Observation of electric-dipole-forbidden infrared transitions in cold molecular ions

Spectroscopic transitions in atoms and molecules that are not allowed within the electric-dipole approximation, but occur because of higher-order terms in the interaction between matter and radiation, are termed dipole-forbidden. These transitions are extremely weak and therefore exhibit very small natural linewidths. Dipole-forbidden optical transitions in atoms form the basis of next-generation atomic clocks and of high-fidelity qubits used in quantum information processors and quantum simulators. In molecules, however, such transitions are much less characterized, reflecting the considerable challenges to address them. Here, we report direct observation of dipole-forbidden, electric-quadrupole-allowed infrared (IR) transitions in a molecular ion. Their detection was enabled by the very long interrogation times of several minutes afforded by the sympathetic cooling of individual quantum-state-selected molecular ions into the nearly perturbation-free environment of a Coulomb crystal. The present work paves the way for new mid-IR frequency standards and precision spectroscopic measurements on single molecules in the IR domain.