Using a simple single-atom system allowed researchers to observe the superfast processes that occur in light-sensitive molecules
What happens inside a molecule when it encounters light? This is a difficult question to answer, partly because it is one of the shortest, fastest, and most elusive moments in nature. Imagine a ray of light reaching a skin cell, or a green dye molecule in a plant absorbing a ray of sunlight, or a material in a solar cell converting light into energy. At that moment, within a fraction of a second, a process begins that changes the molecule from the inside. And when we say fraction, we mean it literally – the entire process lasts only a femtosecond, that is, a millionth of a billionth of a second. How fast is it really? The ratio of a femtosecond to a second is like the ratio of one minute to the age of the universe.
During this brief time, the electrons in the molecule wake up and move, and with them the atomic nuclei. New research suggests a new way to examine these rapid processes by creating a simple system that uses single atoms to simulate what happens in much more complex molecules.
The light at the edge of science
Photochemistry – the study of chemical reactions triggered by light – was born as a scientific field back in the 19th century, when chemists Theodore Grotthuss and John W. Draper determined that for light to lead to a chemical change in a substance, it must be absorbed by it.
It was not until 1905 that another significant development in the field occurred, When Albert Einstein Decipher the the photoelectric effect, which won him the Nobel Prize, and showed that light is made up of discrete packets of energy (quanta). This discovery not only laid the foundation on which the later Quantum theory, but also laid the foundations for understanding how a single photon can excite an electron. It turned out that the energy of the photons had to be high enough to cause the electron to break free from its place in the material and react. You can imagine this as a child trying to kick a ball through a fence: if he kicks too weakly, the ball will get stuck in the fence and stop; if he kicks with just the right force, the ball will go over the fence and fall right behind it; and finally if he kicks harder, the ball will go over the fence and continue moving beyond it.
The ball-and-fence analogy is appropriate for describing the photoelectric effect, in which electrons are displaced from a material, but there are also processes in nature where electrons move within molecules themselves. In these processes, the photons must have an energy that exactly matches that required for the transition. Unlike the ball, which can continue to travel farther, the electrons inside the molecule behave like a book in a library: you can place the book on the shelves, but not between them. If we invest too little or too much energy, and lift the book to a height that is between shelves, we will not be able to place it there and it will return to its original place.
The second half of the twentieth century saw important technological developments that greatly advanced our ability to monitor extremely fast chemical processes. Manfred Eigen, Ronald Norrich, and George Porter were awarded Nobel Prize In 1967, for the development of flash photolysis, which made it possible to measure reactions lasting only a millionth of a second. In 1999, Egyptian-American chemist Ahmed Zewail won Nobel Prize For the development of femtochemistry, a method that made it possible for the first time to record the atoms themselves during a reaction; and in 2023, physicists Anne L'Huillier, Pierre Agostini and Franz Krausz joined the distinguished list of laureates Nobel prize, on the development of even faster cameras.
Understanding these processes is not just a matter of curiosity; photochemistry is an invisible engine that makes life on Earth possible. The most prominent example is, of course, photosynthesis – Nature’s important chemical factory, where plants capture solar energy and convert it into usable chemical energy, in the form of sugar molecules. This process is the basis of the entire food chain as well as the oxygen we breathe.
Scientists are now focusing on creating simple systems that will help explain these processes. For example, scientists around the world are trying to crack the mechanism of The photochemical decomposition of waterThis is one of the greatest aspirations in the field of green energy – the ability to use only sunlight to produce clean hydrogen to use as fuel, without emitting pollutants.
In addition to energy and food, photochemistry is also our protector on Earth. High in the atmosphere, photochemical reactions are what create and preserve the The ozone layer, which absorbs dangerous ultraviolet radiation. So a deep understanding of these tiny molecular movements is the key to controlling the vast processes that shape our lives in this world.
A programmed quantum device
Measuring all the details of a fast chemical reaction is a difficult task. To simplify it, perhaps it could be reproduced proactively within a controlled system. That's what a group of researchers tried to do when they built A programmed quantum device: A small system built to mimic the behavior of a real molecule when exposed to light.
At the center of this device is a single ion – an atom with an electrical charge – trapped in an electric field and surrounded by focused lasers. Ostensibly, such an ion is nothing like a complex molecule and does not have the same composition or mass as the molecule being studied. But from a physical point of view, we do not need to build an exact copy of the molecule. It is enough to create a physical analogy: a system that behaves according to the same laws.
The possible arrangements of the ion and its electrons mimic those of the molecule. Similarly, the motion of the ion mimics the vibrations of the atoms in the molecule. Although the mass of a single atom is very different from that of a large molecule with many atoms, the rate of vibration can be controlled so that its motion mimics the motion of the molecule.
The beauty is that this system can be programmed by physically controlling the state of the atom being tested. In this way, the same device can simulate different molecules if the experimental settings are changed. This way, a simple system of one atom can tell the story of an entire molecule in real time.
One molecule, three ways to react
To test whether the simulation really works, the researchers performed experiments under conditions that simulate three well-known molecules, each of which responds differently to light and whose behavior scientists already know, from previous calculations or experiments. The results were impressive: The simulation was able to reproduce not only the general response of each molecule, but also its small details: how the electrons react, when a transition between states occurs, and how the movement of the atoms changes the image.
With the help of such simulations, we may in the future be able to develop light-sensitive materials that will act at exactly the right time, based on a deep understanding of their chemical reaction to light. This will enable us to create smart drugs that awaken to action within the desired tissue in the body in response to light, extremely efficient solar collectors, and a host of other developments, some of which we are not yet able to even imagine.
More of the topic in Hayadan:
- A quantum logic gate between a photon and an atom that may form the basis of quantum communication
- Universal quantum coupler from Tel Aviv University could reduce the cost of a photonic quantum computer tenfold
- Positive spin - research at the Technion was selected as one of the breakthroughs in photonics in 2023
- A new method for preparing a photonic metamaterial from liquid crystals used for the production of smart windows
One response
Something very challenging.
Interesting not only for chemists, but also for physicists.
A new topic. I think it should be interesting for chemistry teachers and students alike. It's a shame that so few teachers use such interesting material to pass on to our students.