Molecular bonds can be considered as stiff springs which can be bent or stretched. The vibration (bending and stretching) energy lies in the infrared (IR) range. Therefore, through the interaction between IR incident beam and molecules in samples to be tested, through transmission or reflection, molecular bonds information can be extracted from the absorption spectra to obtain their molecular structures.
An ATR sample is portioned next to a crystal, e.g. diamond, with a good contact; an IR beam is directed into the crystal with an incident angle larger than critical angle; it reflects back through attenuated total reflection (ATR), and creates an evanescent wave, since the refractive index of the crystal is larger than the sample. The evanescent wave extends beyond the surface of the crystal into the sample held in contact with the crystal. Typically, the penetration depth is about 0.5-5um deep into the sample.
The above picture schematically show the light path in the ATR setup. And the light path for a traditional FTIR setup is shown below for a comparison.
It is worthy to note that traditional FTIR reflects more the bulk information of a sample while ATR-FTIR is more effective to analyze sample surface composition.
More information about FTIR setup and principles can obtained from the following video.
Various kinds of vibrational bands in an infrared spectrum are observed are outlined in the following chart. The blue colored sections refer to stretching vibrations, and the green colored band encompasses bending vibrations. Infrared spectra in the 1450 to 600 cm-1 region is often called the fingerprint region. 4000 to 1450 cm-1 region is usually due to stretching vibrations of diatomic units, and it is called the group frequency region. The source of the information is from this link.
The following table is useful in obtaining molecular information from FTIR spectra.
And more bond information about other functional groups.