Hi I am new to laser pump-probe spectroscopy and want to understand the concepts better. At the moment I am struggling to understand the following: When one takes absorption measurements with lasers in a pump-probe manner, the spectra that are collected need to be normalized. What does the Normalization mean and how does one do it? Also, why are relative changes important as opposed to absolute changes (in absorption) because I think they should give the same information?
I'm not sure exactly what you mean by normalising as it can mean more than one thing.
At each probe wavelength ideally you measure two signals simultaneously from the same probe pulse which is split into two parts before entering the sample. One part probes the sample at the same place that it is excited and the other probes unexcited sample. You then record the transmission of these two signals and calculate optical density change or work with transmission depending upon choice.
It is clearly necessary to normalise to the pump laser intensity since a more intense pulse will produce a bigger effect. Second the probe pulses have to be monitored and adjusted as necessary (without exciting sample) so that these signals are kept equal or at least in a constant ratio so they can be normalised later.
Pump-probe is just like any other transient absorption measurement (used to be called Flash-Photolysis) and a difference signal is measured. This produces a transient spectrum which might be positive or negative. If you have the exact optical density the you can construct the spectrum of a transient species knowing the ground state spectrum. If there is no ground state absorption where you have a transient spectrum then this must be positive and is the transient spectrum. If it is negative it might be due to transient gain, i.e. the probe causing stimulated emission from the sample.
The schematic shows one way the experiment can be set up. The absorption change at t = 0 is exaggerated for clarity; a 1% change is a huge signal.