At room temperature the thermal population of vibrational excited states is low

At room temperature the thermal population of vibrational excited states is low, although not zero. the initial state is the ground state, and the photons that were scattered will have a lower energy resulting in a longer wavelength than the exciting photon. This shifted scatter is what is what’s observed in Raman spectroscopy.
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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