  Experimental observations suggest that molecular adsorbates exhibit a larger
friction coefficient than atomic species of comparable mass, yet the origin of
this increased friction is not well understood. We present a study of the
microscopic origins of friction experienced by molecular adsorbates during
surface diffusion. Helium spin-echo measurements of a range of five-membered
aromatic molecules, cyclopentadienyl (Cp), pyrrole and thiophene, on a
copper(111) surface are compared with molecular dynamics simulations of the
respective systems. The adsorbates have different chemical interactions with
the surface and differ in bonding geometry, yet the measurements show that the
friction is greater than 2 ps$^{-1}$ for all these molecules. We demonstrate
that the internal and external degrees of freedom of these adsorbate species
are a key factor in the underlying microscopic processes and identify the
rotation modes as the ones contributing most to the total measured friction
coefficient.
