Argon scattering from Ru(0001): Calculations and comparison with experiment

Recently reported measurements of Ar atom scattering from Ru(0001) at thermal and hyperthermal energies exhibited a number of characteristics that are unusual in comparison with other systems for which molecular beam experiments have been carried out under similar conditions: the measured energy losses are unusually small, some of the angular distributions exhibited an anomalous shoulder feature in addition to a broad peak near specular, and quantum mechanical diffraction was observed under conditions for which it was not expected. The data are analyzed and compared to calculations with classical scattering theory. Many of the unusual features observed in the measurements are explained but only upon using an effective surface mass of 2.3 Ru atomic masses, which implies collective effects in the Ru crystal. The bigger effective mass leads to substantially larger Debye-Waller factors and explains and confirms the observations of diffraction features. It also leads to the interesting conclusion that Ru is a metal for which molecular beam scattering measurements in the purely quantum mechanical regime can be performed with a range of atomic and molecular projectiles with masses substantially heavier than He.