Can we draw jablonski diagram for different laser excitations for the same molecule? Is there some proper analytical and qualitative wxplanation for using different excitation sources like 325nm, 514nm, 785nm, 1066nm etc.for Raman Spectroscopy ?
A dispersive Raman spectrometer was used with three different excitation sources (Argon-ion, He-Ne, and Diode lasers operating at 514.5 nm, 633 nm, and 782 nm, resp.). The system was employed to a variety of Raman active compounds. Many of the compounds exhibit very strong fluorescence while being excited with a laser emitting at the UV-VIS region, hereby imposing severe limitations on the detection efficiency of the particular Raman system. The Raman system with variable excitation laser sources provided us with the desired flexibility toward the suppression of unwanted fluorescence signals. With this Raman system, we could detect and specify the different vibrational modes of various hazardous organic compounds and some typical dyes (both fluorescent and nonfluorescent). We then compared those results with the ones reported in the literature and found the deviation within the range of ±2 cm−1, which indicates reasonable accuracy and usability of the Raman system. Then, the surface enhancement technique of the Raman spectrum was employed in the present system. To this end, we used chemically prepared colloidal suspension of silver nanoparticles as substrate and Rhodamine 6G as the probe. We could observe a significant enhancement of Raman signal from Rhodamine 6G using the colloidal solution of silver nanoparticles the average magnitude of which is estimated to be 103.
Journal of Spectroscopy Volume 2015, Article ID 895317, 8 pages http://dx.doi.org/10.1155/2015/895317
Dear Sarjana, as Ladan Ajdanian indicates, the reason of using different laser wavelegths as excitation sources for Raman is a matter of flexibility to cope with fluorescence backgraound problems and also to profit by the resonance conditions (Resonant Raman) when the excitation laser is close to some absorbtion wavelength of the molecule of interest.
Does raman shift depend on the excitation wavelength?
No, it doesn´t, Raman spectra shows Raman shifts, respect to the Rayleigh radiation, which is the same as the laser you choose. Those raman shifts are directly related to the vibrational energy modes of your anlyte molecules.
Can we draw jablonski diagram for different laser excitations for the same molecule?
Yes, of course, the laser radiation would excite the molecules to different virtual excited states. Say that a 325nm photon would excite your molecule at a higher virtual state than a 514nm photon (and so on for 632nm or 1064nm), but these virtual states are not real ones, and the molecule will lose energy (as heat) till it reaches a real vibrational excited state, from there it will relax by emiting a new photon, with a longer wavelength that the original one (Raman Stokes). The emitted photon can also be of smaller wavelength than the excitation one, but in this case the molecule should be originally in a excited state, so that the exciting photon bring it to a virtual state, it loses some energy to reach a stable vibrational excited state and than it relaxes directly to the steady state, now the photon emitted have a energy shift bigger than the original one, because it relaxed not to the original excited state but to the lower energy state, this is the Raman anti-Stokes. Note that you normally measures the Raman Stokes spectra.
Is there some proper analytical and qualitative explanation for using different excitation sources like 325nm, 514nm, 785nm, 1066nm etc.for Raman Spectroscopy ?
Not really, but as Ladan Ajdanian said, working with IR lasers helps to avoid fluorescence, which is an issue due to it hides the Raman peaks.
Remember, the Raman spectra measure shifts, respect to the excitation wavelength, so you have to deal with radiation close to your excitation light and therefore you must be aware that you must choose the right optics (lenses, gratings). For exemple you have to use UV optics if you are working with 325nm lasers, and also the detectors are different. For example the 1064nm laser requires a dedicated ccd detector. It have a huge impact on spectrometer price, and also in its spectral resolution (often the pixels from an IR ccd detector are bigger than those of a visible one).
If you use nanoparticulate or nanostructured metals (mainly Ag or Au) you can improve the Raman signal due to the SERS effect, and there the laser wavelegth election can also have an effect on the final spectra.
Hope it helps.
Best
Dear yes its depend on Raman shift. Usually excitation wavelengths depends upon perdiocity of material to fulfill resonance condition. The interaction of light must obey brags condition so that light deflect and its wavelength match with perdiocity . That's why resonance phenomenon occur.
Рамановский сдвиг не зависит от длины волны используемого источника возбуждения. Сдвиг максимума пика фотолюминесценции зависит от длины волны используемого источника . Это обстоятельство позволяет разделить рамановское излучение и фотолюминесценное излучение
( флюоресценцию).
Fluorescence is dependent on the excitation wavelength. In general, the longer wavelength of a laser the weaker fluorescence; additionally, the detector is also dependent of the light wavelength. For instance, the nice peaks of O-H stretching mode can be detected by a He-Ne laser, but not infrared LD, although the latter generally produce less fluorescence. The longer wavelength laser is not always the best choice. Personally, I always use He-Ne due to its best beam profiles.