  Atomic Force Microscopy - Infrared (AFM-IR) has emerged as a useful technique
for measuring absorption spectra with spatial resolution better than the
optical diffraction limit. The technique relies on the movement of a probe for
atomic force microscopy for detecting the local expansion of a material caused
by the photothermal effect. While AFM-IR is seeing increased application to a
wider range of samples, reports have also appeared in the literature that are
inconsistent with an interpretation of the AFM-IR response simply in terms of
photothermal expansion. The present article addresses the issue by critically
evaluating existing experimental observations. It is concluded that observed
discrepancies arise from the contribution of non-photothermal effects to the
signal, which affect both intensity and spatial resolution of the measurement.
