# 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.

• Welcome to chemistry.SE! Don't hesitate to add your graph. Though I'm no expert in spectroscopy, I'm definite it will help. – M.A.R. Feb 11 '15 at 13:44
• What is the ratio of peak 1 to peak 2? – Jeanno Feb 11 '15 at 19:52
• The ratio can now be seen in the graph above. Note the logscale. – nordmarj Feb 12 '15 at 10:03
• When you say "instrument response" in the question, does that mean the data is for some kind of a blank sample? What can you tell us about the sample chamber? Is it a capillary, a cuvette, or...? Is it aqueous, full of air, or a solvent, or...? How are you measuring the IRF? – Curt F. Feb 12 '15 at 16:25
• I use a piece of semitransparent frosted glass to reflect light from the incoming beam into the detector, to get a crude emulation of scattering. The chamber containing the sample holder is full of air. – nordmarj Feb 13 '15 at 19:05

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.

• When I take the IRF I use a piece of frosted glass reflecting part of the incoming light onto the detector. On the side of the chamber opposite to the detector there is a black plastic wall that possibly could be hit by light from the sample and reflect it back through the sample into the detector. It doesn't look very reflective though. Also the light would have to travel further than 12 cm extra if there was a reflection from there. – nordmarj Feb 13 '15 at 19:03

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:

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.