Lifetime measurement: double peak in instrument response function
I am suspecting some sort of reflection causing this. Does anyone get what I mean here? 


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.