Synopsis: Detection of Ortho-Para Transition in Molecules

Microwave emission from a simple molecule reveals a highly forbidden transition between two nuclear spin states.

Certain symmetric molecules come in two states, ortho and para, which differ in their nuclear spin alignment. Photon-mediated transitions between these spin isomers are strongly forbidden, but researchers have now observed such transitions for the first time. Studying a molecule with an advantageous mixing of ortho and para states, the team detected the microwave emission from several ortho-para transitions and observed intensities 1000 times smaller than those of allowed transitions.

The most familiar ortho and para states are those of the hydrogen molecule, ${\text{H}}_{2}$. At room temperature, about 75% of hydrogen molecules are in the ortho state, with the spins of the two hydrogen nuclei in a symmetric configuration (pointing in the same direction). At lower temperatures, molecular collisions convert most of the ortho-${\text{H}}_{2}$ to antisymmetric para-${\text{H}}_{2}$, as it has slightly less energy. However, radiative ortho-para transitions (where a single molecule emits or absorbs a photon) are extremely unlikely, with a predicted spontaneous-emission rate per ${\text{H}}_{2}$ molecule of less than once per the age of the Universe.

Hideto Kanamori, from the Tokyo Institute of Technology, and colleagues considered disulfur dichloride (${\text{S}}_{2}{\text{Cl}}_{2}$), a helically twisted molecule with relatively strong hyperfine interactions between its nuclear quadrupole moment and its molecular electric field. As a result, certain rotational states in ${\text{S}}_{2}{\text{Cl}}_{2}$ are mixtures of ortho and para nuclear states. The team showed that, through this mixing, a forbidden ortho-para transition can “borrow” intensity from an allowed ortho-ortho or para-para transition. The team verified this prediction by observing the microwave emission lines that are characteristic of several forbidden transitions. The implied radiative transition rate of an isolated ${\text{S}}_{2}{\text{Cl}}_{2}$ molecule is once every few thousand years.

This research is published in Physical Review Letters.

–Michael Schirber

Michael Schirber is a Corresponding Editor for Physics based in Lyon, France.

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