Molecules containing inert elements should not exist. According to the definition of a rare gas, helium, neon, argon, krypton, xenon and radon, the chemical elements located on the far right of the periodic table, are difficult for other elements to bond and form compounds. In fact, people have never observed any natural existence on the earth. of molecules containing inert elements. But about a decade ago, astronomers stumbled upon these compounds in space.
Then, in 2019, observers discoveredThe second type of molecule containing inert elements ——They have been searching for more than thirty years. This molecule, which was first formed after the Big Bang, reveals the chemical composition of the universe long before stars and galaxies formed. The discovery may even help astronomers understand how the first stars were born.
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mostchemical elementcan work withOther elements share electronsForm molecules, but inert elements generally cannot.University of Cologne, Germany(University of Cologne)Astrophysicist Peter Schilke“Noble gases are, in a sense, very 'well rounded,'” said Peter Schilke. This is because the outer electron shells of noble gas atoms are already filled with electrons, so they typically do not exchange electrons with other atoms to form chemical bonds. And form molecules, at least on Earth.
Looking back,Space appears to be an ideal place to search for molecules containing inert elements, because these gases exist in large quantities in the universe. Helium is the second most common element in the universe, after hydrogen, while neon is the fifth. Usually, in interstellar space, the temperature and gas density are in critical or supercritical states. At this time, rare gases will undergo some reactions that cannot occur on earth, such as forming molecules.
Apart fromIn addition to revealing the rudiments of the universe, these strange molecules can also tell scientists about the conditions in interstellar space., providing information about the gases of the interstellar medium.astronomers on thiswith great interest. “The interstellar medium is where stars and planetary systems are born,” laments Maryvonne Gerin, an astrophysicist at the Paris Observatory and co-author of an article on the interstellar medium that was published in 2016 Annual Review of Astronomy and Astrophysics.
For decades, astronomers have been searching for aSpecial molecules containing inert elements: Helium hydride (HeH+). Helium hydride is composed of helium and hydrogen, two of the most common elements in the universe, so it is considered likely to exist in space. Although naturally occurring helium hydride has never been found on Earth, scientists were able to combine the two types of atoms in the laboratory about a century ago. This compound seems to be what astronomers are searching for. But the discovery of an even more exotic molecule caught them off guard.
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Discovery of rare gases on Earth
On Earth, scientists have been trying to synthesize molecules containing inert elements for nearly a century. In 1925, experimentalists successfully bonded helium and hydrogen to obtain helium hydride, or HeH+. To an astronomer, this is just a molecule; but to a chemist, it should be classified as a molecular ion due to its electrical properties.
In 1962, chemistNeil Bartlett(Neil Bartlett) Combined xenon with fluorine and platinum to obtain a mustard-colored compound for the first time. Because it contains electrically neutral molecules, astronomers and chemists are happy to say that it is filled with molecules of inert elements. However, no naturally occurring molecules containing inert elements have ever been observed on Earth.
Interstellar news
The content of argon in the Earth's atmosphere is more than 20 times that of carbon dioxide, but it is rarely mentioned. In fact, argon is the third most abundant element in the gas you exhale. Nitrogen makes up 78% of the Earth's atmosphere, oxygen makes up 21%, and argon makes up the bulk of the remaining 1% of the gas mixture.
However,No one expected to find interstellar molecules containing argon. Mike Barlow, an astrophysicist at University College London, led a team that accidentally discovered argon hydride (ArH+, that is, argon hydrogen ions). “The discovery was purely accidental,” Barlow said.
anotherInert elements also contributed to the discovery. In 2009, the Herschel Space Observatory entered space. During operations, it was cooled by refrigerated liquid helium tanks that could last four years. This allows the observatory to observe far-infrared light emitted by distant objects without interference from its own heat dissipation. Many molecules can absorb and emit far-infrared light, so this band is ideal for discovering new space molecules.
Within a year of Herschel's launch, astronomers began to noticeThere is some substance in interstellar space that has an absorption peak at 485μm, and such spectral lines have never been observed before. “No one can figure out what it is,” says astrophysicist David Neufeld of Johns Hopkins University. He co-authored an article published in the Annual Review in 2016.
Schilke consulted colleagues at Universität zu Köln and elsewhere. “We sat in the office and listed all the possible molecules on the whiteboard, including argon hydrogen ions,” he says. No known molecule had a wavelength that matched the observed wavelength of 485 μm.
Meanwhile, Barlow's team is using data from the Herschel Space Observatory to study the Crab Nebula, the remnant of a massive star that our ancestors saw exploded in 1054. The explosion produced argon and other “metals” – what astronomers define as any element heavier than helium.
In the argon-rich gas of the nebula,Barlow and his colleagues discovered two unknown spectral lines. One of them is the mysterious line of 485μm that everyone has seen, and the wavelength of the other is exactly half of the former, which is a characteristic of diatomic molecules. Barlow identified it as argonium and published his discovery in 2013. This is the first molecule containing an inert element ever discovered in nature. (Barlow points out that in the title of his Science paper, the editor changed “molecule” to “molecular ion” at the last minute.)
The discovery was shocking.“When we heard the news, we were just stunned!” Neufeld said. After all, astronomers have observed the same 485μm spectral line elsewhere. “When I first heard about this discovery, I was embarrassed because we hadn't seen it before,” Schilke said.
Scientists have been misled by this confusion.They thought they knew the wavelength of argon hydrogen ions because, decades ago, scientists had synthesized argon hydrogen ions in the laboratory and measured their spectra. But the molecules in the lab contained argon-40, by far the most common argon isotope on Earth. But that's only because the argon we breathe comes from the radioactive decay of potassium-40 in rocks.
In the universe, this is not the case. “Argon-36 is by far the most abundant isotope in the interstellar medium,” Silk said. “We were stupid not to notice it.” The wavelengths at which argon hydrogen ions made from argon-36 absorb and emit light It's slightly different than the argon hydrogen ions you get with argon-40, which is why the scientists didn't find it.
When they confirmed the existence of interstellar argon hydrogen ions, Hilke, Neufeld, Gerin and colleagues tried to explain how the molecule formed. “It's a molecule that doesn't look like a molecule,” Silk said. Just as argon atoms are very different from other atoms. This bizarre property also has practical value.
The origin of interstellar argon hydrogen ions
Based on standard extrapolations of chemical reactions in space,Scientists know that the formation of interstellar argon hydrogen ions requires two steps.First, cosmic rays (high-speed charged particles) will steal an electron from the interstellar argon hydrogen ion, turning it into an argon ion (Ar+). Subsequently, the argon ions can be converted from hydrogen molecules (H2) takes away a hydrogen atom and combines to form argon hydrogen ion (ArH+); this is because hydrogen atoms tend to bond with argon ions rather than with other hydrogen atoms.
However,Argon hydrogen ions are structurally unstable, and the hydrogen molecules required to form compounds can also destroy their structure.Therefore, the existence conditions of rare gas element molecules have very strict requirements on the ratio of hydrogen atoms to argon atoms: if there are too few hydrogen atoms, the two atoms cannot effectively collide, and argon hydrogen ions will be difficult to form. Too many hydrogen atoms will quickly coordinate with a small amount of argon atoms, changing the normal structure of argon hydrogen ions. This ratio is crucial for the formation of argon hydrogen ions. This harsh condition is actually very useful – it can be used to determine which interstellar clouds are unlikely to form new stars and planets.
In our galaxy,There are two main types of interstellar gas: atomic gas and molecular gas.The former is more common and consists mainly of individual hydrogen and helium atoms. Because the atomic gas structure is dispersed, new stars are rarely formed. Instead, most stars form in denser gaseous environments where atoms clump together to form molecules.
It is not easy to distinguish whether the main structure of interstellar clouds is atomic gas or molecular gas.But argon hydrogen ions can be distinguished. “Argon hydride ions can track high-purity atomic gases,” Silk said. In fact, although argon hydride ions are classified as molecules, they only exist in atomic gases that are 99.9 to 99.99 percent pure.
Cosmic rays cause the formation of argon hydrogen ions.Therefore, the density of argon hydrogen ions in interstellar space is a measure of the number of cosmic rays traveling through the Milky Way. “There are more cosmic rays than we previously thought,” Green said. This is important not only for Captain Kirks who wish to minimize their exposure to damaging radiation on interstellar travel in the future, but also for scientists studying the chemistry of interstellar matter, as cosmic rays are also other The first step in the formation of molecules.
the first molecule in the universe
After the discovery of interstellar argon hydrogen ions, astronomers are still chasing the simplest molecule of the noble gas element predicted by theorists decades ago – helium hydride.“This is the first chemical bond formed in the universe,” said astrophysicist Stephen Lepp of the University of Nevada, Las Vegas.
Helium hydride molecules were able to form because hydrogen and helium were the two main elements that emerged after the Big Bang.In the embryonic stage of the universe, the temperature was extremely high, and electrons captured by hydrogen and helium elements were immediately stripped away by the high-energy radiation generated by the extremely high temperature. As the universe expands, the temperature gradually decreases. About 100,000 years after the Big Bang, each helium nucleus captured two electrons and became electrically neutral. Hydrogen ions (H+) and helium atoms (He) combined to form the first molecule in the universe – HeH+.
To this day, no one has detected helium hydride in the early universe.Because this requires spanning more than 13 billion light-years of space, going back to the beginning of time, and identifying the faint spectral lines of this molecule – which is equivalent to completing an unprecedented feat! However, in April 2019, astronomers led by Rolf Güsten of the Max Planck Institute for Radio Astronomy in Germany claimed to have discovered this search in the Milky Way. long-standing molecules.
Instead of using a spacecraft, Gusten's team made the discovery using a special aircraft.The aircraft flies above nearly all water vapor in the atmosphere to ensure that infrared radiation is not blocked. The SOFIA Stratospheric Observatory for Infrared Astronomy (SOFIA) uses a high-resolution spectrograph telescope to search for their long-awaited molecules in the Milky Way. This device successfully detected the far-infrared spectral signature of helium hydride at 149 μm.
Gusten and his colleagues searched in the NGC7027 nebula in the constellation Cygnus. No one before had succeeded, but they succeeded!About 600 years ago, an aging star called a red giant shed its atmosphere. Our sun will do the same in about 7.8 billion years. The dying star exposed its hot core with a temperature of 190,000 Kelvin (340,000 degrees Fahrenheit). It glowed fiercely and radiated extremely strong ultraviolet light, which stripped electrons from helium atoms to form He+. He+ Combined with the neutral hydrogen atoms inside the nebula, HeH is obtained+. In the early universe, the opposite was true: charged hydrogen and electrically neutral helium combined to form HeH+. But the end result is the same, the products are HeH+the first type of chemical substance formed after the Big Bang.
“It finally brings a long story to an end,” said astronomer Paul Goldsmith. He works at NASA's Jet Propulsion Laboratory and was not involved in the discovery. The detection results confirmed that the calculations predicting the existence of this strange molecule were correct, making it more credible that this molecule existed in the embryonic stage of the universe.
In addition to helium hydride, there may be other molecules containing inert elements in space.In space, there are many more neon atoms than argon atoms, so there may be neon hydrogen ions (i.e. NeH+). If so, the abundance and location of neon hydrogen ions will shed further light on the environment of the interstellar medium. In addition, krypton is extremely rare, so krypton ions cannot threaten “Interstellar Superman” at all. As for xenon, it is even rarer.
However, the universe is so vast, and the temperature and density vary greatly across the universe, which is completely different from that of the Earth. Perhaps, in the corner of some distant interstellar cloud, the most unlikely atoms have come together to form structures stranger than any molecule we have discovered so far, waiting only for a brave observer in the depths of the universe. Their spectral signatures are detected.
Author: Ken Croswell
Translation: Bian Ying
Reviewer: wnkwef
Original link:Space is the place for impossible molecules