 The sun is continuously delivering energy to the earth. Interestingly, the amount of sun energy is larger than the amount of energy needed by all people using current electricity on earth. How can we efficiently use the energy of the sun? One way is to use solar panels, which will result in direct energy, which are a large amount of energy on sunny days, less on cloudy days, and basically nothing during the night. Of course, the human energy need is not directly reflected in this variation of sun energy during sunny and cloudy days. So we need a way to store the energy. Trees are good in storing energy. During photosynthesis, the pigments in the leaf convert the sun energy into chemical energy by converting carbon dioxide and water into sugar. Sugar is stored in the trees and the sugar is a form of chemical energy. When we burn the trees again, the sugar is consumed again and energy will be released, which we can see and feel when we, for example, enjoy a campfire. Unfortunately, the process of photosynthesis is very slow, so it will take a very long time before a tree is useful to give enough energy to be useful. An alternative way to store the sun energy is by producing hydrogen. In this type, the sun is used to split water into hydrogen. Hydrogen can be stored and subsequently the hydrogen can be burned and the energy can be released, which can, for example, be used to drive a car around. Moreover, oxygen is produced as a side product by generating hydrogen. Moreover, if the hydrogen is burned again in the car, for example, oxygen is consumed and water is released. So it is a very clean way of energy. The only way that comes out is water. So how is now hydrogen produced? So water can split the sunlight into oxygen and hydrogen, which has been discovered by Fuyutzima and Honda roughly 40 years ago. However, this process doesn't occur by itself. A catalyst is needed. At the moment, this process is still very low in efficiency and therefore not used on large scale. For example, it is not known why one catalyst works better than the other. Many scientists are trying to understand this process, but they are doing this basically by trial and error process. What we do is a different approach. We try to really understand the mechanism. So we can really look at the process during the processes happening. How is water split into hydrogen and oxygen? So how can we do that? So with my ERC-Finnert research group, we use newly developed laser-based spectroscopy methods where we can only look at the water molecules present at the interface. We don't see the much larger amount of water molecules present below the catalyst. For example, we can see how the water molecules bind to the interface. And we can try to link that to the efficiency of the catalyst. The Sun needed to initiate the water splitting process is in our case mimicked by a laser pulse. But what's subsequently happening? To understand that, you want to look at the process at the time scale the process is taking place. This principle has nicely be illustrated by Moebridge in 1878. At that time, there was a big debate going on if a horse is during caloping once with always four feet from the ground or that it always has one foot on the ground. So Moebridge could show by fast camera that a horse is basically flying during caloping. So we use the same approach. However, as molecular processes are taking place on picosecond time scale, we cannot simply use a fast photo camera. So we have to use laser-based methods with sub picosecond laser pulses. Where we use one laser pulse to initiate the process and the second laser pulse to make pictures of the moving molecules. So in this way, we can obtain a movie about how this photocatalytic mechanism is working at the interface. If we understand how the process is working, hopefully new and better-designed catalyst can produce so that the hydrogen production can be more efficiently, so that sunlight can be stored more efficiently. Hydrogen produced in this way is a very clean source of energy and environmentally friendly, which can be used, for example, for public transport, for driving a car around or for electricity and heating at home. If hydrogen can be produced more efficiently, we don't need the large amount of fossil fuels as nowadays, but we result in less pollutants and therefore make life a better place to live on.