Single photon excitation - emits 1 photon at a time. Better to use Pulsed excitation, where we can clearly see the single photon purity.

In case of multi-photon excitation, it is like coherent behavior. For example, we can see the emission characteristics arising from multiple emitters.

The best way to understand is studying co-relation experiments which explains clearly about single or multi photon characteristics.

Thanks, Devaraj

So, in multi photon excitation several photons may interact with the molecules

simultaneously and make them reach the excited state. Then relax back with emitting photons. So, will those photons be with longer wavelength than the excited ones?.

Hi Dr. Azeez, I really appreciate it. It is really added a lot to my knowledge.

Thanks,

Mahmud,

In single photon excitation, the energy of the photon used for excitation matches the S0-S1 energy gap whereas in multi-photon the energy of the photon used for excitation is either half or one third of the S0-S1 energy gap in a molecule.

Since the wavelength of excitation is different, dye photobleaching rates are different and light penetration depths can also be different.

Hi, Kishore,

So, why the energy of the photon in multi-excitation is either one half or one third of S0-S1 gap?.

Multiple photons are used to cause excitation from S0 state to S1 state. For example, in two photon excitation two photons of half the energy would be needed to excite an electron from S0 to S1 state.whereas in three photon excitation three photons of one-time third energy would be needed to excite an electron from S0 to S1 state.

Since multiple photons need to be absorbed simultaneously by a molecule, higher intensity of light would be needed. It is a non-linear process.

In the case of multi photon excitation one need high intensity laser to provide more number of photons to be absorbed simultaneously to excite atoms or molecules. However if the laser intensity is even more than the required for excitation. Then it can ionize the atoms or molecules and produce plasma. This indicates that multi photon absorption is mainly dependent on laser intensity. That is on number of photons available for interaction.

Thanks, Rai.

So, how to know the exact intensity?.

Mahmud Sefannaser

" So, will those photons be with longer wavelength than the excited ones?."

This is bit complicated to explain. In one part, it is possible the emission can be at longer wavelength. But it is the combination of following factors:, the material properties, its dimension, shape, excitation wavelength and temperature.

In order or fs.