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 suggests
that the GLO+F method is accurate enough to describe vibrational excitation
as measured in the experiments. The GLO+F calculations also suggest
that the promoting effect of raising Ts on the measured vibrational excitation is due to an electronically
nonadiabatic mechanism. However, by itself, enabling energy exchange
with the surface by modeling surface phonons and ehp’s leads
to reduced vibrational excitation, further decreasing the agreement
with experiment. The simulated gain peak is quite sensitive to energy
shifts in calculated vibrational excitation probabilities and to shifts
in a specific experimental parameter (the chopper opening time). While
the GLO+F calculations allow important qualitative conclusions, comparison
to quantum dynamics results suggests that, with the quasi-classical
way of describing nuclear motion and the present box quantization
method for assigning the final vibrational state, the gain peak is
not yet described with quantitative accuracy. Ways in which this problem
might be resolved in the future are discussed.