I am experiencing some unusual behavior in the transient absorption kinetics of a napthalene and napthalimide based sample. In two consecutive scan the kinetics at any particular wavelength is different. Also, it exhibits anomalous rise and decay behavior. I was wondering if this is an effect of sample trapping at the focal volume. If it is, then how to get rid of this behavior?
If there is reason to suspect that the laser beam itself is bringing about chemical reactions, one way to check this out is by expanding the incident laser beam on the sample to a larger area by using a negative (concave) lens. By using various expanded beam diameters one may be able to determine if there is threshold irradiance at which chemical reaction starts taking place. Also use a thermocouple to see if there are temperature changes in the sample due to laser irradiation.
The easiest way to get rid from this trouble is to flow the sample using a flow cell, OR use a rotating cell. By doing so, you will continuously change the spot on sample and hence the degradation can be reduced/vanished.
The effect can be weakened between the laser beam and the sample a propeller is placed making the irradiation intermittent thus reducing the effect of irradiation.
Dear Nita.. you have to figure out the lifetime of your photoproduct. This will give you the information on whether you can do your experiments at low rep-rate of the laser. In case, your product is long-lived (seconds or longer), even sample flowing will be of no use as you will accumulate the product in your reservoir with time. Flowing the sample out remains the only solution, but that needs a huge amount of sample. You can refer to following paper:
https://www.ncbi.nlm.nih.gov/m/pubmed/1061102/
In case you are dealing with film samples, and this damage is caused by laser induced heating, then you can try moving the sample in 2D plane and secondly, you can also replace you quartz substrate with sapphire.
Laser trapping is easiest to deal with. You can play with many parameters: pump focal volume, pump power, concentration of your sample, flow rate etc. Do remember to filter the sample with micron filters. To achieve more signals at low pump power, the sample length can be increased (OD
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