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Rotational cooling of molecular ion-electron collisions measured using laser technology

When it is free in cold space, the molecule will spontaneously cool by slowing down its rotation and losing rotational energy in quantum transitions.Physicists have shown that this rotational cooling process can be accelerated, slowed down or even inverted by collisions of molecules with surrounding particles.googletag.cmd.push(function() { googletag.display(‘div-gpt-ad-1449240174198-2′); });
Researchers at the Max-Planck Institute for Nuclear Physics in Germany and the Columbia Astrophysical Laboratory recently conducted an experiment aimed at measuring the quantum transition rates caused by collisions between molecules and electrons.Their findings, published in Physical Review Letters, provide the first experimental evidence of this ratio, which has previously only been estimated theoretically.
“When electrons and molecular ions are present in a weakly ionized gas, the lowest quantum-level population of molecules can change during collisions,” Ábel Kálosi, one of the researchers who conducted the study, told Phys.org.”An example of this process is in interstellar clouds, where observations show that molecules are predominantly in their lowest quantum states. The attraction between negatively charged electrons and positively charged molecular ions makes the electron collision process particularly efficient.”
For years, physicists have been trying to theoretically determine how strongly free electrons interact with molecules during collisions and ultimately change their rotational state.However, so far, their theoretical predictions have not been tested in an experimental setting.
“Until now, no measurements have been made to determine the validity of the change in rotational energy levels for a given electron density and temperature,” explains Kálosi.
To gather this measurement, Kálosi and his colleagues brought isolated charged molecules into close contact with electrons at temperatures around 25 Kelvin.This allowed them to experimentally test theoretical assumptions and predictions outlined in previous works.
In their experiments, the researchers used a cryogenic storage ring at the Max-Planck Institute for Nuclear Physics in Heidelberg, Germany, designed for species-selective molecular ion beams.In this ring, molecules move in racetrack-like orbits in a cryogenic volume that is largely emptied from any other background gases.
“In a cryogenic ring, stored ions can be radiatively cooled to the temperature of the ring walls, yielding ions filled at the lowest few quantum levels,” explains Kálosi.”Cryogenic storage rings have recently been built in several countries, but our facility is the only one equipped with a specially designed electron beam that can be directed into contact with molecular ions. The ions are stored for several minutes in this ring , a laser is used to interrogate the rotational energy of molecular ions.”
By choosing a specific optical wavelength for its probe laser, the team could destroy a small fraction of the stored ions if their rotational energy levels matched that wavelength.They then detected fragments of the disrupted molecules to obtain so-called spectral signals.
The team collected their measurements in the presence and absence of electron collisions.This allowed them to detect changes in the horizontal population under the low temperature conditions set in the experiment.
“To measure the process of rotational state-changing collisions, it is necessary to ensure that there is only the lowest rotational energy level in the molecular ion,” Kálosi said.”Hence, in laboratory experiments, molecular ions must be kept in extremely cold volumes, using cryogenic cooling to temperatures well below room temperature, which is often close to 300 Kelvin. In this volume, molecules can be isolated from ubiquitous molecules, Infrared thermal radiation of our environment.”
In their experiments, Kálosi and his colleagues were able to achieve experimental conditions in which electron collisions dominate radiative transitions.By using enough electrons, they could collect quantitative measurements of electron collisions with CH+ molecular ions.
“We found that the electron-induced rotational transition rate matches previous theoretical predictions,” Kálosi said.”Our measurements provide the first experimental test of existing theoretical predictions. We anticipate that future calculations will focus more on the possible effects of electron collisions on the lowest energy-level populations in cold, isolated quantum systems.”
In addition to confirming theoretical predictions in an experimental setting for the first time, the recent work of this group of researchers may have important research implications.For example, their findings suggest that measuring the electron-induced rate of change in quantum energy levels could be crucial when analyzing the weak signals of molecules in space detected by radio telescopes or chemical reactivity in thin and cold plasmas.
In the future, this paper could pave the way for new theoretical studies that more closely consider the effect of electron collisions on the occupation of rotational quantum energy levels in cold molecules.This could help to figure out where electron collisions have the strongest effect, making it possible to conduct more detailed experiments in the field.
“In the cryogenic storage ring, we plan to introduce more versatile laser technology to probe the rotational energy levels of more diatomic and polyatomic molecular species,” adds Kálosi.”This will pave the way for electron collision studies using large numbers of additional molecular ions. Laboratory measurements of this type will continue to be complemented, especially in observational astronomy using powerful observatories such as the Atacama Large Millimeter/Submillimeter Array in Chile. ”
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Post time: Jun-28-2022