Observing Chemical Reactions in Ultra-Slow Motion

At ultra-low temperatures, molecules have so little energy that, in effect, their reactivity is slowed down.

Dec 05, 2019 · 1 min read

Artistic illustration of molecules and chemical bonds. Credit: MasterTux - Pixabay

A chemical reaction, simply stated, is the conversion of reactants into products. Consider the following simple chemical reaction where nitrogen dioxide decomposes into nitric oxide and oxygen.

2NO_2 \rarr 2NO + O_2

This reaction, in reality, is believed to progress in two steps as shown below,

2NO_2 \rarr NO_3 + NO

NO_3 \rarr NO + O_2

However, only the products $NO$ and $O_2$ can be detected. This is because, in the first intermediate step of the reaction, the production of $NO_3 + NO$ is transient and takes place within, literally, no time. Instantaneously, $NO_3$ decomposes into $NO_2 + O_2$ . Therefore, measure of transient intermediate steps of a chemical reaction is very difficult and in many cases, impossible. Knowledge of these intermediate steps of a chemical reaction is very useful. This will allow “quantum-state control” over the outcome of chemical reactions of interest to us.

Researchers have now come closer to achieving this quantum-state control by directly observing transient intermediates in a chemical reaction. They achieved this feat by cooling down the reaction (Hu et al., 2019) to almost “absolute zero degree kelvin”. At this ultra-low temperatures, molecules have so little energy that, in effect, their reactivity is slowed down. Therefore, an otherwise transient intermediate will become stable for a longer time in ultra-cold conditions.

Researchers trapped potassium-rubidium gas molecules in a chamber at an extremely low temperature of only $500×10^{-9}$ kelvin or 500 nanokelvin. In this chamber, gas molecules were constantly reacting with each other. researchers were able to detect the intermediate $K_2Rb_2$ molecules for the first time along with the reactants and products as per the following reaction.

2KRb \rarr K_2Rb_2 \rarr K_2 + Rb_2

According to the author’s who performed these experiments and published it in Science, this research opens up many avenues. Specifically, in quantum mechanics which is a field of science that is used in engineering and machine designing.

References

M. Hu et al., Direct observation of bimolecular reactions of ultracold KRb molecules. Science. 366, 1111-1115 (2019). 10.1126/science.aay9531. context