A small fraction of the molecules is in vibrationally excited states

A small fraction of the molecules is in vibrationally excited states. The Raman scattering from a vibrationally excited molecules will leave the molecule in the ground state. The photon that was scattered will appear with a higher energy. This anti-Stokes-shifted Raman spectrum is always weaker than the Stokes-shifted spectrum, at room temperature it has enough for vibrational frequencies less than about 1500 cm-1. The Stokes and anti-Stokes on a spectrum contain the same frequency information. The anti-Stokes spectrum can be used when the Stokes spectrum is not directly observable.
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