Vibrational Excitation of H2 Scatteringfrom Cu(111): Effects of Surface Temperature and of Allowing EnergyExchange with the Surface

摘要

In scattering of H2 from Cu(111), vibrational excitation has so far defied an accurate theoretical description. To expose the causes of the large discrepancies with experiment, we investigate how the feature due to vibrational excitation (the “gain peak”) in the simulated time-of-flight spectrum of (v = 1, j = 3) H2 scattering from Cu(111) depends on the surface temperature (Ts) and the possibility of energy exchange with surface phonons and electron–hole pairs (ehp’s). Quasi-classical dynamics calculations are performed on the basis of accurate semiempirical density functionals for the interaction with H2 + Cu(111). The methods used include the quasi-classical trajectory method within the Born–Oppenheimer static surface model, the generalized Langevin oscillator (GLO) method incorporating energy transfer to surface phonons, the GLO + friction (GLO+F) method also incorporating energy exchange with ehp’s, and ab initio molecular dynamics with electronic friction (AIMDEF).Of the quasi-classical methods tested, comparison with AIMDEF suggeststhat the GLO+F method is accurate enough to describe vibrational excitationas measured in the experiments. The GLO+F calculations also suggestthat the promoting effect of raising Ts on the measured vibrational excitation is due to an electronicallynonadiabatic mechanism. However, by itself, enabling energy exchangewith the surface by modeling surface phonons and ehp’s leadsto reduced vibrational excitation, further decreasing the agreementwith experiment. The simulated gain peak is quite sensitive to energyshifts in calculated vibrational excitation probabilities and to shiftsin a specific experimental parameter (the chopper opening time). Whilethe GLO+F calculations allow important qualitative conclusions, comparisonto quantum dynamics results suggests that, with the quasi-classicalway of describing nuclear motion and the present box quantizationmethod for assigning the final vibrational state, the gain peak isnot yet described with quantitative accuracy. Ways in which this problemmight be resolved in the future are discussed.