STED microscopy principle and emission of photon.

[Rustem]

Good day, I'm student and i would like to ask a question about how molecule can go to the ground state from excited state, by absorbing the photons, in the end red shifted photons with lower energy are released. This is the principle of STED microscopy, that is used to limit amount of observed material. But me and teacher can not explain this situation. We were tough that if molecule absorbs photons/energy then it gets excited, but this situation is vice versa. I would be glad for any responses and answers, thanks! 

Edited September 26, 2019 by Rustem

[swansont]

You are correct, absorbing a photon does not get you from an excited state to the ground state. In stimulated emission, the incident photon induces an excited system to de-excite and emit a photon. i.e. one photon in, two photons out. 

In this technique, you aren't going to the lowest energy state — you drive the molecule into a higher-energy vibrational state (so these photons have less energy that spontaneously-emitted photons), meaning they are red-shifted, as you note, and can thus be filtered out of the detection (by wavelength or possibly, I suppose, by polarization)

The energy-level diagram from wikipedia shows what's going on

https://en.wikipedia.org/wiki/File:STED_Jablonski.jpg

 

Stimulated emission is the concept behind lasers. You can excite an atom via some method and then induce it to give up its photon by hitting it with photons of the proper wavelength. One photon in, two photons out. Repeat many, many times.

 

 

 

 

[Rustem]

37 minutes ago, swansont said:

You are correct, absorbing a photon does not get you from an excited state to the ground state. In stimulated emission, the incident photon induces an excited system to de-excite and emit a photon. i.e. one photon in, two photons out. 

In this technique, you aren't going to the lowest energy state — you drive the molecule into a higher-energy vibrational state (so these photons have less energy that spontaneously-emitted photons), meaning they are red-shifted, as you note, and can thus be filtered out of the detection (by wavelength or possibly, I suppose, by polarization)

The energy-level diagram from wikipedia shows what's going on

https://en.wikipedia.org/wiki/File:STED_Jablonski.jpg

 

Stimulated emission is the concept behind lasers. You can excite an atom via some method and then induce it to give up its photon by hitting it with photons of the proper wavelength. One photon in, two photons out. Repeat many, many times.

 

Thank you for the answer!