Thus far, they were observed only in a handful of neutral diatomics, such as H 2, N 2 and O 2, using high-pressure samples and/or very long absorption path lengths 14, 15, 16. Studies of vibrational transitions in molecules, however, are attractive as they probe different spectral domains and dynamic regimes from those in studies of atomic systems 5, 8, 12.ĭipole-forbidden vibrational transitions in molecules 13 are several orders of magnitude weaker than dipole-forbidden optical transitions typically used in atoms 2, 3, rendering their observation challenging. By contrast, to the best of our knowledge no dipole-forbidden vibrational-that is, IR-spectra of molecular ions have been reported so far. Indeed, many of the currently most precise spectroscopic experiments rely on dipole-forbidden electronic transitions in Coulomb-crystallized atomic ions 2, 11. Together with ultracold atoms in optical lattices 3, they represent one of the most advanced systems used in state-of-the-art precision spectroscopic measurements. Trapped cold ions spatially localized in a Coulomb crystal 10 with sufficiently strong confinement to allow Doppler-free excitation in the Lamb–Dicke regime fulfil these requirements. Moreover, studies should be performed in a well-controlled and isolated environment. Experiments need to allow for long interrogation times to minimize line broadening induced by the finite measurement time. Systems suited for precise spectroscopic measurements need to exhibit narrow spectral lines. Fundamental questions, such as a possible time variation of fundamental physical constants 6, the magnitude of the dipole moment of the electron 7, the existence of additional fundamental interactions 8 and the effects of parity-violating interactions in chiral molecules 9, can now be addressed by molecular spectroscopy at an unprecedented precision. Recent technological advances in the cooling and manipulation of molecules have opened up perspectives for new types of precision measurements. The present work paves the way for new mid-IR frequency standards and precision spectroscopic measurements on single molecules in the IR domain 5. 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. Here, we report direct observation of dipole-forbidden, electric-quadrupole-allowed infrared (IR) transitions in a molecular ion. In molecules, however, such transitions are much less characterized, reflecting the considerable challenges to address them. Dipole-forbidden optical transitions in atoms form the basis of next-generation atomic clocks 2, 3 and of high-fidelity qubits used in quantum information processors and quantum simulators 4. These transitions are extremely weak and therefore exhibit very small natural linewidths. 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 1.
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