Synopsis

Ejected Electron Slows Molecule’s Rotation

Physics 17, s134
Sometimes a rotating molecule can transition to a new state only if an electron carries away some of the molecule’s angular momentum.
MPIK

Small molecular ions are important in atmospheric chemistry and astrophysics. The diatomic carbon ion C2 is a heavily studied example that has been a source of mystery: When highly excited, most molecular ions have a wide range of lifetimes before converting to a neutral form. But many C2 ions shed their electron with a specific lifetime of about 3 milliseconds. Now Viviane Schmidt of the Max Planck Institute for Nuclear Physics (MPIK) in Germany and her colleagues have solved the mystery by discovering a molecular process that involves a change in angular momentum [1, 2]. If the C2 molecule spins rapidly enough, a certain electronically excited state transitions to a C2 state only if the departing electron takes away some of the molecule’s angular momentum, a requirement that leads to the measured lifetime.

For the conversion from C2 to C2 to occur, the final state must have lower energy than the initial state. However, in a rapidly rotating molecule, the energies of the electronic states differ from those in a nonrotating molecule. Schmidt and her colleagues found theoretically that when C2 has 155 or more quanta of angular momentum, a certain excited electronic state has less energy than the C2 state to which it would normally convert. The transition is impossible unless the ejected electron removes enough angular momentum to shift the final state’s energy below the initial state’s energy.

The researchers’ theory for such “rotationally assisted” transitions showed that processes requiring a transfer of six units of angular momentum are responsible for the 3-millisecond C2 lifetime the team observed at the MPIK Cryogenic Storage Ring. Schmidt expects similar processes to occur in other highly excited molecules both in the atmosphere and in nuclear-fusion plasmas.

–David Ehrenstein

David Ehrenstein is a Senior Editor for Physics Magazine.

References

  1. V. C. Schmidt et al., “Autodetachment of diatomic carbon anions from long-lived high-rotation quartet states,” Phys. Rev. Lett. 133, 183001 (2024).
  2. V. C. Schmidt et al., “Unimolecular processes in diatomic carbon anions at high rotational excitation,” Phys. Rev. A 110, 042828 (2024).

Subject Areas

Atomic and Molecular PhysicsChemical Physics

Related Articles

Probing the Rotational Doppler Effect with a Single Ion
Atomic and Molecular Physics

Probing the Rotational Doppler Effect with a Single Ion

A light beam with orbital angular momentum can produce the rotational analog of the Doppler effect on an ion. Read More »

Quantum Coherence Boosts Quantum Work
Atomic and Molecular Physics

Quantum Coherence Boosts Quantum Work

By manipulating a nitrogen vacancy’s single spin, researchers have shown that the more coherent the system is, the more work can be extracted from it. Read More »

Qubits Manipulated on the Fly
Quantum Information

Qubits Manipulated on the Fly

A way to address the individual ions of a rotating ion crystal could allow scientists to perform quantum simulations in which each qubit can be carefully controlled. Read More »

More Articles