Infrared spectroscopy or IR spectroscopy is essentially a way to see what is apparently invisible, which in this case refers to atoms and molecules. It is the study of matter in which how infrared light interacts with a molecule is observed. Infrared light refers to the type of light which has a frequency lower than visible light but has a longer wavelength. Absorption, reflection and emission are three ways in which the analysis is conducted.
With the IR spectroscopy method, the infrared light frequencies absorbed by a molecule can be accurately detected. Since these light frequencies match the vibration of bonds inside the molecule, the latter absorbs the frequencies. The energy needed for exciting this molecular bond and making them vibrate with greater amplitude is only found in the infrared region. However, it must be noted that only a polar molecular bond will interact with electromagnetic infrared radiation.
The infrared spectrum can be essentially divided into near, mid and far regions, based on how they are related to the visible spectrum. The high energy and near IR region have 0.8-2.5 μm wavelength or 14000-4000 cm-1 and can lead to harmonic or overtone vibration. The mid IR region on the other hand has 2.5-25 μm wavelength or 4000-400 cm-1 and can help in studying fundamental vibrations as well as associated rotational vibrational structure. The far IR region is right next to the microwave region and has 25-1000 μm wavelength or 400-10 cm-1. Being low energy, this is useful for rotational spectroscopy.
IR spectroscopy can make use of samples in different physical states, such as solid, liquid and gas. The methods of sample preparation are different for each and are elaborated below:
The IR spectroscopy principle is based on some fundamental concepts. It is common knowledge that atoms connected by chemical bonds make up a molecule. The motion of atoms and bonds can be compared to springs and balls, which involve vibration and this vibration is known as the natural frequency of vibration.