Physicists find charge separation in a molecule consisting of two identical atoms

Physicists from the University of Stuttgart show the first experimental proof of a molecule consisting of two identical atoms that exhibits a permanent electric dipole moment. This observation contradicts the classical opinion described in many physics and chemistry textbooks.

This statement bugs me for two reasons. One is the “contradicts classical opinion” statement, because statements like this usually are a matter of context, and the contradiction occurs when you strip the context. In physics, most equations come with caveats. There are few that apply universally; most are derived under a set of assumptions or meant to apply under specific conditions. However, there are some who try to apply the equations under conditions that violate the assumptions and should not be surprised when the equation fails. And I think this is one of those cases. The “no permanent electric dipole moment” argument is one of symmetry. As long as the symmetry is maintained, there is no EDM, and electrons distributions a these cases should ensure that symmetry. But what happened in this research is a way was found to violate that symmetry — by putting one of the pair of atoms into a Rydberg state (high energy level, maximum angular momentum for the state, which makes the atom physically large). The electron is far from the nucleus and the other electrons can’t compensate. That’s pretty neat, and I think we should celebrate that, rather than the sensationalistic “They said it couldn’t be done!” half-truth.

The other part is the use of “permanent”. This is an excited state. It’s not permanent, though Rydberg states tend to be long-lived. Though that may also be a terminology issue, with permanent simply meaning lives long enough to be measured.

Here’s a better (IMO) write-up on the phenomenon.

Electrons out of balance