New molecular bonding mechanism predicted in exotic stars’ atmospheres.
Magnetism may be the secret to a strong marriage ― at least between certain atoms in the atmospheres of white dwarf and neutron stars. Computer simulations reveal that a new, more powerful chemical bond should be induced by the stars’ ferocious magnetic fields. If harnessed at lower fields in the lab, similar effects in ‘magnetized matter’ could be exploited for quantum computing.
Chemists classify strong molecular bonds into two types: ionic bonds, in which electrons from one atom hop over to another, and covalent bonds, in which electrons are shared between atoms. But Trygve Helgaker, a quantum chemist at the University of Oslo in Norway and colleagues accidentally discovered a third bonding mechanism when they simulated how atoms behave under extreme magnetic fields of around 10^5 Tesla--ten thousand times greater than those generated on Earth.
The team first examined how the lowest energy state, or ground state, of a hydrogen molecule, containing two atoms, was distorted by the field. The dumbbell-shaped molecule reoriented its axis to lie parallel to the direction of the field and contracted, making its bond even more stable, says Helgaker. More surprisingly, when one of the electrons was given some energy, the molecule flipped so that its axis lay perpendicular to the field. “We always teach students that when an electron is excited like this, the molecule falls apart,” says Helgaker. “But here we see a new type of bond keeps the atoms hanging together.”
The glue holding the atoms together is provided by the way that electrons dance around the magnetic field lines, explains Helgaker. “The way electrons move relative to the field, and their kinetic energy, can become as important for chemical bonding as the electrostatic attraction between the electrons and the nuclei,” he says. Depending on their geometry, molecules will reorient to allow electrons to rotate around the direction of the magnetic field. The team also report that a similar effect should occur between two ground state helium atoms, in Science today1.
Though these new states could well exist in the atmospheres of some white dwarfs and neutron stars, it will be tough to spot them, says astrophysicist Dong Lai at Cornell University, in Ithaca, New York. The team will need to extend their model to see how likely it is that these states remain bound at the high temperatures seen in these atmospheres, and exactly how they would modify the spectra of light coming from the stars, he says. “This is an important step, but several more are needed to see how relevant this is in astrophysics,” says Lai.
Closer to home, it is virtually impossible to generate such high magnetic fields because they are accompanied by drastic changes in chemistry, with atoms shrinking by around 25% in size, notes Helgaker: “The experimental apparatus would cease to be an apparatus in these extreme conditions!”
Nonetheless, the findings boost hopes that ‘magnetized matter’ in the lab could have dramatic new properties that may be exploited. In particular, in 2009, physicists created a new weakly bound state2, originally posited by atomic physicist, Chris Greene at the University of Colorado at Boulder and colleagues3, called a “Rydberg molecule”. These molecules are candidates for building quantum computers and they are highly sensitive to magnetic effects, Greene notes: “That means we could use magnetic fields as a knob to tightly control the strength of the binding, to manipulate them to store and erase quantum memory as needed.”
Lange, K. K., Tellgren, E. I., Hoffmann, M. R. & Helgaker, T. Science 337, 327–331 (2012).
Bendkowsky, V. et al., Nature, 458, 1005-1008 (2009).
Greene, C. H., et al. Phys. Rev. Lett. 85, 2458 (2000).
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Merali, Z. Magnetic fields boost atoms’ bonding skills [back by 1pm pls]. Nature (2012). https://doi.org/10.1038/nature.2012.11045