a method of obtaining submicron resolution ir absorption data from a sample surface . a probe microscope probe interacts with the sample surface while a tunable source of ir radiation illuminates the sample - tip interaction region . the source is modulated at a frequency substantially overlapping the resonant frequency of the probe and may be modulated at the contact resonance frequency of the probe when the probe is in contact with the sample surface . the modulation frequency is continually adjusted to account for shifts in the probe resonant frequency due to sample or other variations . a variety of techniques are used to observe such shifts and accomplish the adjustments in a rapid manner .