  The dynamic characteristics of a tip oscillating in the nc-AFM mode in close
vicinity to a Cu(100)-surface are investigated by means of phase variation
experiments in the constant amplitude mode. The change of the quality factor
upon approaching the surface deduced from both frequency shift and excitation
versus phase curves yield to consistent values. The optimum phase is found to
be independent of distance. The dependence of the quality factor on distance is
related to 'true' damping, because artefacts related to phase misadjustment can
be excluded. The experimental results, as well as on-resonance measurements at
different bias voltages on an Al(111) surface, are compared to Joule
dissipation and to a model of dissipation in which long-range forces lead to
viscoelastic deformations.
