Isotope effects in kinetics of low-temperature adsorption of hydrogen on the Mo(110)surface

The kinetics of H-2 and D-2 low-temperature (down to 5 K) adsorption on the Mo(110) surface have been studied by a molecular beam method. An effusion molecular beam is directed onto the surface of the cooled sample and the intensity of the flux of scattered (or desorbed) molecules is measured with a QMS detector. This intensity has been used to determine the sticking probability. Two main isotope effects have been revealed in adsorption kinetics, as follows. (1) The sticking probability S of D-2 on the Mo(110) surface is higher than that of H-2, which may be a sign of the phonon mechanism of energy loss of impinging molecules. (2) The coverage dependence of the sticking probability S(theta) for the H-2/Mo(110) system displays a sharp and deep minimum near theta=0.5, whereas no such a dramatic feature is found in the case of the D-2/Mo(110) system. The differences between the quantum properties of H-2 and D-2 molecules seem to play a significant role in these effects. Essential dissimilarities are found between hydrogen adsorption kinetics on the Mo(110) and W(110) surfaces. (C) 2000 Published by Elsevier Science B.V. All rights reserved.