Dear Peter,

Thanks for the interest. I've appended the graph of attempted fit employing IRF. What bothers me the most is the wave in the range of 0-200 micro seconds. I cannot get rid of it even if I take 3 or more exponents. From my point of view this is due to incorrect IRF, but I might be wrong.

Detection method is TCSPC.

Greetings,

Arturas

I'm pretty sure that it is TCSPC, at least that's what they write in instruction manual. The detector is extended red PMT from Hamamatsu. I'll try measuring the IRF at lower count rate as you suggested and see what happens.

Thanks a lot!

Arturas

1. The shape of the recorded IRF looks unusual. Its flat on the top that is unusual. Most of the time such problems are related to the shape of the IRF.

Reduce the concen of ludox solution and record the IRF. See if the shape of IRF changes. This can be a problem if the concentration is very high.

Also reduce the slits if you are using large slits for recording IRF.

Just check if ludox absorbs 980 nm.

You could also use an alternative method for recording IRF.

2. Its not only the count rate of IRF that is important (as suggested by Peter) but also of the fluorescence decay that is important. Ensure that TAC stop count rate is 1% of TAC start count rate for Poisson statistics to be valid.

3. I hope it is not a data analysis problem. Check the shift parameter in your fits, are you giving a sufficient window.

Thank you Ravi for the tips. I'll try to measure the absorption of ludox and remeasure the IRF.

Best wishes,

Arturas

@ Arturas

I have seen this kind of misfit occurs when there is a problem with the IRF. The fit  doesn't improve even if you increase the no. of exponentials.

Your IRF FWHM seems pretty long ~ 100 us. I just checked up the Horiba's nanoled pulse width. It is

@Peter

I have not seen this kind of IRF profile with LEDs and diode lasers I have used.

Since I have not used high power pulse LEDs or fast switched laser diodes, can't say about them.