Raman Side Projects
FSRS setup guide: FAQ’s
So, you want to set up a FSRS table? You should! We think it’s a fantastic technique with unique capabilities for learning a lot about reaction dynamics in various systems. Apparently other people agree, as we get lots of questions about setting up and running FSRS (rhymes with scissors) experiments. Here are our thoughts on the most common questions…
First of all, before you get started, please read the papers on the setup.
These are all excellent places to start. Then, here are some additional thoughts:
The Laser Table:
The most common question we get asked is “How do you generate your continuum?” This is a surprising question, as continuum generation is quite common in ultrafast spectroscopy. There are several ways to do it, but we have always generated our continuum in sapphire plates. We’ve used either lenses or focusing mirrors to focus and recollimate the beam. We’ve also found that with our amplifiers, adding an aperture before the first lens, which picks out the most intense part of the fundamental, is helpful for beam stability. And probe stability is crucial to running a successful experiment.
Another issue results from people trying to convert existing pump-probe systems into FSRS system. Frequently this results in not allocating sufficient power to generate the Raman pump. No matter how you make it, creating a picosecond pulse from a femtosecond pulse is going to use lots of power. We devote 80-90% of our 800uJ amplifier output to the Raman pump on both of our tables.
We’ve also been asked about what pulse powers to use. This is a very sample dependent question, and you can see the range of powers we’ve successfully used in all of the published papers. We always check the power dependence of our signals to make sure we are not above the linear regime.
We set the actinic pump power equal to a photoalteration parameter (F) of 1. Here’s the equation:
where E is the pulse energy in photons,
is the molar extinction coefficient (M-1 cm-1), is the photochemical quantum yield, r is the 1/e2 radius of the Gaussian beam (cm), and NA is Avogadro’s number. This equation and it’s derivation can be found in the thesis of Anne B. Myers, 1984.
The Raman pump power needs to be quite low (100-200 nJ) when directly on resonance, and can be as high as 2-3 uJ off resonance. It is crucial to check the linearity of the signal in this region.
Once you’ve got a Raman signal, remember that the Raman gain must always be normalized to the probe intensity. We always report our data as a percent gain: (Raman pump on counts) / (Raman pump off counts)
This is the only way to correctly obtain accurate relative Raman intensities. By normalizing, your final Raman spectrum should always have the same intensities as a cw Raman spectrum. Doing otherwise will result in incorrect power dependencies.
This sample properties and setup obviously depend a lot on the experiment. However, a sample concentration with OD = 1 is ideal. Any higher concentration and you won’t pump the back of the sample, any lower and you lose out on Raman signal. We match our cell thickness to the most concentrated sample we can make.
It’s also crucial to flow your sample during experiments, at a rate sufficient to replenish the volume between laser shots. We have found that using cuvettes with stir-bars can be a problem when there are photoproducts with moderately long lifetimes. Flowing obviously requires more sample volume.
Ok, that’s it for the FAQs. If you’ve got any other questions, or any additional comments, please let us know!
- Renee Frontiera, 4/2009