per centimeter (kayser)
Wavenumberexact by definition
The reciprocal centimetre equals exactly 100 reciprocal metres, since a centimetre is exactly 0.01 metres. It is the standard x-axis of infrared and Raman spectroscopy, where the mid-infrared region runs from about 400 to 4000 cm⁻¹ and a carbonyl stretch sits near 1700. The conversion is exact, following from the exact definition of the centimetre.
Watch out: Spectroscopic wavenumber is proportional to photon energy, so spectra plotted in reciprocal centimetres are linear in energy while spectra plotted in wavelength are not. The two views compress and stretch different parts of the same spectrum, which is why band shapes look different between them.
| 1 cm⁻¹ | 100 m⁻¹ |
Sometimes called the kayser, after the spectroscopist Heinrich Kayser, though the name never displaced the plain reciprocal centimetre in practice.