Chemical reactions triggered by light are ubiquitous in the world around us. From fading paintings to skin cancer, all these processes involve a type of reaction known as a photochemical reaction. Probably the most famous of all photochemical reactions is photosynthesis, a process that converts light energy into chemical energy. Plants absorb sunlight and use the energy this provides to liberate oxygen from water, which maintains the chemical composition of our atmosphere and is ultimately responsible for life on Earth.
However, not all photochemical reactions are as useful and desirable as photosynthesis. The chemical structure of our DNA means that it absorbs light very readily, particularly UV light emitted from the sun. Typically, this is of little consequence as DNA has incredibly efficient photoprotection mechanism to convert this light energy to harmless heat but if anything goes wrong during this process, the results can be fatal. Unwanted reactions can occur between the constituent chemicals of DNA, resulting in the formation of potentially carcinogenic compounds that lead to skin cancers.
Understanding the details of photochemistry is therefore incredibly important when it comes to making efficient solar cells, trying to find ways to perform ‘artificial’ photosynthesis in the lab and providing an insight into why people may be prone to skin cancers and how we can protect against them. The challenge is finding a way to study photochemical reactions in the lab.
It turns out light is not just responsible for initiating these reactions but also provides us with a powerful tool for studying them, in the form of lasers. Lasers are an ideal light source for doing photochemistry experiments as, not only do they produce high-energy beams, but they can be used as a monochromatic light source, where they only produce one colour of light at a time. It is because of this that, following the development of the first laser in the 1960s, they have become such a popular tool in laboratories for studying the interaction of light with molecules.
There are endless possibilities with the types of experiments you can do with lasers in the lab but a common technique that is used to study how molecules react with light are called ‘pump-probe’ experiments. In these experiments, one laser pulse acts a ‘starting gun’ and provides the energy to trigger the reaction and pulses from another laser can be used to watch what happens in the reaction.
One of the big challenges in photochemistry is the fact that many photochemical reactions occur incredibly quickly. Some photochemical reactions occur on a timescale of 10s to 1000s of femtoseconds. A femtosecond is 10-15 s, or a thousand trillionths of a second. A femtosecond is such a miniscule fraction of a second that, the number of femtoseconds in one second is equivalent to the number of seconds in over 30 million years.
If you want to photograph something moving quickly, in order to get a clear photo rather than just a blurred stripe, the shutter speed of a camera needs to be quicker than the movement of what you are observing. The same is true of using lasers to observe molecular reactions, if you want to get a clear picture of how the molecules are moving, your laser pulses need to be faster than the speed of your molecular reaction.
This is why the development of femtosecond lasers systems has revolutionised the way in which we study chemical reactions. Rather than just being able to take a snapshot of what happens at the beginning and the end of a reaction, we can now watch exactly how the reaction evolves and changes with time.
Femtosecond experiments have been performed on a huge number of molecules now and have given us an insight into many different aspects of chemistry. From understanding the fundamental steps involved in DNA’s photoprotection mechanism, to how sunscreens can help protect our skin, the use of light to probe the inner workings of molecules is likely to remain a fruitful area of research.
Rebecca Ingle is a second year PhD student in the Bristol Laser Group at the University of Bristol. Her research interests involve studying how molecules reaction with light, in both the gas and solution phase, using a combination of laser experiments and computational chemistry methods. You can find more about some of the latest research from the Bristol Laser Group here: http://www.bristoldynamics.com/