## PRINCIPLES OF MID-INFRARED SPECTROSCOPY

Mid-infrared (mid-IR) spectroscopy involves measurement of the absorption of
electromagnetic radiation over the wavenumber range of 4000–400 cm^{−1}
(which corresponds to the wavelength range of 2.5–25 µm) caused by the promotion of
molecules from the ground state of their vibrational modes to an excited vibrational
state.
The most commonly used parameter to denote the energy of the transitions is the wavenumber,
i.e., the number of waves per centimeter. The wavelength, λ (µm), and wavenumber,
*ṽ* (cm^{−1}), of radiation are related by the
expression:
v
∼
=
10
4
/
λ
(1)