The study of excitation of molecules from the ground state to a higher vibrational energy state is called infrared or vibrational spectroscopy. The infrared region lies between the visible and microwave regions extending from 0.75 micro meter (750 nm) to 1000 micro meter . The infrared radiations are characterized both by wavelength (micro meter) and wave number (c/m).
The infrared region is divided into three parts.
(1) NEAR INFRARED
This region extends from 0.75 to 2.5 micro meter, with wave- numbers 12,500- 4000 c/m. In this region majority of the absorption bands are due to overtones of hydrogen stretching vibrations.
(2) MIDDLE OR FUNDAMENTAL INFRARED
This region extends from 2.5 to 16 micro meter with wave numbers 4000- 625 c/m. Molecular vibrations and rotations take place in this region.
(3) FAR INFRARED
This region lies in a range from 16 to 1000 micro meter with wave numbers 625 — 10 c/m. The skeletal vibrations and rotations take place in this region.