As David Walwark said : could you reduce the repetition rate of the excitation laser ? If the baseline intensity drops compared to the 2ns decay, this suggest that most of the fluorescence yield in due to a long lived decay.

Can you please convert this to a linear (y) log(x) plot?

Is ~100 counts a normal background level for your system or is that abnormally high?

Dear Uma Nudurupati ,

I tried doing that, the plot is quite random. Can you please explain how the log plot will help my understand the lifetime problem.

Also Thank you for your response.

Dear David Walwark ,

That is exactly the problem I am facing. All my other samples under same conditions reach ~0, so this is abnormally high.

Do you think the excited electrons do not reach the ground state or reshuffling of the electrons in the band gap is occurring. Please let me know what you think.

Also thank you for the response.

Hi @Phurpa

i just wanted an estimate of the lifetime. That’s why I suggested the linear log plot. If this is fit to a mono exponential- what’s the lifetime like ?

I suggest you measure this sample again with a much slower repetition rate. If you do that then you may be able to resolve the time constant for a slower decay. If the slow decay does not become apparent with a slower pulse/repetition rate then there might be some other source of error.

Dear Uma Nudurupati ,

with mono exponential fitting the lifetime I get is 2.22 ns with X^2=1.045.

Hie@Phurpa and David,

It could be due to the weak singlet state emission from the excited state.

As David Walwark said : could you reduce the repetition rate of the excitation laser ? If the baseline intensity drops compared to the 2ns decay, this suggest that most of the fluorescence yield in due to a long lived decay.

Dear David and Robert,

I did not reduce the rate but increased the time span from 50 ns to 200 ns.

The following is the graph I got. The lifetime was 3.9 ns in this case. In both cases however, the baseline doesn't drop all the way to 0. Can someone explain what I am missing here. Also I will try to reduce repetition rate.

Thank you.

Phurpa

Do not worry about the change in the lifetime. The signal is so noisy that the result of the fit fluctuates. What is the emission wavelength ? Reduce the repletion rate by a factor of 10, even without changing the time span.

Dear Robert Bernard Pansu ,

Thank you for so many suggestions. The excitation wavelength was 375 nm and emission was 670 nm. I also performed the experiment with excitation of 475nm. I got the following result.

Thank you. The question about the wavelength was because I am surprised by the importance of the noise. In the data. 670nm is not a difficult wavelength where detectors are not sensitive. For fluorescence yield measurement it is important that the absorbance over 1cm is between 0.05 and 0.1, for plenty of reasons. For fluorescence lifetime, you can multiply the Absorbance by 5 (and the collected intensity). The repetition are of the excitation is the key of your long lived component question.

Dear Phurpa Dema Thungon,

I am almost sure that the long tail of the emission decay of your nanoclusters is due to phosphorescence. Please check the following reference:

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