Photoluminescence Excitation (PLE) vs PL spectroscopy

[Jalo]

I'm not sure I understand what information we can get from Photoluminescence Excitation (PLE) that we don't get from regular PL.

Let's imagine we have a sample with emissions between 700nm and 850nm. If I'm not mistaken in a typical PL experiment you'd just use a laser source with a wavelength lower than 700nm in order to observe all the transitions from the material. Would there be a difference between the obtained spectra if I used a 300nm and 600nm laser source wavelength?

Also, on a typical PLE for this same material which wavelengths would be of interest? Would I want to vary the laser source's wavelength between 850nm and 700nm? What information would I be able to extract from the obtained spectra that I wouldn't in typical PL?

Cheers

 

[Henryk]

In many cases, the photoluminescence involves an internal energy transition between light absorption and emission events. The emitted photon has then lower energy than the absorbed one. The photoluminescence emission let's you investigate the higher energy state of this process.

The excitation energy has to match an absorption band to a state that internally decays to the luminescence emission band. If your emission is in 700 to 850 nm, and you use 300 or 600 nm, you may or may not observe luminescence.

 

[Jalo]

The excitation energy has to match an absorption band to a state that internally decays to the luminescence emission band. If your emission is in 700 to 850 nm, and you use 300 or 600 nm, you may or may not observe luminescence.

If I'm exciting on a wavelength lower than where my emission is shouldn't I always obtain luminescence, with the extra energy from the inciding photons leading to non radiative relaxation of the electrons until they match an energy equal to radiative transitions of the material?

 

[Henryk]

If I'm exciting on a wavelength lower than where my emission is shouldn't I always obtain luminescence

Not always. You need to absorb light first and that requires that the photon energy matches energy from the ground state (plus possible vibrational/rotational) energy to a higher energy first. If the incident photon energy does not match an absorption band of the substance, it won't get absorbed. The PLE is the technique that let's you scan the excitation energy (wavelength). I've done these kind of measurements myself and there is a definite PLE spectrum.

 

[Jalo]

Not always. You need to absorb light first and that requires that the photon energy matches energy from the ground state (plus possible vibrational/rotational) energy to a higher energy first. If the incident photon energy does not match an absorption band of the substance, it won't get absorbed. The PLE is the technique that let's you scan the excitation energy (wavelength). I've done these kind of measurements myself and there is a definite PLE spectrum.

What I don't understand is how is it different to excite the sample with a 300nm photon vs a 600nm photon for a material whose main transitions take place at for example 800nm, since both have a higher energy than the main transition that takes place in the sample. Shouldn't both photons be absorbed by the material and then relaxe through non radiative paths until they reach the main transition level, and from there decay to the ground state?

I ran a PLE experiment recently as well for a sample with emission around 800nm and while the PL spectrum shape didn't change throughout the experiment it was more intense for some wavelengths than others (with a peak at 760nm for some reason)