Best: Basic scheme for the keep watch over of ion-molecule reactions the use of microwave radiation with the instance of the He++ CO –> He + C+ + O response. A 𝜋 pulse is a pulse which transfers all the molecular inhabitants coherently from one state to every other. Backside: Temporal evolution of the inhabitants within the floor rotational state (black, left vertical scale) and the primary excited rotational state (pink, proper vertical scale) led to by means of the microwave pulse. The period of the microwave pulse is used to exactly track the inhabitants of those two rotational states. Credit score: Martins et al.
A key purpose of ongoing analysis rooted in molecular physics is to know and exactly keep watch over chemical reactions at very low temperatures. At low temperatures, the chemical reactions between charged debris (i.e., ions) and molecules spread with extremely rotational-state-specific fee coefficients, that means that the velocity at which they continue strongly is dependent upon the rotational states of the concerned molecules.
Researchers at ETH Zürich have not too long ago presented a brand new solution to keep watch over chemical reactions between ions and molecules at low temperatures, using microwaves (i.e., electromagnetic waves with frequencies starting from 300 MHz to 300 GHz). Their proposed scheme, defined in a paper revealed in Bodily Overview Letters, involves using microwave pulses to govern molecular rotational-state populations.
“During the last 10 years, we’ve evolved a technique with which ion-molecule reactions can also be studied at very low temperatures, underneath 10 Okay, comparable to the prerequisites in large molecular clouds within the interstellar medium, the place these kind of reactions play a key function,” Valentina Zhelyazkova, corresponding writer of the paper, informed Phys.org.
“Experimental effects we received on a lot of reactions involving undoubtedly charged atoms or molecules published that some reactions nonetheless happen as regards to absolutely the 0 of the temperature scale and, crucially for our present paintings, that the reactivity strongly is dependent upon the level of rotation of the molecules.”
As a part of their fresh find out about, Zhelyazkova, Fernanda B. V. Martins, Frédéric Merkt, and their colleagues sought to milk the robust affect of the level of rotation of the molecules at the chemical reactions happening at temperatures as regards to absolute 0. In particular, they leveraged this robust dependence to cause the inhibition of a response by means of converting the rotational state of molecules the use of brief microwave radiation pulses.
“We studied low-temperature reactions between undoubtedly charged atoms and molecules,” defined Merkt. “The reactivity in those techniques is dependent upon the quantum state of the molecule.”
Credit score: Bodily Overview Letters (2025). DOI: 10.1103/PhysRevLett.134.123401
To reveal their proposed means, the researchers first cooled molecules right down to their rotational floor state. On this state, the molecules are identified to be extra reactive than when they’re within the so-called first excited rotational state.
“We then used a microwave pulse so as to add a quantum of rotation to the molecule and excited it to this much less reactive state,” stated Martins. “We monitored the reactivity by means of looking at the choice of merchandise shaped in keeping with unit of time and noticed a discount when the microwave pulse was once became on.”
General, the findings collected by means of Zhelyazkova, Martins, Merkt and their colleagues confirmed that other rotational ranges of a molecule can react at other speeds. Additionally, the researchers effectively managed the chilly ion-molecule chemical reactions the use of microwave pulses.
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“We confirmed that the reactivity can also be changed by means of thrilling the molecule from its rotational floor state to the primary excited rotational state the use of brief pulses of microwave radiation,” stated Zhelyazkova. “Additionally, we demonstrated that microwaves can be utilized to decelerate a response thru a nonthermal mechanism. In maximum packages of microwaves in chemical synthesis, the microwaves are used to thermally warmth the molecules up, which at all times makes them extra reactive.”
The new paintings by means of Zhelyazkova, Martins, Merkt and their colleagues may quickly open new probabilities for analysis, because it gives a treasured new solution to exactly controlling and learning ion-molecule chemical reactions at low temperatures. In the meantime, the researchers plan to refine their means and use microwaves to strengthen reactivity.
“In our subsequent research, we plan to organize molecules in decided on nonreactive quantum states and use brief microwave pulses as a reactivity cause,” added Merkt.
Additional information:
Fernanda B. V. Martins et al, Microwave-Managed Chilly Chemistry, Bodily Overview Letters (2025). DOI: 10.1103/PhysRevLett.134.123401.
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