Comparative dynamical study of the HD and H2 isotopomers interacting with Pd(111) and Cu(110) surfaces

We study the dynamics of HD and H-2 molecules interacting with Pd(1 1 1) and Cu(1 1 0) using the classical trajectory method based on potential energy surfaces obtained from Density Functional Theory calculations. Our results predict a negligible isotopic effect on the dissociative adsorption probability on Pd(1 1 1) whereas on Cu(1 1 0), the adsorption probability for HD(nu(i) = 0) is slightly lower than for H-2(nu(i) = 0), mainly due to its lower initial vibrational zero point energy. The final rotational energy distribution of scattered HD and H-2 molecules are very similar. This shows that the asymmetric mass distribution of HD, barely affects the fraction of initial translational energy transferred to rotation during the scattering process. Our calculations point to the larger number of open rotational excitation channels for HD, as the main cause of rotational excitation probabilities larger than for H-2. The theoretical apparent rotational temperature, T-rot, of HD molecules scattered from Pd(1 1 1) at impact energy E-i = 75 meV, is in good agreement with the experimental value. In contrast, for Cu(1 1 0) the theoretical T-rot is much lower than the value measured for Cu(1 0 0). Possible reasons for such a discrepancy between theory and experiments are discussed. (C) 2009 Elsevier B.V. All rights reserved.