Raman or fluorescence spectral lines?
2nd May 2018 17:22 UTCAdolf Cortel
A laser of 520 nm can help to discriminate between both types of spectral lines. Some years ago these lasers appeared in the marked as relatively inexpensive modules, pointers or bare diodes. A green laser of 520 nm is not convenient for Raman spectrometers because of the big width of the emission “line” (about 2 nm at half height), which gives also broad Raman lines and undesirable laser light not blocked well by a longpass filter.
However, this laser of 520 nm coupled to a Raman spectrometer (at least to a homemade one fitted with a longpass filter), is useful because it allows exciting the fluorescence of a sample in a similar way than the conventional laser of 532 nm. The lines or bands due to fluorescence appear at the same position (pixel o wavelength) than the ones which can mask the Raman signals when a 532 nm laser is used, but the true Raman lines appear displaced (and broad). Instead of a green laser of 520 nm, a violet (405 nm) or blue lasers can be used, but in some cases the sample can not be excited and fluoresce in the same way.
Here come some preliminary trials. The first picture shows the setup: a 520 nm laser module has been coupled to a homemade Raman spectrometer. This spectrometer and some complementary information can be found at: https://youtu.be/C-C7DtaSPb0
In the next picture the Raman spectra of naphtalene and diamond (both are strong Raman scatterers) with 532 and 520 nm lasers are compared ( please, note that the X-axis have not been calibrated, the only purpose is the comparison of the spectra). With the 520 nm laser the Raman lines appear displaced and broad as corresponds to the width of the line of the exciting laser
The last picture shows the comparison of the fluorescence of an europium based phosphor in the coating of an energy saving bulb, excited by the lasers at 532 and 520 nm. The spectral lines appear at the same position and the width of the fluorescence lines is independent of the width of the exciting wavelength.
