  We study diffusion of small light particles in a solvent which consists of
large heavy particles. The intermolecular interactions are chosen to
approximately mimic a water-sucrose (or water-polysaccharide) mixture. Both
computer simulation and mode coupling theoretical (MCT) calculations have been
performed for a solvent-to-solute size ratio five and for a large variation of
the mass ratio, keeping the mass of the solute fixed. Even in the limit of
large mass ratio the solute motion is found to remain surprisingly coupled to
the solvent dynamics. Interestingly, at intermediate values of the mass ratio,
the self-intermediate scattering function of the solute, F_{s}(k,t) (where k is
the wavenumber and t the time), develops a stretching at long time which could
be fitted to a stretched exponential function with a k-dependent exponent,
\beta. For very large mass ratio, we find the existence of two stretched
exponentials separated by a power law type plateau. The analysis of the
trajectory shows the coexistence of both hopping and continuous motions for
both the solute and the solvent particles. It is found that for mass ratio
five, the MCT calculations of the self-diffusion underestimates the simulated
value by about 20 %, which appears to be reasonable because the conventional
form of MCT does not include the hopping mode. However, for larger mass ratio,
MCT appears to breakdown more severely. The breakdown of the MCT for large mass
ratio can be connected to a similar breakdown near the glass transition.
