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To look further at the interpretation of the
2 bands at 1105 and 1175 cm-1 , the raw spectra of the high
fluorescence Beryl and an Emerald crystal have been superimposed
on the background corrected spectra shown above. Note the great
difference of the exposure times to get a signal in the range of the CCD.
The blue curve below is the raw signal of the blue spectrum reported
above. Note that the CCD is completely saturated above 1000 cm-1
. If the integration time for this Beryl is decreased (red curve), two
high intensity peaks appear around 1105 and 1175 cm-1 . The
Raman spectrum below 1000 cm-1 is barely visible for this
integration time. In order to get the signal for an Emerald
crystal in the range of the CCD, a very short integration time (1 ms)
must be used. The same two peaks are already present in the spectrum for
integration time with very high intensities. The color of Emerald
is due to impurities of chromium 3+ which give here a high
fluorescence. If a green laser is used for Raman excitation, the
position of the chromium peak do not move (in nanometers) and thus this
fluorescence should appear well above the Raman spectrum. Chromium
fluorescence is a disadvantage of He-Ne laser for Raman spectroscopy.
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