Microwave pulses can control ion-molecule reactions at near absolute zero

A key objective of ongoing research rooted in molecular physics is to understand and precisely control chemical reactions at very low temperatures. At low temperatures, the chemical reactions between charged particles (i.e., ions) and molecules unfold with highly rotational-state-specific rate coefficients, meaning that the speed at which they proceed strongly depends on the rotational states of the involved molecules.

Researchers at ETH Zürich have recently introduced a new approach to control chemical reactions between ions and molecules at low temperatures, employing microwaves (i.e., electromagnetic waves with frequencies ranging from 300 MHz to 300 GHz). Their proposed scheme, outlined in a paper published in Physical Review Letters, entails the use of microwave pulses to manipulate molecular rotational-state populations.

“Over the past 10 years, we have developed a method with which ion-molecule reactions can be studied at very low temperatures, below 10 K, corresponding to the conditions in giant molecular clouds in the interstellar medium, where these types of reactions play a key role,” Valentina Zhelyazkova, corresponding author of the paper, told Phys.org.

“Experimental results we obtained on numerous reactions involving positively charged atoms or molecules revealed that some reactions still take place close to the absolute zero of the temperature scale and, crucially for our current work, that the reactivity strongly depends on the degree of rotation of the molecules.”

As part of their recent study, Zhelyazkova, Fernanda B. V. Martins, Frédéric Merkt, and their colleagues sought to exploit the strong influence of the degree of rotation of the molecules on the chemical reactions occurring at temperatures close to absolute zero. Specifically, they leveraged this strong dependence to trigger the inhibition of a reaction by changing the rotational state of molecules using short microwave radiation pulses.

“We studied low-temperature reactions between positively charged atoms and molecules,” explained Merkt. “The reactivity in these systems depends on the quantum state of the molecule.”

To demonstrate their proposed approach, the researchers first cooled molecules down to their rotational ground state. In this state, the molecules are known to be more reactive than when they are in the so-called first excited rotational state.

“We then used a microwave pulse to add a quantum of rotation to the molecule and excited it to this less reactive state,” said Martins. “We monitored the reactivity by observing the number of products formed per unit of time and saw a reduction when the microwave pulse was turned on.”

Overall, the findings gathered by Zhelyazkova, Martins, Merkt and their colleagues showed that different rotational levels of a molecule can react at different speeds. Moreover, the researchers successfully controlled the cold ion-molecule chemical reactions using microwave pulses.

“We showed that the reactivity can be modified by exciting the molecule from its rotational ground state to the first excited rotational state using short pulses of microwave radiation,” said Zhelyazkova. “Moreover, we demonstrated that microwaves can be used to slow down a reaction through a nonthermal mechanism. In most applications of microwaves in chemical synthesis, the microwaves are used to thermally heat the molecules up, which always makes them more reactive.”

The recent work by Zhelyazkova, Martins, Merkt and their colleagues could soon open new possibilities for research, as it offers a valuable new approach to precisely controlling and studying ion-molecule chemical reactions at low temperatures. Meanwhile, the researchers plan to refine their approach and use microwaves to enhance reactivity.

“In our next studies, we plan to prepare molecules in selected nonreactive quantum states and use short microwave pulses as a reactivity trigger,” added Merkt.

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