Phase model and data. Lower plot: Phase data recorded at three different cantilever-surface gap heights (only every twentieth marker is shown), the model ϕ (solid black line) and the baseline ϕ
(dashed black line) included in the model to account for the phase response of the photothermal excitation ϕ
and the measurement electronics ϕ
. Upper plot: The difference between the model and the experimental data, Δ; to extract the eigenfrequencies and quality factors from the data, Δ was minimized using a Levenberg-Marquardt algorithm.