Lifetime components in phosphorescence decay

I'm studying the phosphorescence decay of diacetyl in acetonitrile (it has a high quantum yield of intersystem crossing). The fluorimeter than I'm employing has microsecond resolution and, since the decay of the phosphorescence isn't too long, I have to measure the IRF (width of 1-2 $\mu$s) in order to perform a reconvolution fit. However, I get a biexponential fitting with a long time (like 20 $\mu$s) and a very short time (around 30 ns). Does that make sense? I thought that the short lifetime could be due to fluorescence, but I get it even in the decay of the righmost peak while using a long pass filter. Moreover, I don't know if it makes sense to have a lifetime with more resolution that the limit of the aparatus.


Does this make sense or has any meaning? Maybe is an artifact or I'm making some mistake with the measurements?


Edit: I'm using the Spectrofluorometer FS5 from Edimburgh Instruments, and employing its software (Fluoracle) to make to fittings. The light source is a flash lamp. By reconvolution/convolution I mean a fitting that takes into account the IRF (instrument response factor), as the lifetime of the decay approaches the duration of the IRF.


The fittings look like this:


[![Monoexponential](https://i.stack.imgur.com/npgKB.png)](https://i.stack.imgur.com/npgKB.png)
Figure 1: monoexponential fitting


[![Biexponential](https://i.stack.imgur.com/Lc6Te.png)](https://i.stack.imgur.com/Lc6Te.png)
Figure 2: biexponential fitting




Phosphorescence should be at a longer wavelength than any fluorescence so carefully using a filter or grating should remove any fluorescence. Also measuring at different wavelengths will change percent of any fluorescence still getting through a filter. Is there a tiny bit of scattered light being mixed in with the phorphorescence? (Some types of glass filters do fluoresce but usually you need to excite then directly).


How are you doing the convolution, Marquardt type non-linear least squares iterative convolution is normal. What sort of chi-squared are you getting for the fit, what type of weighting is used? How does a 1 exponential vs 2 compare? What do the residuals look like and so on. Are you fitting to a background also? However, 30 ns is really too short to be confident that it actually exists if the irf is 1-2 microsecs.


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