I'm studying the optical properties of materials, I measured the Absorption (by UV-Vis), Excitation and Emission of the same samples. The absorption peak is at 347 nm and the range is 305-396 nm, whereas the excitation peak is at 405 nm and the range is 250-430 nm. Why the absorption and excitation peaks are different and what is the possible reason for the small range of absorption and a wide range of excitation? Kindly help me to sort out this query
Thank you so much
Dear Abdur Raheem,
In general, absorption spectra and photoluminecence excitation spectra will only be the same if a single chromophore absorbs light at an excited state and it relaxes quickly (and completely) to always the same emission state (exciton), independent on the excitation energy (Kasha's rule). There are many reasons why ABS and PL excitation spectra may be different:
1) Inhomogeneous samples with aggregated chromophores will lead to absorption due to aggregates that may lead to excitons of different quantum yields, due to PL quenching by aggregation;
2) PL measurements are more sensitive than ABS, so that maybe very weak absorption still leads to measurable PL, while at high ABS optical density the amount of absorbed light actually saturates, and you don't see a corresponding increase in absorbed light intensity (and therefore PL intensity).
3) The ABS spectra may have contributions from different exciton transitions, but their emission efficiency at the measured PL wavelength can be very different.
I hope this is an useful statring point for your study of this in many available books on PL spectroscopy...
Paulo
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