Most Raman spectrometer has the laser converging into focus with the sample at center like this.

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Usually how many millions of H2O atoms will be covered by one spot size? What will happen if the laser beam would be made parallel instead of converging. Will it increase the counts of the spectrum? Will it improve the spectral resolution? Or will both become worse?

Water Raman:

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  • $\begingroup$ Do you have any own thoughts about this? Please elaborate! $\endgroup$
    – Karl
    Nov 24, 2020 at 8:45
  • 3
    $\begingroup$ The collection optics image a small volume in the sample, you need to get as many photons into this region as possible to get strong signals. This normally means focussing the laser, but parallel beam would work if intensity is sufficient. The resolution is limited by slits and grating or CCD pixels per mm and grating, as appropriate , assuming that you laser bandwidth is narrow, as it usually is. You could try going to 2nd order to increase resolution but intensity will be lower. (You should look up Confocal parameter and Gaussian beams to understand laser focusing ) $\endgroup$
    – porphyrin
    Nov 24, 2020 at 9:13

1 Answer 1


Spectral resolution is not dependent on the laser intensity. It is determined by the state of the sample ( gas, liquid or solid) and more importantly by the components of the instrument itself. For example, what is the slit width anf the number of lines on the grating? What type of detector is there? If it is charged coupled device (CCD) then the spectral resolution will also be dependent on it.

  • $\begingroup$ So what happens if the laser is made parallel covering more sample than just spot size? What will happen to the spectrum? $\endgroup$
    – Jtl
    Nov 24, 2020 at 8:32
  • $\begingroup$ It might affect the intensity only. Some surface Raman experiments which use polarized light are angle sensitive. $\endgroup$
    – AChem
    Nov 24, 2020 at 16:36
  • $\begingroup$ I disagree with the statement that spectral resolution is independent of laser intensity. You can have power broadening (saturation effect) and Doppler broadening due to the wave front curvature for example. It is of course true that these effects are probably much smaller than the instrument effects you mention. $\endgroup$
    – Paul
    Nov 24, 2020 at 18:37
  • $\begingroup$ Right, those effects must be quite small for the Raman spectrometer this OP is using (as seen in his other posts). $\endgroup$
    – AChem
    Nov 24, 2020 at 19:01

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