During IR spectroscopy/vibrational spectroscopy, compounds become analyzed by infrared radiation passing through the sample. This occurs over an array of different frequencies. The beam of infrared radiation is then reflected through a detector. The next step is measuring the absorptions created by each type of bond in the compound.   

There are three main ways a molecule can absorb radiation. Each of them involves an increase of energy that is proportional to the light absorbed. The first way occurs when absorption of radiation causes a higher rotational energy level in rotational transition. The second way is a vibrational transition that occurs on absorption of quantized energy (creates a higher vibrational energy level). The third way happens when electrons of molecules become raised to a high electron energy (known as electronic transition).