I have heard many times that the absorbance of the sample must be kept low when measuring electron dynamics in dye-semiconductor interfaces by transient absorption spectroscopy. Some say between 0.1-0.5, others say between 0.5-1. What range is the best and why? What effects are to be avoided? Can I optimize the transient absorption signal by playing with the absorbance of the sample and the power of the laser?
One effect that comes into play for samples with high concentration/strong absorbance is that the generated signal can be again absorbed by the sample thus falsifying your recorded signal spectrum. This does not only hold for dye/semiconductor systems but more or less all other samples in time-resolved spectroscopy as well.
Another thing that is important is possible interaction of your molecules on the SC surface. Dpending on your specific system dynamics of samples with high or low dye-loading of , e.g., nanoparticles may be different and depending on your sample preparation you will have a largely inhomogeneous signal composition which should be avoided.
In addition, for high concentrations and especially also for high intensity laser pulses you might generate multiple charge separations on the nanoparticples. Again, depending on your system, this can change the dynamics of the individual events and may cause things like, e.g., exciton-exciton annihilation.
You can definitely play with the laser fluence to optimize your signal but I strongly suggest to record an intensity dependence of your signal. You need to consider that you might have multi-photon events for strong laser pulses (which again effect your dynamics). Moreover you can easily destroy your samples at high fluences.
Note also that especially for structured surfaces you can have local-field surface-enhancement effects which can influence the excitation process of your sample and often you can exploit these effects in reducing the excitation intensity.
Dear Valeria,
I guess, reading this most recent review paper by:
Lukas Gallmann, Jens Herrmann, Reto Locher, Mazyar Sabbar, Andre Ludwig, Matteo Lucchini and Ursula Keller
"Resolving intra-atomic electron dynamics with attosecond transient absorption spectroscopy"
Molecular Physics, Vol. 111, Nos. 14-15, pp. 2243-2250, 2013
ought to ultimately answer your question...
Respectfully yours,
Jewgeni
http://www.ulp.ethz.ch/content/dam/ethz/special-interest/phys/quantum-electronics/ultrafast-laser-physics-dam/publications_awards/publications/2013/352__Molecular_Physics_2013__Invited_attosecond_TAS.pdf
One effect that comes into play for samples with high concentration/strong absorbance is that the generated signal can be again absorbed by the sample thus falsifying your recorded signal spectrum. This does not only hold for dye/semiconductor systems but more or less all other samples in time-resolved spectroscopy as well.
Another thing that is important is possible interaction of your molecules on the SC surface. Dpending on your specific system dynamics of samples with high or low dye-loading of , e.g., nanoparticles may be different and depending on your sample preparation you will have a largely inhomogeneous signal composition which should be avoided.
In addition, for high concentrations and especially also for high intensity laser pulses you might generate multiple charge separations on the nanoparticples. Again, depending on your system, this can change the dynamics of the individual events and may cause things like, e.g., exciton-exciton annihilation.
You can definitely play with the laser fluence to optimize your signal but I strongly suggest to record an intensity dependence of your signal. You need to consider that you might have multi-photon events for strong laser pulses (which again effect your dynamics). Moreover you can easily destroy your samples at high fluences.
Note also that especially for structured surfaces you can have local-field surface-enhancement effects which can influence the excitation process of your sample and often you can exploit these effects in reducing the excitation intensity.
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