The overall sample deformation in AFM-IR is on the order of picometers 7 and experimental parameters are chosen to not change sample properties (e.g. not to cause phase transitions or damage to the sample) during the experiment. Hence, we can split the time dependent, volumetric heat source in to a spatial and a temporal component g(x, t) = g V (x)g t (t). The time domain variation in the illumination intensity is described by g t (t), while g V (x) describes the location and intensity of heating, i.e., location and magnitude of light absorption inside the sample (in the following discussion, we neglect optical effects such as interference upon the signal. These effects can be added by multiplying the absorption coefficient with the local light intensity to arrive at a modified g V 32 ). Splitting g(x, t) allows us to also split the convolution in equation ( ) into two parts, one that describes the mode amplitude and shape in r and z, and one that describes the time domain behaviour of each mode: