Atomic and molecular collisions with surfaces: Comparisons of Ar and N ₂ scattering from Ru(0001)

Recently reported molecular beam studies of Ar and N₂ scattering from Ru(0001) at thermal and hyperthermal energies exhibited a number of characteristics that are unusual in comparison to other systems for which molecular beam experiments have been carried out under similar conditions. For both systems the measured energy losses were unusually small. In the case of the Ar measurements some of the angular distributions exhibited an anomalous shoulder feature in addition to a broad peak near the specular direction, and quantum mechanical diffraction was observed under conditions for which it was not expected. These measurements are analysed and compared to calculations with a mixed quantum-classical scattering theory. This theory uses classical mechanics to describe the translational and rotational degrees of freedom while internal molecular vibrational modes are treated quantum mechanically. 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 large effective mass, because it leads to substantially larger Debye-Waller factors, 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, where diffraction and single-phonon creation are dominant, should be possible not only with He atoms, but with many other atomic and molecular species with larger masses.