energy only on top of the first atomic layer. Additional preparation steps help to integrate the probes into defined crystalline sites on the surface or at an interface. Different detection geometries allow for PAC spectroscopy along an perpendicular to all surface directions at variable temperatures (4K to 500K) and variable magnetic field (0.01T to 0.8T).
A beamline upgrade currently under consideration includes nuclear polarisation of the isotope beam provided by ISOLDE and therefore allows for β-NMR spectroscopy in a wide range of sample environments realized in three end-stations. The UHV and low temperature station will remain for surface PAC and will be extended for β-NMR spectroscopy. An additional station will be equipped with strong differential pumping allowing for online β-NMR and online PAC spectroscopy in volatile matter, such as biochemically relevant aqueous solution. The third station will be open for movable experiments requiring rare polarized ions or UHV environment.