



Slow down molecules to see their reaction in detail

Chemical reactions convert reagents into products via an intermediate state where bonds break and form. Often too short to be observed, this phase has so far escaped detailed observation. By "freezing" the rotation, the vibration and the movement of the reagents (here potassium-rubidium molecules) at a temperature of 500 nanokelvin (temperature barely higher than the temperature of absolute zero), the number of energetic states allowed for the products is limited. "Trapped" in the intermediate phase longer, researchers can then observe this phase directly with photoionization detection. Credits: Ming-Guang Hu

Bibliography:



Direct observation of bimolecular reactions of ultracold KRb molecules



M.-G. Hu, Y. Liu, D. D. Grimes, Y.-W. Lin, A. H. Gheorghe2, R. Vexiau4, N. Bouloufa-Maafa4, O. Dulieu4, T. Rosenband2, K.-K. Ni,



Science 29 Nov 2019:

Vol. 366, Issue 6469, pp. 1111-1115

DOI: 10.1126/science.aay9531

Chemical reactions are extremely fast processes, involving complex and successive molecular interactions. Until now, researchers could only observe the beginning and end phases of a chemical reaction, without being able to observe the course of the reaction itself. Recently, however, chemists at Harvard University have been able to cool molecules to such a low temperature that they have been slowed down to the point where researchers have the opportunity to observe the chemical reaction in detail. A feat that could pave the way for new technologies, from civil engineering to quantum computing.It's 500 nanokelvins, a few millionths of a degree above absolute zero. The icy nature of this configuration is important because at these temperatures, molecules tend to slow down to the point of almost stopping. For a chemical reaction to occur, slow molecules are usually not indicated.But in this case, the reduction in temperature and speed gave the team led by Harvard University the opportunity to see something that had never been observed before: the moment when two molecules meet and form two new molecules. The results were published in the journal Science .The chemical reactions take only one picosecond, which makes it very difficult to observe what happens during this period of time. Even ultra-fast lasers acting as cameras can usually capture the beginning and the end of a reaction, but not what happens in the middle. Slowing the reaction through extremely cold temperatures obtained by the team was therefore the ideal solution.The coldest temperature in the universe is absolute zero, a temperature that is experimentally impossible to reach. But it is possible to seriously approach it. Ultra-low temperatures mean very low energies, and therefore a much slower reaction: two rubidium potassium molecules chosen for their plasticity have been delayed during the reaction phase for a few microseconds.A technique known as photoionization detection was then used to observe what was happening between the two molecules, providing researchers with valuable real data to inform their models and assumptions. Being able to observe chemical reactions so closely and at such a fundamental level opens up the possibility of designing new reactions as well; an almost limitless number of combinations is imaginable, potentially useful in all areas, from material construction to quantum computing.