Chemists predict ghostly molecule

You might want to leave the lights on tonight; scientists say ghosts can exist. Theorists predict conditions to create a so-called ghost molecule—a single atom with electron density that resembles that of unusual two-atom molecules (Phys. Rev. Lett. 2018, DOI: 10.1103/PhysRevLett.121.113203).

Besides expanding chemists’ understanding of chemical bonding, these oddities could serve as components of quantum computers, if researchers can find a way to make them.

These ghosts resemble trilobite molecules, which Chris H. Greene of Purdue University and colleagues predicted in 2000. The trilobites consist of a Rydberg atom—one in which electrons are in very high energy states—and a second atom whose presence causes the Rydberg atom’s energy levels to mix together to create an unusual pattern of electron density that looks like a fossilized trilobite. Researchers at the University of Oklahoma made a trilobite molecule from cesium atoms in 2015.

Greene’s group is now predicting that the same effect can be achieved using an excited hydrogen atom and a sequence of electric and magnetic field pulses to perturb its energy levels. Because there is no second, ground-state atom, the team calls the effect a “ghost bond” or “ghost molecule.”

Through computational simulations, they predict the resulting trilobite-like electron density would last several milliseconds at 10 K. Like with non-ghost trilobite molecules, these densities would be large—as much as 2,500 angstroms long—and have permanent dipole moments on the order of 1,000 Debye.

Harvard University theoretical physicist Hossein Sadeghpour, who first predicted the existence of trilobite molecules with Greene, says “it will take awhile” to make the ghost version, but he thinks it is possible. He and Greene agree detecting them may be as hard as making them because it is difficult to image these high energy states. Greene proposes using electron momentum spectroscopy or X-ray diffraction spectroscopy, but he thinks it could be a stretch even for those techniques.