I have taken both the absorption and excitation spectrum of europium (3+) doped phosphors (5, 10 and 20mol%). In the excitation spectrum with an increase in Europium substitution the intensity of charge transfer band is around 300nm but in absorption spectra the absorbance decreases with europium doping. Why does this happen?
Absorption spectrum is just plot of absorption intensity as a function of wavelength usually from 200nm to 800 nm covering the electronic energy levels of molecules, Absorption spectra are recorded for any absorbing material.
Excitation spectra are recorded only for fluorescent materials apart from their usual absorption spectra.
In fact for fluorescent materials, there are two types of spectra namely fluorescence emission spectra and excitation spectra.
In emission spectra we record emission spectrum by fixing the excitation wavelength at a particular wavelength.Or emission wavelength arising due to radiative relaxations to various lower energy levels from a particular energy level to which molecules are excited.
In excitation spectra we scan excitation wavelength by fixing emission wavelength. In other words, excitation spectrum will give all the wavelengths absorbed by molecule which will provide a particular emission wavelength.
If we compile all the excitation spectra corresponding to all the emission wave length we will get spectrum which is almost same as the absorption spectrum. We say almost since we will not get signature of absorption if that absorption does not yield fluorescence emission.
Excitation and emission spectra of fluorescent material is needed to model the luminescent centres completely and to find out which wavelength provide the maximum fluorescence emission.. This is useful for example to identify pump wavelength for laser action.
V P N Nampoori
I am not sure to fully understand your question but If I understand, you excite your material @ 300 nm and look for emission of Eu3+ @~ 620 nm. And you also look at the absorption around 300nm.
The more there is Europium, the more there is luminescence @ 620 nm for the same excitation , and the less there is absorption ?
My supposition is adding europium to the material, and then, the energy transfer from CT band to europium increase (in probability), then the luminescence and so the excitation. For the diminution of the absorption, I have no strong clue, but maybe adding quantitativ part of europium change the CT band of the material.
I hope it helps
Romain
I too bit confused with your query, What I understand is that you are observing an increase in the intensity of CT band @300 nm with increase in conc. of Eu3+ in the excitation spectrum and the reverse in absorption spectrum.
My first question is, did you measured both absorption and excitation in same state (solid).
Coming to the difference between absorption and excitation, these are indeed different. There is no rule that all the energy absorbed must be emitted (Non-radiative transitions may occur). In your case as Intensity of CT band in the excitation spectrum increases with increase in Eu concentration, it is clear that CT band is contributing to emission.
That means you are getting a peak at 300 nm in excitation spectrum, only because that energy is used for emission of Eu3+ at around 615 nm (where you are monitoring). I think now you got the reason why absorption spectra doesn't shows that peak.
The difference between an excitation spectrum and absorption spectrum? In the excitation spectrum you do a scan on a specific-emission wavelength monitored, e.g. Eu3+ at 612 nm, and the absorption spectrum you do not need to monitor in a specific wavelength and it scans only. Therefore there may be band shift when comparing two kinds of spectra, the effects on probabilistic energy transfer phenomena can increase or decrease the absorption coefficients of Einstein and influence the intensities of the bands. I did not understand so well your second question, you have Europium-doped phosphor with 5, 10 and 20%. An assumption, if the intensity of the charge transfer band is increasing in the excitation spectrum by increasing of the doping europium, can be related possibly to the covalent interaction of Eu3 +-ligand involving orbitals of atoms of the ligand and f orbitals. This can be the more excited europium ions (increased doping) transfer more load. Must take into account in which emission wavelength is being monitored the excitation spectrum. In the case of the absorption spectrum, is probably having an luminescence quenching . You have to take into account the type of equipment and experimental conditions in order to make comparisons. Otherwise this discussion has to be changed. These books can help you: 1) M. D. Lumb, Luminescence spectroscopy. 2) G. Blasse, B.C. Grabmaier, Luminescent Materials.
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