How do we choose excitation wavelength of Laser for PL measurement?
On what do its selection depend on?
I am trying to understand the basic concept for the PL spectroscopy, hence any fundamental explanation will be very much helpful to me.
Thank you.
The excitation wavelength does not affect the the luminescence of a pure compound, since the relaxation of the Frank Condon state toward the emissive state is faster than most other processes. There are exceptions : emission from S2 state in azulene and phtalocyanines, some photochromic reactions, some photoaquation reaction, the photoionisation.
But if the sample is a mixture of different compounds changing excitation will change relative excitations.
Respected Sir,
I think that you will get the answer as the liquid crystal is also such kind of material. This is also our work.
Article
Electro-optical, UV absorbance, and UV photoluminescence analysis of Se95In5 chalcogenide glass microparticle doped ferroelectric liquid crystal
Journal of Applied Physics (Impact Factor: 2.21). 01/2014; 115(21):214103-214103-7. DOI: 10.1063/1.4880997
I recommend you first study an elementary textbook on spectroscopy.
Then there is a more senior book that I attach
In a few words: the excitation wavelength should be tuned to the absorbance band of the material under investigation.
An "Excitation spectrum" scans the excitation wavelength for a fixed emission wavelength and can allow optimisation of the excitation conditions. This is available on commercial "double monochromator" fluorimeters!
emission spectra do depends upon excitation spectra in many cases. this phenomena is known as EERS (Edge Excitation Red Shift). it usually occurs when the reorientation relaxation time is larger than the fluorescence time so the total fluorescence emission spectrum depends upon fluorescence emission from differently solvated species governed by Franck-Condon excited-state distribution which is a is a function of excitation energy. (attachment- page no 4547)
Respected Dr. Harry
You are absolutely right, it is an exception but even i observed it in some of my compounds (donor acceptor system based on carbazole). so i thought, i must share about this.
mr Singh
You now give a different explanation namely the molecule is a double-component with a donor and acceptor part. Sure then the excitation spectrum is not similar to the absorption spectrum
Respected Dr. Harry
My explanation is same as given in the paper (J. Phys. Chem. A 2010, 114, 4542–4549). In the paper also, there is a bipolar system. Probably we have more chances of observing EERS in bipolar molecules as their are more number of excited state species. As i did not record the excitation spectrum, so i am sorry about that.
Regards
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