Instructions for a Time-resolved spectroscopy lab.
As you are about to see, the instructions are written not as a list of actions you have to take but rather as a set of questions. There are also same hints to help you here and there. The main idea of this form of lab instruction is that one usually “digests” and fastens own thoughts much better than somebody’s commands. Also, I would like to present to you the multi-dimensional field of technical and conceptual requirements of a pump-probe experiment rather than drug you through a narrow well-defined path of a single particular differential absorption experiment. Last but not least, I am far from sure that my experimental approach is the most optimal. I would appreciate your thoughts and ideas, which I will sort and put together for my own, youth and future students benefits.
1.
Typically, one starts an experimental session with measurement of the absorption spectrum of the sample. Determine the optical density at the excitation and the probe wavelength. What are the arguments to choose the optical density at the pump wavelength? Take a careful look on the shape of the sample absorption in the spectra region of probe. Remember: if sample absorption will change the probe spectrum one has to have either both probe and reference beams passing through the same sample or an identical (very similar) absorbing “dummy” sample in the reference channel.
2.
Consider the expected differential absorption signal in relation to signal/noise ratio, probability of excitation of a molecule (chromophore), saturation of absorption, possible non-linear excitation effects, and potential sample degradation. What should be the concentration of the sample you want to study?
Assume extinction of 10
5
L/(Mol*cm) or cross-section

=10
16
cm
-2
. By the way, which of the above two would be of a stronger absorber?
3.
Estimate excitation photon density before the experiment. What is it be in terms of energy per pulse, average power and excitation beam diameter on the sample?
How will you vary the excitation photon density in the experiment if needed?
What should be the range of variation?
4.
Measure the excitation beam lateral shape (beam diameter).
5.
Choose the means to block the detector from the transmitted or scattered excitation beam. Consider options of a spatial filtering, a spectral instrument, a cut-off filter or an additional cell with a solution of a dye that will transmit the probe and block the excitation.
6.
What should be the probe beam size on the sample? Are there any limitations on the pulse energy (average power) of the probe? on the photon density in the probe? What ultimate signal/noise can you get for a particular energy of probe pulse? Measure the probe energy and beam diameter.
7.
What are the important considerations to choose the polarization of the probe beam? Regarding optical elements? Regarding spectral instrument to be used?
8.
Check the spatial overlap of the excitation and probe beams over the whole length of the delay line to be used. What procedure would you suggest for this?
9.
What is the angle between excitation and probe beams? Why is it important?
Calculate, what are the limitations in temporal resolution as functions of the angle and beam diameters?
10.
What temporal resolution (instrumental response function) do you need for the measurements? Consider time-bandwidth relation for the spectral selectivity of excitation. What should be the instrumental response duration in relation to the

time scale of the process you would like to measure (assume a single exponential decay with the characteristic time

)? What should be the overall time scale of a measurement and what delay line you have to use? What should be the sampling density in relation to the number of measured points; signal/noise and time in will take to have the trace measured?
11.
Determine the zero-delay position for your experiment. Optimise the excitation and/or probe pathways to fit the time span you want to measure to the length of the delay line.
12.
Measure the cross-correlation trace for the instrumental response function. You have to get temporal and spatial overlap in the non-linear crystal for this measurement and the right phase matching angle and be sure that the detector
(typically, Si-photodiode) will see and will sense the cross-correlation signal.
Watch the polarizations of excitation and probe in relation to those needed for generating the cross-correlation signal. Consider optical elements in the excitation and probe beams when measuring cross-correlation. What cut-off filters should you use for this measurement? How will you choose the phase-matching angle of the non-linear crystal? How will you ensure spatial overlap of pump and probe in the non-linear crystal?
13.
What should be the mutual polarization of pump and probe? What dynamic information would you like to obtain in the experiment? What should be the polarization purity (contrast)? Measure the contrast for both pump and probe beams. Where in the set-up should you measure the contrast?
14.
Insure the spatial overlap of the excitation and probe beams in the sample. What methods could you suggest for this?
15.
Have everything sorted out? Time to get the kinetics measured!
16.
Check the limits of linear excitation. Choose the excitation to be used in the rest of the measurements.
17.
Measure kinetics with parallel-, perpendicular- and Magic Angle orientation of excitation polarization in respect to the probe polarization. Calculate the anisotropy (orientational) dynamics from all possible combinations of relative polarization. Calculate the Magic angle (populational) dynamics and compare it with the measured.
18.
Analyse the kinetics of depolarisation and depopulation. Use instrumental function for de-convolution if you have appropriate software or as a guideline for the part of the kinetics to be discarded if de-convolution will not be done. How many exponential processes are you expecting to fit the measured trace? Compare the characteristic decay time of a single exponential decay function with the fast estimate of 1/e decay time. For the advanced students, calculate the error bars of the fitted decay time.
19.
Speculate what are the processes you have just recorded in the test sample?