= Raman spectrum
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= Raman spectra
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{synonym}
A \b[Raman spectrum] records the strength of <Raman scattering> as a function of scattered optical frequency or, usually, <Raman shift> relative to the excitation. For vacuum wavelengths in nanometres, a common wavenumber convention is
$$
\Delta\widetilde\nu=10^7\left(\frac1{\lambda_0}-\frac1{\lambda_s}\right)\,\mathrm{cm^{-1}}.
$$
Here the numerical wavelengths are in nanometres. <Stokes Raman scattering> has positive shift under this convention; <Anti-Stokes Raman scattering> has negative shift. The strong unshifted <Rayleigh scattering> line is not a vibrational Raman band.
For an original chosen excitation $\lambda_0=500\,\mathrm{nm}$ and a $1500\,\mathrm{cm^{-1}}$ vibration, the Stokes wavenumber is $20000-1500=18500\,\mathrm{cm^{-1}}$, giving $\lambda_s\approx540.54\,\mathrm{nm}$. The corresponding anti-Stokes wavelength is $10^7/21500\approx465.12\,\mathrm{nm}$. Changing the excitation changes these optical wavelengths while the same assumed transition retains its Raman shift. The <One-mode Raman spectrum example> gives an existing level diagram and a separate numerical spectrum.
The vertical quantity must be stated too: detector counts per spectral bin, calibrated scattered power density and a stimulated gain ratio are different observables. <Femtosecond stimulated Raman spectroscopy> can plot the <Raman pump-on pump-off gain ratio> rather than spontaneous photon counts. Its primary apparatus disperses probe and reference beams onto an array and calibrates shifts with known solvent lines.<Femtosecond Broadband Stimulated Raman: A New Approach for High-Performance Vibrational Spectroscopy by David W. McCamant and others>[Materials and Methods, Instrumentation/Figure 2 and absolute-shift calibration; Data Collection, equation 4]{ref}
<Spectral-density conversion between wavelength and Raman shift> also changes the density's units: rebadging the horizontal axis alone need not preserve a spectral integral. Background, excitation rejection, instrument response and <spectral resolution> affect observed band heights and widths. An integrated band area is not determined by peak height alone. A chemical inference should compare calibrated positions and responses under specified conditions, rather than identify a material from one unqualified peak.
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