The difference in energy between the absorbed and re-emitted photons corresponds to the energy required to excite a molecule to a higher vibrational mode. Anti-Stokes Raman scattering is another inelastic scattering process and only occurs from molecules starting in excited vibrational states; it results in light scattered with higher energy. Light scattered elastically (no change in energy between the incoming photons and the re-emitted/scattered photons) is known as Rayleigh scattering. Generally, the difference in energy is recorded as the difference in wavenumber () between the laser light and the scattered light which is known as the Raman shift. A Raman spectrum is generated by plotting the intensity of the scattered light versus . Like infrared spectroscopy, Raman spectroscopy can be used to identify chemical compounds because the values of are indicative of different chemical species (their so-called chemical fingerprint). This is because the frequencies of vibrational transitions depend on the atomic masses and the bond strengths. Thus, armed with a database of spectra from known compounds, one can unambiguously identify many different known chemical compounds based on a Raman spectrum. The number of vibrational modes scales with the number of atoms in a molecule, which means that the Raman spectra from large molecules is complicated. For example, proteins typically contain thousands of atoms and therefore have thousands of vibrational modes. If these modes have similar energies (), then the spectrum may be incredibly cluttered and complicated. Not all vibrational transitions are Raman active, meaning that some vibrational transitions do not appear in the Raman spectrum. This is because of the spectroscopic selection rules for Raman spectra. In contrast to IR spectroscopy, where a transition can only be seen when that particular vibration causes a net change in dipole moment of the molecule, in Raman spectroscopy only transitions where the polarizability of the molecule changes along the vibrational coordinate can be observed. This is the fundamental difference in how IR and Raman spectroscopy access the vibrational transitions. In Raman spectroscopy, the incoming photon causes a momentary distortion of the electron distribution around a bond in a molecule, followed by re-emission of the radiation as the bond returns to its normal state. This causes temporary polarization of the bond and an induced dipole that disappears upon relaxation. In a molecule with a center of symmetry, a change in dipole is accomplished by loss of the center of symmetry, while a change in