Question

If you excite a fluorescent sample with light of 310 nm, in addition to the peaks from the light emitted by the molecule of interest, at what other wavelengths (in nm) would you expect to see peaks in the spectrum? Why?

Answer 1

If a sample is excited at 310 nm, the one peak expected to be obtained around 349 nm which is called Raman peak. Raman scattering occurs from all solvents and the shift in energy between the excitation wavelength and the Raman peak depends on solvent. The Raman peak of water is always located approx. 3400-3600 cm^-1 lower in energy than the excitation wavelength which can be calculated of the excitation wavelength is known. Raman peak is obtained due to inelastic scattering of the incoming light from the solvent, also known as Raman scattering (or Stokes scattering if there is a loss of energy).

Raman scattering occurs from all solvents and the shift between the excitation wavelength and the Raman peak is always a constant amount of energy. As a result, peaks resulting from Raman scattering shift spectrally when changing the excitation wavelength, which can be used to identify the Raman peak. There fore if we excite a sample at 310 nm, we obtain Raman peak at 349 nm and when exciting at 350 nm the Raman peak is located around 398 nm. While exciting at 280 nm generates a Raman peak at 311 nm. Raman scattering has a rather low yield. Raman peaks are therefore only seen when the gain or slit bandwidth of the instrument is increased to compensate for low fluorescence signals.