You record an electronic absorption spectrum in a spectrophotometer and you record a PLE spectrum in a fluorimeter

Both PLE and electronic absorption spectrum will be identical for a pure single fluorescent compound.

If the compound is non-fluorescent then one cannot record a PLE spectrum but can successfully record an electronic absorption spectrum.

These spectra will be useful in determining if the sample is

- is pure

- Contains a mixture of species

            i.e.,  it contains a non-fluorescent impurity or 

            a fluorescent impurity (with little more effort).

If the sample contains a single pure fluorescent compound then both PLE and electronic absorption spectrum will be identical.

If the sample contains a fluorescent compound and an non-fluorescent impurity then one sees two bands in absorption spectrum and a single band in PLE spectrum.

If the sample contains a fluorescent compound and a fluorescent impurity then one sees two bands in absorption spectrum and a single band in PLE spectrum depending on the emission wavelength. If the emission wavelength is changed then one sees another band.

Absorption spectrum gives you a range of all wavelengths in which the given sample absorbs whereas PLE spectrum give you the range of wavelengths over which the sample absorbs and emits at the chosen emission wavelength. The chosen emission wavelength is the wavelength at which the PLE spectrum is being recorded.

In both cases you are looking at ground state to excited state transitions.

There are many examples where the sample after heat treatment or optical "bleaching" with prolonged exposure to the laser absorption spectrum remains practically unchanged, while the PLE varies greatly. So after radiation of a sample of polymer on one wavelength, the photoluminescence can cease to be excited on other wavelength.

Processes of aging of samples at their long keeping on air also lead to change of PLE (for example, in case of porous silicon). This may be due either to the desorption of hydrogen or complexes SiH, SiH2, SiH3, or with oxidation and can be explained by two reasons. One of them — growth of concentration of the centers of a fast recombination (the torn-off silicon communications) which are formed during the desorption process, the second — destruction of the luminescing substance on a surface of silicon threads which components are desorbing complexes. Moreover, it is established that nature of change of PLE in the course of aging of porous silicon depends on the wavelength of exciting light.

Based on the foregoing, I agree with the answer that was given to you earlier (V. N. Ravi Kishore V.).

These two techniques are related to emission and absorption processes respectively. All their differences in that. All answers given above are pretty completed, I add only that these two kinds of spectrum are usually very different for non-equilibrium time-resolved measurements (meaning the pump-probe measurements for the absorption spectrum).

Dear Mykhailo, you rightly noticed the difference for non-equilibrium time-resolved measurements.

Best regards, Leonid.

very thanks. all of your answers was very useful!

regards

Dear friends, you are right with your explanation, but there are some little nuances regarding the difference between absorption spectroscopy and PLE method. In the UV/Vis absorption one wavelength area is scanned with one reference sample simultaneously and each measured point result from other wavelength (wl). The observed wl is equal to the excited wl. However at a fluorescence spectrometer you set one specific emission wavelength (observe its change) and scan the excitation wl area of your interest to observe the Photo-Luminescence-Excitation spectrum. Additionally you have to scan the reference sample with the same preferences in advance. PLE offers you one special advantage which may be needed sometimes, if you want to observe which excitation cause your desired emission wl.

cheers, Sergej