  In the scanning tunnelling microscope, the many-body Kondo effect leads to a
zero-bias feature of the differential conductance spectra of magnetic
adsorbates on surfaces. The intrinsic line shape of this Kondo resonance and
its temperature dependence in principle contain valuable information. We use
measurements on a molecular Kondo system, all-trans retinoic acid on Au(111),
and model calculations to discuss the role of instrumental broadening. The
modulation voltage used for the lock-in detection, noise on the sample voltage,
and the temperature of the microscope tip are considered. These sources of
broadening affect the apparent line shapes and render difficult a determination
of the intrinsic line width, in particular when variable temperatures are
involved.
