In the Non-fluorescent case the excitation photon’s energy is not sufficient to raise the valence electron to even the first excited state. In other words, the molecule cannot absorb the photon into its electronic shell structure. However, Raman scattering and Rayleigh scattering can still occur, since these phenomena do not require absorption as a precursor.
In the Fluorescent case we still have the same excitation wavelength, but a different molecule with a different set of electronic and vibrational energy levels. Now the photon may be absorbed by the molecule, raising it to an energy level defined by: 1) the electron resides in the S1 state and 2) the molecule as a whole is in some transient vibrational state. The electronic absorption phenomenon is fast (femtoseconds) due to low inertia, so to speak, of electrons. The subsequent relaxation dynamics that the molecule and its surroundings go through in order to “relax” the molecule to the lowest vibrational level while staying electronically in the S1 level take typically less than 100 ps. After this relaxation, the molecule’s valence electron is just waiting to return to the S0 state. This return, which entails the molecule losing internal energy, can occur either by emitting a new photon (Fluorescence) or non-radiatively by giving up the energy to the surrounding molecules (heating). As depicted, the wavelength of the fluorescence photon varies according to the vibrational energy level that the molecule ends up upon emission. This, combined with various broadening mechanisms, produces the typically quite featureless spectral shape of fluorescence. The mean lifetime of the S1 state determines the decay time of the fluorescence “glow” produced by an ensemble of molecules after a short excitation pulse. For more comprehensive and detailed physical picture of photoluminescence, see (Valeur & Berberan-Santos, 2013).
The mechanism of Raman scattering (Long, 2002) is different from the photoluminescence mechanism described above. There are a few different models, both classical and quantum-mechanical, that help to picture the essential properties of the phenomenon. In the most rudimentary quantum picture, only the initial and final states of the system are considered. We now consider the Stokes type of Raman scattering where the scattered photon’s energy is less than that of the excitation photon. In the initial state, our system is composed of the excitation photon and the molecule in ground state. In the final state, the system is composed of the Raman photon and the molecule in some nonzero vibrational state. What happens between these states is dismissed, but it is assumed that from start to finish, the process takes only on the order of picoseconds (this assumption is phenomenologically legitimate based on a huge number of measurement results). The emitted photon’s energy is the excitation photon’s energy minus the final vibrational energy of the molecule. In other words, the molecule has transitioned from the lowest vibrational state to some upper vibrational state, having been induced to do so by the electromagnetic perturbation provided by the excitation photon. Not all such transitions are “allowed” but it is required that the vibrational states and the polarization of the photon fulfil certain symmetry. It is these restrictions, in addition to interatomic bond lengths and strengths, that define the Raman spectrum of a molecule and also enable polarization-sensitive measurements.
The anti-Stokes Raman scattering is otherwise analogous with the Stokes type, but the molecule starts from a nonzero vibrational state and ends up in the ground state. This time the scattered photon’s energy is the sum of the excitation photon’s energy and the vibrational energy given up by the molecule. The positions of the anti-Stokes peaks in the emission spectrum are a mirror image of the Stokes peaks with respect to the laser line. The peak intensities depend on the populations of the upper vibrational states, i.e. temperature of the molecule ensemble.
A given molecule’s Raman scattering and photoluminescence mechanisms differ drastically in their response to excitation laser’s central wavelength and spectral linewidth. The spectral shape of fluorescence is unaffected by these parameters (as long as the photon energies are sufficient for absorption, but also not too high). On the other hand, the Raman spectrum as a whole follows any changes in the laser’s center wavelength, and the emission lines directly reflect the line shape of the laser. Raman bands are in effect broadened and shifted versions of the laser line.
Long, D. A. (2002). The Raman Effect: A Unified Treatment of the Theory of Raman Scattering by Molecules. Chichester: Wiley-Blackwell.
Valeur, B.;& Berberan-Santos, M. N. (2013). Molecular Fluorescence – Principles and Applications. Weinheim: Wiley-VCH.