2.2 ILENA Setup

Figure 2.1: ILENA setup

The experiment ILENA at the University of Bern [11,12,13,14] (see Figure 2.1) consists of an ion source, a beam guiding system, a sample stage with housing and with an alkali dispenser unit (not used for this study), and a detection unit. All these units are contained in a single vacuum chamber pumped by a turbomolecular pump. Ions are formed in an electron impact ion source (Nier type), with the intensity of the primary ion beam of pA. Molecular ions of oxygen or hydrogen were used rather than atomic ions because they can be produced far more efficiently in this system. The ions are then deflected into a cylindrical analyzer with an energy width of the ion beam at the sample of (FWHM) [11]. The energy analyzer focuses the ion beam on the entrance aperture of the sample housing. Two diaphragms limit the beam size to 1mm and the beam divergence to . The impact angle of the ion beam on the conversion surface can be varied between and with respect to the surface normal. The reflected beam is recorded with a two-dimensional position-sensitive micro channel plate (MCP) detector with a viewing angle of in the azimuthal direction and in the polar direction. A retarding potential analyzer (RPA) consisting of three grids is mounted in front of the MCP detector. The detector unit, including the RPA, is shielded electrostatically and can be rotated independently of the conversion surface around the same axis. The outer grids of the RPA are grounded to shield the inner grid, which can be biased to suppress positive ions. An additional grid in front of the MCP detector at negative potential with respect to the MCP detector serves to reject secondary electrons originating from the preceding grids and the converter surface. The MCP detector may be floated to a negative high voltage with respect to the converter surface to eliminate negative particles. After baking out the vacuum chamber a residual gas pressure of mbar is achieved. During operation the pressure may rise into the low mbar range as a result of the test gas leaking into the ion source chamber. The sample can be heated in order to remove adsorbates from the surface. A filament next to the surface allows scattering experiments to be carried out with insulating surfaces that would charge up otherwise.