Lifetime measurement: double peak in instrument response function

I am measuring flourescence lifetimes with a picosecond pulse laser, using the LifeSpec II with an MCP detector. Instead of a single peak rapidly decaying, as would be expected, my IRF contains two peaks. I am suspecting some sort of reflection causing this. Does anyone get what I mean here? Could add a graph soon if that might help.


EDIT: I have realized this might not be an error, but I would still like to know why.


![The IRF](https://i.stack.imgur.com/K5JLx.png)





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> I am suspecting some sort of reflection causing this. Does anyone get what I mean here? 
> 
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The time between the two peaks looks like 0.4 nanoseconds.


This time (0.4 ns) should be compared to the dimensions of the vessel (chamber, cuvette, I don't know the right word) and the speed of light in the medium being observed (water, air, diamond).


So for example in vacuum or gas the light would travel 0.4ns x (0.3m/ns) = 0.12m.


In water where the index of refraction is 1.33 more like 0.09m.


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It is quite common to observe secondary peaks or "bumps" in the instrumental response function for a laser excitation. It is not always easy to pinpoint the origin of the second peak, but it is typically *not* associated with a reflection in the instrument. If any reflection were to be involved, it would be one within the cavity of the laser itself. It is however more often the case that these secondary peaks are just an adventitious product of the laser production process (no laser is perfect). In fact, manufacturers sometimes will suggest keeping an eye on the relative ratio of these peaks in the IRF to gauge the health status of your laser.


For instance, observe the secondary peak in the red line below from a PicoQuant FluoTime instrument with a picosecond laser:


[![An example from PicoQuant](https://i.stack.imgur.com/p9XW3.jpg)](https://i.stack.imgur.com/p9XW3.jpg)  

([original source](https://www.picoquant.com/?ACT=32&f=decay_analysis.jpg&fid=27&d=7309))


You can also see an example of a much more bumpy IRF in a leaflet published by Edinburgh, the makers of your own instrument [LifeSpec II spectrometer](http://www.edinburghphotonics.com/files/file/technical-specifications/Lifespec%202%20Flyer.pdf).


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