STICKING OF RARE-GAS ATOMS ON THE CLEAN RU(001) SURFACE

Absolute values of the initial sticking coefficients of rare gas atoms (Ne, Ar, Kr, Xe) on a flat, clean Ru(001) surface have been determined with thermal beams and a highly sensitive thermal desorption method. The sticking coefficients increase with increasing mass of the atoms. Their decrease with increasing gas temperature is stronger the lighter the atom; different surface temperatures within the accessible range do not measurably affect the sticking efficiency. At a gas temperature of 300 K and a surface temperature of 6.5 K the initial sticking coefficients are 0.004 for Ne, 0.13 for Ar, 0.25 for Kr, and 0.71 for Xe. Forced oscillator calculations treating the substrate phonons quantum mechanically have been performed. With the well depths derived from experiment, and other reasonable input parameters, absolute values and functional forms of the sticking coefficients can be reproduced. The low values are due to the high elastic reflection probability which is a consequence of the inefficient energy transfer and the phonon quantization. The calculated Debye-Waller factors at zero gas and surface temperature are 0.92 for Ne, 0.36 for Ar, 0.14 for Kr, and 0.01 for Xe. A classical interpretation of the sticking data is impossible at least for Ne and Ar.