| We study the coupling of torsional and normal cantilever oscillations and their effect on the imaging process of a frequency-modulated atomic force microscope by means of molecular dynamics simulations. We show that the bending and torsional modes of the cantilever are coupled if the tip is near the surface and connect this coupling to the damping of the cantilever oscillation. The strength of the coupling is determined roughly by the strength of the lateral forces on the closest approach of the tip. Energy is transferred from the normal to the torsional excitation which can be detected as damping of the cantilever oscillation. Energy is actually dissipated by the usually uncontrolled mechanical damping of the torsional excitation. For high Q factors, the transferred energy is not completely dissipated during one cycle. The question, what happens to the remaining energy of the lateral degree of freedom in the long run, is addressed by studying a simplified two-dimensional point-mass model. We show that in succeeding cycles, energy is transferred back into the normal degree of freedom. The observation of the energy swapping process (amplitude and frequency) can therefore give additional information of the surface structure, especially on lateral forces. |
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