High-resolution He-scattering apparatus for gas–surface interaction studies

Abstract

A high‐resolution apparatus designed for the study of elastic and inelastic scattering of thermal helium atoms from crystal surfaces is presented. The highly expanded He nozzle beam has an energy spread ΔE/E of about 1.4% and is collimated to 0.2°. The angle subtended by the detector opening as seen from the sample is also 0.2°. Beam intensities as low as 10⁻⁶ of the specular beam intensity from a low‐temperature clean Pt(111) surface are detectable. Pseudorandom chopping with a resolution of 2.5 μs (flight path 790 mm) is used for time‐of‐flight (TOF) analysis of the scattered helium. The base pressure in the sample chamber is in the low 10⁻¹¹ mbar. The capabilities of the apparatus are demonstrated for physisorbed Xe adlayers on Pt(111). The results presented are obtained by using He scattering in various modes: coherent inelastic, coherent elastic, and incoherent (diffuse) elastic. This technique allows for a nondestructive nearly exhaustive characterization of the thermodynamics, structure, and dynamics of physisorbed adlayers on arbitrary substrates.