In Raman spectroscopy one looks at scattering from the sample. Scattering can occur from a virtual state or from a resonant state. As already mentioned, It gives information about the vibrational modes of a sample. The frequency of scattered light differs from the frequency of incident light and the difference corresponds to the vibrational modes of the sample.

In fluorescence spectroscopy one looks at emission from the excited state of the sample. There is lifetime associated with fluorescence. Here the excitation wavelength must match the energy difference between the energy levels i.e., resonant condition needs to be satisfied.

There are some more differences depending on whether the sample is an atom or a molecule or a crystal.

I will give you half an answer.  Raman spectroscopy gives you the vibrational frequencies of a molecule (well, not all of them, because some vibrational modes may have no Raman cross section.) You send in photons (typically visible light).  The photons are absorbed and re-emitted, but in the process they absorb or release an amount of energy corresponding to one or more quanta of a vibrational mode, so the light is shifted in frequency on being re-emitted.  Considering large numbers of photons being scattered by a sample, some of them absorb or lose an amount of energy corresponding to the quantized energy of each mode. Said differently, you send in light at frequency w, and out comes like at frequencies w +/- w1 where w1 is the frequency of some vibrational mode, and light corresponding to every vibrational mode is emitted.  The frequency shifts are the molecular vibrational frequencies, and are in part as the frequencies at which infrared light is absorbed by the sample.  I say 'in part' because some vibrational modes contribute only to IR or only to Raman or to neither.

In Raman spectroscopy one looks at scattering from the sample. Scattering can occur from a virtual state or from a resonant state. As already mentioned, It gives information about the vibrational modes of a sample. The frequency of scattered light differs from the frequency of incident light and the difference corresponds to the vibrational modes of the sample.

In fluorescence spectroscopy one looks at emission from the excited state of the sample. There is lifetime associated with fluorescence. Here the excitation wavelength must match the energy difference between the energy levels i.e., resonant condition needs to be satisfied.

There are some more differences depending on whether the sample is an atom or a molecule or a crystal.

From the answers above, it is important to note that Raman is a scattering technique, which is inelastic because of interaction with the vibrations of the atoms in a molecule or solid (not a single atom). It  can occur nonresonantly, and therefore instantaneously, and the selection rules which govern it require a change in polarisability of the bond at the zero point of the vibration, as opposed to IR spectroscopy, which similarly probes vibrations, but as it is an electric dipole transition it requires a change in electric dipole. Raman therefore tends to be good for symmetric, polarisable moieties, whereas IR is better for asymmetric, polar bonds.

This does a comparison of Raman and IR in the context of clinical applications.

“Raman Microscopy: Complement or Competitor?”,

Hugh J. Byrne, Ganesh D. Sockalingum and Nick Stone,

in ”Biomedical Applications of Synchrotron Infrared Microspectroscopy: A Practical Approach”, David Moss, (Editor), RSC Analytical Spectroscopy Monographs No. 11 (2011) ISBN: 978-0-85404-154-1

Photoluminescence, is a radiative relaxation of an already electronically excited state. It is governed by the selection rules of an electronic dipole transition and has a characteristic lifetime (transition probability), although this can be very strongly affected by the competing nonradiative processes (quenching).

so when i want analyse spectrum of pl , is that accurs in broad scan area? 

It depends on the material (and temperature). In general Raman bands are very narrow, although the full spectrum, containing many bands, can span a range of a few hundred nanometers.

What is your material?

thanks for your answers. i using raman spectroscopy to gemmology. my material right now is ruby. i wanna compare natural and synthetic ruby by PL spectrum

thanks for your answers. i using raman spectroscopy to gemmology. my material right now is ruby. i wanna compare natural and synthetic ruby by PL spectrum

Then you should go into the absorption maximum to see the PL emission. It is sometimes possible to see raman bands superimposed on PL spectra, but if you change the excitation wavelength by a small amount, then the PL should stay the same, but ant Raman lines should follow the change in excitation wavelentght, as in Raman, it is the shift which is important.

Mr. Mahdavi,

The physical background of PL is absorption of electromagnetic light, while Raman is process based on scattering of light. Nevertheless that you can obtain PL and Raman within the frame of one and the same optical arrangement of the instrumental scheme. The difference is in the monochromator residue. However the Raman signal is weak comparing with PL-one.

I work with plasmonic metal nanoparticles and have also been confused about this distinction as it sometimes appears that the terms are used interchangeably. Raman scattering as I understand it is the result of excited electrons in the metal re-emitting light at a stokes shifted wavelength, and photoluminescence is also stokes shifted so I don't quite see what the difference is (since they both seem like the only other option besides rayleigh scattering). Is the only distinction that Raman scattering resolves the energy differences due to vibrational states? If so then are these just different methods of measuring the same phenomenon?

@ Zachary Hallenbeck - further to the previous responses, and in answer to you question, light scattering, and resonant absorption of a photon, followed by emission of a Stokes shifted photon, are completely different physical phenomenon, and are governed by different selection rules.

Light scattering is governed by the real component of the refractive index, or polarisability, which is enhanced in the region of a material absorption (described by the Kramer-Kronig relationship). This polarisability is modulated by the intrinsic vibrations of a material, such that the photon energy can couple to a vibrational state, either generating (Stokes) or annihilating (anti-Stokes) a vibrational quantum. The former results in a decrease in the photon energy, the latter an increase. This is the Raman shifting of light due to scattering.

Photoluminescence, is linked to the imaginary component of the refractive index, or the material absorption. The absorption is an electric dipole transition and follow the selection rules of this process (e.g. change in dipole moment). An electron is promoted to n excited state, in which it has a finite lifetime, before is relaxes to the ground state, either radiatively (PL) or non radiatively (internal conversion), generating heat. Because the change in electronic state results i a relaxation of the lattice to a lower energy configuration, the relaxation energy (or emitted photon) is of lower energy (also called after the Irish scientist George Stokes).

Differences between Raman and PL

Raman does not need a resonant excitation.

Raman can be performed at any wavelength

PL has a finite and characteristic lifetime...

Raman specifically provides a spectrum of (Raman active) vibrational modes.

Depending on the material, PL can be dominated by traps or defect states which act as combination site...

The temperature dependence of each is different

....and many more

PL spectroscopy is as a result of the excitation of electrons from valence band to conduction band and coming back emitting light. Raman is however extracted from emitting light with different frequency from vibrational modes of molecules.