As the element that makes up 75 percent of all the mass in the Universe, and more than 90 percent of all the atoms, we’re all pretty well acquainted with hydrogen.

But the simplest and most abundant element in the Universe still has some tricks up its sleeve, because physicists have just created a never-before-seen form of hydrogen - negatively charged hydrogen clusters.

To understand what negatively charged hydrogen clusters are, you first have to wrap your head around their far more common counterparts - positively charged hydrogen clusters.

Positively charged hydrogen clusters are pretty much exactly what they sound like - positively charged clusters of a few or many hydrogen molecules.

Known simply as hydrogen ion clusters, they form at very low temperatures, and can contain as many as 100 individual atoms.

Physicists confirmed the existence of hydrogen ion clusters some 40 years ago, and while a negative counterpart to these clusters boasting large numbers of atoms were theorised, no one could figure out how to create one.

But that didn't stop a team of physicists led by Michael Renzler from the University of Innsbruck in Austria from giving it a shot.

First, they injected cold liquid helium droplets with hydrogen (H2) molecules. This caused the mixture to form clusters with a neutral charge.

Next, they exposed these H2-infused droplets to an electron beam, and that caused some of the hydrogen molecules to ionise, and be flung out into the surrounding vacuum as negatively charged hydrogen ions.

Soon, nearby hydrogen molecules started clustering around the negatively charged ions, and the researchers discovered that these newly formed groups could boast a few, or many molecules each.

As is often the case with the most elusive structures in particle physics, these negatively charged hydrogen clusters existed only for an incredibly fleeting moment - several microseconds (1 microsecond = 0.000001 seconds).

But that was enough time for the team to determine their geometric structures.

As Michael Schirber reports for the American Physical Society, the researchers found that the clusters only had odd atom numbers, ranging from n= 5 to n = 129.

The most stable clusters had a central, negatively charged H− ion core sounded by shells that were completely filled up by hydrogen molecules.

"The odd values implied that the clusters were a combination of several H2 molecules and a single H− ion core, held together through an induced dipole attraction," Schirber explains.

The most commonly recorded clusters had the atomic numbers n = 25, n = 65, and n = 89.

These 'magic numbers', as the team calls them, correspond to symmetrical, almost solid arrangements of H2 molecules, such as a 20-sided icosahedron. (You can see one of these formations in the image at the top of the page.)

So why does all this matter? For years now, researchers have suspected that these large hydrogen clusters could form naturally in outer space, either in cold, dense clouds, or the atmospheres of gas planets.

Now that we know that negatively charged hydrogen clusters aren't impossible, and what their most common and stable form could be, it could make it easier to identify them in nature for the time.

And at a time when researchers are questioning if dark matter - the stuff that hypothetically makes up 27 percent of the observable Universe - even exists, we need all the help we can get when it comes to deciphering the smallest constituents of the space around us.

The research has been published in Physical Review Letters. </s> For some time, scientists have been fascinated by the concept of metallic hydrogen. Such an element is believed to exist naturally when hydrogen is placed under extreme pressures (like in the interior of gas giants like Jupiter). But as a synthetic material, it would have endless applications, since it is believed to have superconducting properties at room temperature and the ability to retain its solidity once it has been brought back to normal pressure.

For this reason, condensed matter physicists have been attempting to create metallic hydrogen for decades. And according to a recent study published in Science Magazine, a pair of physicists from the Lyman Laboratory of Physics at Harvard University claim to have done this very thing. If true, this accomplishment could usher in a new age of super materials and high-pressure physics.

The existence of metallic hydrogen was first predicted in 1935 Princeton physicists Eugene Wigner and Hillard Bell Huntington. For years, Isaac Silvera (the Thomas D. Cabot Professor at Harvard University) and Ranga Dias, a postdoctorate fellow, have sought to create it. They claim to have succeeded, using a process which they described in their recently-published study, “Observation of the Wigner-Huntington transition to metallic hydrogen“.

Such a feat, which is tantamount to creating the heart of Jupiter between two diamonds, is unparalleled in the history of science. As Silvera described the accomplishment in a recent Harvard press release:

In the past, scientists have succeeded in creating liquid hydrogen at high temperature conditions by ramping up the pressures it was exposed to (as opposed to cryogenically cooling it). But metallic hydrogen has continued to elude experimental scientists, despite repeated (and unproven) claims in the past to have achieved synthesis. The reason for this is because such experiments are extremely temperamental.

For instance, the diamond anvil method (which Silvera and Dias used a variation of) consists of holding a sample of hydrogen in place with a thin metal gasket, then compressing it between two diamond-tipped vices. This puts the sample under extreme pressure, and a laser sensor is used to monitor for any changes. In the past, this has proved problematic since the pressure can cause the hydrogen to fill imperfections in the diamonds and crack them.

While protective coatings can ensure the diamonds don’t crack, the additional materials makes it harder to get accurate readings from laser measurements. What’s more, scientists attempting to experiment with hydrogen have found that pressures of ~400 gigapascals (GPa) or more need to be involved – which turns the hydrogen samples black, thus preventing the laser light from being able to penetrate it.

For the sake of their experiment, Professors Ranga Dias and Isaac Silvera took a different approach. For starters, they used two small pieces of polished synthetic diamond rather than natural ones. They then used a reactive ion etching process to shave their surfaces, then coated them with a thin layer of alumina to prevent hydrogen from diffusing into the crystal structure.

They also simplified the experiment by removing the need for high-intensity laser monitoring, relying on Raman spectroscopy instead. When they reached a pressure of 495 GPa (greater than that at the center of the Earth), their sample reportedly became metallic and changed from black to shiny red. This was revealed by measuring the spectrum of the sample, which showed that it had become highly reflective (which is expected for a sample of metal).

As Silvera explained, these experimental results (if verified) could lead to all kinds of possibilities:

In short, metallic hydrogen could speed the revolution in electronics already underway, thanks to the discovery of materials like graphene. Since metallic hydrogen is also believed to be a superconductor at room temperature, its synthetic production would have immense implications for high-energy research and physics – such as that being conducted by CERN.

Beyond that, it would also enable research into the interior’s of gas giants. For some time, scientists have suspected that a layer of metallic hydrogen may surround the cores of gas giants like Jupiter and Saturn. Naturally, the temperature and pressure conditions in the interiors of these planets make direct study impossible. But by being able to produce metallic hydrogen synthetically, scientists could conduct experiment to see how it behaves.

Naturally, the news of this experiment and its results is being met with skepticism. For instance, critics wonder if the pressure reading of 495 GPa was in fact accurate, since Silvera and Dias only obtained that as a final measurement and were forced to rely on estimates prior to that. Second, there are those who question if the reddish speck that resulted is in fact hydrogen, and some material that came from the gasket or diamond coating during the process.

However, Silvera and Dias are confident in their results and believe they can be replicated (which would go far to silence doubts about their results). For one, they emphasize that a comparative measurement of the reflective properties of the hydrogen dot and the surrounding gasket suggest that the hydrogen is pure. They also claim their pressure measurements were properly calibrated and verified.

In the future, they intend to obtain additional spectrographic readings from the sample to confirm that it is in fact metallic. Once that is done, they plan to test the sample to see if it is truly metastable, which will consist of them opening the vise and seeing if it remains in a solid state. Given the implications of success, there are many who would like to see their experiment borne out!

Be sure to check out this video produced by Harvard University that talks about the experiment:

By Matt Williams - 

 Matt Williams is the Curator of the Guide to Space for Universe Today, a regular contributor to HeroX, a science fiction author, and a Taekwon-Do instructor. He lives with his family on Vancouver Island in beautiful BC. </s> State of hydrogen predicted to be a room temperature superconductor reportedly seen – but the methods used are being questioned

It has been over 80 years in the making but the metallic form of hydrogen has finally been created in the lab at extreme pressures. Isaac Silvera and Ranga Dias from Harvard University in the US estimate that they subjected hydrogen to 495 gigapascals (GPa), 4.95 million times the standard pressure exerted by Earth’s atmosphere. ‘It’s the first time that metallic hydrogen has existed on Earth,’ Silvera tells Chemistry World.

Yet, with this achievement long prized in the high pressure science community, Silvera’s rival experimentalists have responded with extreme scepticism. Eugene Gregoryanz, whose University of Edinburgh team took hydrogen to 380GPa in 2016, expects the study to be debunked within a year. Doubts centre on approximations used by the Harvard researchers in measuring the pressures involved.

It’s the first time that metallic hydrogen has existed on Earth Isaac Silvera, Harvard University

Modern calculations suggest metallic hydrogen could be a valuable room temperature superconductor and stable on returning to atmospheric pressure, and in its dense form also potentially useful as a rocket propellant. University of Cambridge materials scientist Chris Pickard is one of those trying to predict metallic hydrogen’s properties, and the pressure at which it might form. ‘Interestingly, it’s been settling down around the pressure that [Silvera and Dias] have found,’ Pickard says.

Crushing hydrogen so comprehensively means squeezing it between micrometre-scale points in diamond anvil cells (DACs). At such extreme conditions, however, diamonds regularly break, especially because they become brittle when hydrogen works its way among their carbon atoms. Consequently, Silvera and Dias were extremely cautious, starting by making anvils from synthetic diamonds that are less likely to have defects than natural ones.

‘We remove five or six microns of the surface and the possible defects that might be there,’ Silvera explains. ‘Then we coat the diamonds with alumina as a diffusion barrier against hydrogen. We never let the diamonds get above 80K, because at low temperature you supress diffusion. We put a lot of attention into getting good alignment. And we don’t shine intense laser beams at the sample until the end, because it’s known that lasers break diamonds.’ However, the final measure casts a shadow over what researchers can learn from the experiment, because they rely on lasers to gather information.

More illumination needed?

Silvera and Dias assert that their hydrogen sample is metallic because it became reflective, which they can see with laser beams that don’t damage the diamond. They also use lasers to measure the pressure the hydrogen and diamond experience, largely by monitoring variations in the light wavelengths the materials absorb, which indicate structural changes. However, above 335GPa that does require damaging beams, and so past that point they only risked one pressure measurement, when they had observed reflection.

The changes in reflectivity are too big to believe it’s actually hydrogen Eugene Gregoryanz, University of Edinburgh

The resulting pressure estimations in the paper are ‘strange, if not incorrect’, according to Mikhail Eremets from the Max Planck Institute for Chemistry in Mainz, Germany. Dias and Silvera find that pressure changes in direct proportion to the force applied by turning the screws that crush the anvils together. That’s not what Eremets has observed in his own high pressure hydrogen research. The Harvard scientists extrapolate this relationship to get the 495GPa measurement, he explains, whereas if they’d used the more conventional pressure–force relationship they would get 590GPa. That stretches plausibility, he underlines.

‘They might be lucky,’ Eremets says. ‘They used good diamonds, maybe it helped. But we’ve done the same for three years at least. In our experience it’s impossible to reach such high pressures.’ He’s therefore ‘very surprised’ the paper was published.

Gregoryanz’s many objections include – but go beyond – pressure measurements. He also questions whether the Harvard team even saw metallic hydrogen, and suggests they might instead be measuring a rhenium gasket used in the DAC. ‘The changes in reflectivity are too big to believe it’s actually hydrogen,’ Gregoryanz says.

Peer pressure

Paul McMillan from University College London notes Silvera’s team is ‘known internationally for careful and pioneering work’. ‘Although the researchers record the existence of a metallic state, they could not conclude on the crystalline nature of the sample,’ he adds. ‘Now the worldwide community, including extreme conditions researchers, planetary physicists and energy scientists, will look forward to discussing and debating these new data.’ Pickard echoes this. ‘As a landmark scientific result, this will have to stand up to the scrutiny of the experimental groups, but it’s very exciting,’ he says.

Silvera stands by his results but accepts that the pressure measurement may be revised. ‘In high pressure research things aren’t always well calibrated,’ Silvera says. ‘If people disagree, they should go to measure it and try to show that it’s different than what was claimed.’ Among the next steps for his own team will be measuring the high pressure hydrogen sample’s conductivity directly.

In the meantime, many have already visited Silvera’s lab to see the DAC containing a 10µm diameter hydrogen particle that they believe is unique in the planet’s history. ‘After we made it we told the staff here at Harvard,’ Silvera says. ‘We got 200 or 300 people come to see it, it went on all day.’
Scientists have created a new form of hydrogen at temperature near absolute zero. These anions or clusters are formed by an electron addition to liquid helium nanodroplets doped with molecular hydrogen or deuterium.