  We study the 2D motion of colloidal dimers by single-particle tracking and
compare the experimental observations obtained by bright-field microscopy to
theoretical predictions for anisotropic diffusion. The comparison is based on
the mean-square displacements in the laboratory and particle frame as well as
generalizations of the self-intermediate scattering functions, which provide
insights into the rotational dynamics of the dimer. The diffusional anisotropy
leads to a measurable translational-rotational coupling that becomes most
prominent by aligning the coordinate system with the initial orientation of the
particles. In particular, we find a splitting of the time-dependent diffusion
coefficients parallel and perpendicular to the long axis of the dimer which
decays over the orientational relaxation time. Deviations of the
self-intermediate scattering functions from pure exponential relaxation are
small but can be resolved experimentally. The theoretical predictions and
experimental results agree quantitatively.
