Dynamic force spectroscopy using the frequency modulation technique with constant excitation

We investigate the measurement principle of a dynamic force microscope utilizing the frequency modulation technique with a constant excitation (CE) mode which has been successfully applied in vacuum, air, and liquids. The basic principles of this mode are comparable with the constant amplitude (CA) mode, but in difference to the CA mode the excitation amplitude is kept constant in the CE mode. Using an theoretical approach we show how the measured quantities of the CE mode - the frequency shift and the oscillation amplitude - are related to the tip-sample interaction force. Based on this analysis we demonstrate how dynamic force spectroscopy experiments can be done also in the CE mode.