Spectroscopy in general means the detection and analysis of an absorbed, emitted or scattered radiation by a substance. In case of infrared (IR) spectroscopy the absorption of IR radiation causes the excitation of molecular vibrations and rotations. Put more simply, an infrared beam is directed towards a sample and vibrations in the molecules are excited. The beam is then absorbed at the specific energy of the vibration and loses in intensity at the corresponding wavenumber. To get information about the chemical composition of a compound or the structure of an adsorbed species on a sample, a spectrum is recorded from about 4000 to 400 cm-1 and the intensity of the absorbed or transmitted light is plotted against the wavenumber. The absorption can be measured from the transmitted (e.g. nanoparticles, liquids, gases) or the reflected IR beam (e.g. single crystals). However, not all molecules and functional groups can be detected in an infrared spectrum, since the excitation requires a change in the molecules dynamic dipole moment. In general, polar molecules like CO, CO2 and N2O are IR-active, while non-polar molecules like N2 are IR-inactive. 


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