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

摘要: Spectroscopic transitions in atoms and molecules that are not allowed within the electric-dipole approximation, but occur because of higher-order terms interaction between matter radiation, termed dipole-forbidden(1). These extremely weak therefore exhibit very small natural linewidths. Dipole-forbidden optical form basis next-generation atomic clocks(2,3) high-fidelity qubits used quantum information processors simulators(4). In molecules, however, such much less characterized, reflecting considerable challenges to address them. Here, we report direct observation dipole-forbidden, electric-quadrupole-allowed infrared (IR) a molecular ion. Their detection was enabled by long interrogation times several minutes afforded sympathetic cooling individual quantum-state-selected ions into nearly perturbation-free environment Coulomb crystal. The present work paves way for new mid-IR frequency standards precision spectroscopic measurements on single IR domain(5).