RESUMEN
We perform bimodal atomic force microscopy measurements on a Br-doped NaCl (001) surface to investigate the mechanisms behind frequency shift and energy dissipation contrasts. The peculiar pattern of the dissipated energy in the torsional channel, related to frictional processes, is increased at the positions of Br impurities, otherwise indistinguishable from Cl ions in the other measured channels. Our simulations reveal how the energy dissipates by the rearrangement of the tip apex and how the process is ultimately governed by lateral forces. Even the slightest change in lateral forces, induced by the presence of a Br impurity, is enough to trigger the apex reconstruction more often, thus increasing the dissipation contrast; the predicted dissipation pattern and magnitude are in good quantitative agreement with the measurements.