Quantum reflection of He^* on silicon

A cold beam of He^* (2 ³S₁) atoms is used at grazing incidence to study the quantum reflection on a flat polished silicon surface. We measure the reflectivity as a function of the normal incident velocity component between 3 and 30 cm∕s. Our result is in reasonable agreement with a calculation of the attractive van der Waals surface potential using the dielectric function of Si and the dipole polarizability of He^*. We discuss the influence of the conductivity and of a thin oxide layer on the potential. By comparing our data to those previously measured with Ne^* atoms, we are also able to confirm the scaling of the reflectivity with atomic mass.