A Proposal for the Origin of the Near-Ubiquitous Fluorescence in Raman Spectra

Every analyst who has recorded Raman spectra is aware of the difficulties in accessing the information from the spectra when samples fluoresce. From the instrument’s point of view, the fluorescent emission is the same as the Raman emission; that is, there is an excitation laser wavelength and there is emission to the long wavelength side of the laser. If the spectrum exhibits sharp lines, and the lines follow the excitation wavelength, the emission is Raman. If the emission is intense and sharp lines are hidden by the noise from a high background, the emission is fluorescence. And finally, if the laser wavelength is changed, the broad intense emission will remain at the same position in nanometers. In this column, I describe what I believe may be the origin of this fluorescence emission and support my conjecture with some measurements of polycyclic aromatic hydrocarbons (PAHs). Understanding the origin of these interfering backgrounds may enable you to design experiments with less interference, avoid the laser illuminations that make things worse, or both.