Question: Could you explain your ultrafast (femtosecond) experiment in detail?

Rebecca Ingle answered on 12 Nov 2014:

A femtosecond is 10^-15 seconds which is an incredibly short amount of time. However, molecular motions such as rotation and vibration can occur on timescales up to ~ 10^-14 seconds, so if you want to see these kinds of things happening, you need a laser that can go faster than what you’re looking for.

A lot of chemical reactions also happen quicker than 10^-12 s, particularly if the reaction is an intramolecular reaction e.g. the movement of a hydrogen from one part of the molecule to another. Again, if you really want to see these things happening in ‘real life’ you need these ridiculously quick lasers.

So how does the experiment work? It’s quite simple in theory, we use two different laser pulses, one we use to trigger the reaction and the other we use to watch what happens during the reaction. We just shine both lasers through a sample (a liquid one in our case) and then into a detector to see how the sample changes.

Often, we’re interested in UV-induced photochemistry, so we’ll ‘pump’ the system (provide it with the energy to reaction) with UV light and then use either UV or infrared laser light to see what products are formed or which of the reactants disappear.

How you make the femtosecond pulses is really quite complicated as you start to get all sorts of weird effects when your pulses are that short. It involves doing lots of things like stretching the length of the pulses in time, amplifying them, compressing them again etc. which is why these types of laser systems are so expensive and there aren’t many of them around.