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, but 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.
A more prominent application for Raman spectroscopy is determining the chemical composition of unknown substances. The laser used in a Raman spectrometer causes specific parts of the target molecule to vibrate; thus, specific chemical bonds and structures display characteristic Raman peaks. Many factors determine the wavenumber shift and intensity at which each functional group’s peak or peaks will be found. Some functional groups are more Raman active than others and will produce more intense peaks.