3
$\begingroup$

enter image description here

I hit the above plastic sliding toys with 532nm Laser in a Raman spectrometer and the following are the spectra of each. This is the blue sliding toy showing the right Raman for the plastic.

enter image description here

But when I hit the red sliding toy, there is no Raman signature detected, instead there is a broad Fluorescence.

enter image description here

What happen exactly at the molecular level when the red object get fluorescence from the 532nm laser but not the blue object?

And if I paint the red object with blue paint, would the fluorescence be suppressed producing the spectrum for the blue object?

$\endgroup$

1 Answer 1

7
$\begingroup$

The laser excites both a Raman signal and fluorescence in both materials. In the blue dyed material whatever dye is used has a low fluorescence yield so that the Raman signal is seen on top of the fluorescence signal. In the lower figure the signal is about ten times bigger, and the fluorescence swamps the Raman signal. In this case the fluorescence yield of the red dye must be large.

If you add paint and the laser passes through it to the material underneath then the sum of any Raman and sum of any fluorescence will be seen. Clearly, if the paint is so thick as to absorb all the laser only signal from this will be seen.

$\endgroup$
4
  • $\begingroup$ Can we say that in the blue dye, the gap between S0 and S1 levels where the electrons can get excited have more energy than the red dye that's why the green laser photons can't be excited from S0 to S1 electronic levels in the blue dye? $\endgroup$
    – Jtl
    Aug 27, 2022 at 9:00
  • 3
    $\begingroup$ Not exactly. The laser excites both the dye in the blue and in the red material. The blue emission seems to have a max at about $550$ nm (its hard to tell as there are no values on your plot) and the dye in the red material at about $590$ nm. Thus the blue dye has a larger S1-S0 energy gap than the red dye but both are less than the energy of the laser's photon. (i'm guessing that $532$ nm is off the left of either plot.) $\endgroup$
    – porphyrin
    Aug 27, 2022 at 10:05
  • $\begingroup$ If both the blue and red S1-S0 energy gap is less than the energy of the laser's 532nm photon. How come the green laser didn't cause muchg fluorescence in the blue dye when the S1 was reach and transitional down non-radiatively and then emitting fluorescence photon as it goes from S1 to S0 (like in the red dye)? $\endgroup$
    – Jtl
    Aug 27, 2022 at 10:26
  • 1
    $\begingroup$ Because different types of molecules have different fluorescence quantum yields. This can depend on several factors such as energy gap to triplet, and /or presence of heavy atoms. $\endgroup$
    – porphyrin
    Aug 27, 2022 at 16:12

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.