Isotope effects on direct and indirect processes of hydrogen abstraction from Cu(111)

We investigate and discuss isotope effects on hydrogen abstraction from Cu(111) by performing quantum dynamics calculations that consider both direct (an impinging hydrogen atom directly abstracts an adsorbed hydrogen atom) and indirect (the impinging hydrogen atom diffuses across the surface before it abstracts an adsorbed hydrogen atom) processes quantum mechanically. Our calculation results show that the abstraction reaction probabilities Preac are 0.742(D-on-D)>0.640(H-on-D)>0.594(H-on-H)>0.533(D-on-H). (Note that the term 'X-on-Y' indicates abstraction of Y atoms adsorbed on the surface by gas-phase X atoms.) Furthermore, the mean normal translational and vibrational energies for H-on-D are larger than those for D-on-H, while the mean parallel translational energy for D-on-H is comparable with that for H-on-D. This implies that the decrease in the abstraction reaction probability for D-on-H, as compared with that for H-on-D, is mainly due to the decrease in the frequency of occurrence of the direct process in abstraction reactions. (author)