wavenumber \(\sigma\), \(\tilde{\nu }\)

https://doi.org/10.1351/goldbook.W06664
The reciprocal of the @W06659@, \(\lambda \), or the number of waves per unit length along the direction of @P04881@. The SI unit is \(\text{m}^{-1}\), but a commonly used unit is \(\text{cm}^{-1}\). Symbols \(\overset{˜}{\nu }\) in a vacuum, \(\sigma \) in a medium.
Sources:
Green Book, 2nd ed., p. 30 [Terms] [Book]
PAC, 1985, 57, 105. (Names, symbols, definitions and units of quantities in optical spectroscopy (Recommendations 1984)) on page 109 [Terms] [Paper]
PAC, 1996, 68, 2223. (Glossary of terms used in photochemistry (IUPAC Recommendations 1996)) on page 2283 [Terms] [Paper]