I've been running visible pump (750 nm), broadband IR probe (3000 cm-1 - 1000 cm-1) time resolved experiments with ~40 fs short pulses and the FWHM of the instrument response function is ~120 fs. I'm using CaF2 windows as the substrate of my samples. When I do a time resolved experiment with just a CaF2 substrate, there is a transient feature that is long lived till about 2 ps and spans cross the entire probe energy range. Because the instrument response is 120 fs, I was expecting the signal around time zero to die off within ~200 fs.

The visible pump is not focused and the pump fluence is not large enough to do multiphoton excitations (bandgap of CaF2 is about 11 eV).

I'm struggling to find a way to explain the 2 ps long feature originating from CaF2 and would like to know if there are any suggestions?

Thank you