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 . 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 2,3 and of high-fidelity qubits used in quantum information processors and quantum simulators 4 . 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 a orded 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 5 .Recent technological advances in the cooling and manipulation of molecules have opened up perspectives for new types of precision measurements. 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.Systems suited for precise spectroscopic measurements need to exhibit narrow spectral lines. Experiments need to allow for long interrogation times to minimize line broadening induced by the finite measurement time. Moreover, studies should be performed in a well-controlled and isolated environment. 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. 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. Indeed, many of the currently most precise spectroscopic experiments rely on dipole-forbidden electronic transitions in Coulomb-crystallized atomic ions 2,11 . By contrast, to the best of our knowledge no dipole-forbidden vibrational-that is, IR-spectra of molecular ions have been reported so far. 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 .Dipole-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. Thus far, they were observed only in a handful of neutral diatomics, suc...

Recent mass measurements of light atomic nuclei in Penning traps have indicated possible inconsistencies in closely related physical constants like the proton-electron and deuteron-proton mass ratios. These quantities also influence the predicted vibrational spectrum of the deuterated molecular hydrogen ion in its electronic ground state. We measure the frequency of the v = 0→9 overtone transition of this spectrum with an uncertainty of 2.9 parts-per-trillion through Doppler-free two-photon laser spectroscopy. Leveraging high-precision ab initio calculations we convert our measurement to tight constraints on the proton-electron and deuteron-proton mass ratios, consistent with the most recent Penning-trap determinations of these quantities. This results in an unprecedented precision of 21 parts-per-trillion for the value of the proton-electron mass ratio.

State-selected Coulomb-crystallized molecular ions were employed for the first time in ion-molecule reaction studies using the prototypical charge-transfer process N + 2 + N 2 → N 2 + N + 2 as an example. By preparing the reactant ions in a well-defined rovibrational state and localizing them in space by sympathetic cooling to millikelvin temperatures in an ion trap, state-and energy-controlled reaction experiments with sensitivities on the level of single ions were performed. The experimental results were interpreted with quasi-classical trajectory simulations on a six-dimensional potential-energy surface which provided detailed insight into translation-to-rotation energy transfer occurring during charge transfer between N 2 and N + 2 .

Radiation damage is considered to be the major problem that still prevents imaging an individual biological molecule for structural analysis. So far, all known mapping techniques using sufficient short wavelength radiation, be it x rays or high energy electrons, circumvent this problem by averaging over many molecules. Averaging, however, leaves conformational details uncovered. Even the anticipated use of ultrashort but extremely bright x-ray bursts of a free electron laser shall afford averaging over 10{6} molecules to arrive at atomic resolution. Here, we present direct experimental evidence for nondestructive imaging of individual DNA molecules. In fact, we show that DNA withstands coherent low energy electron radiation with deBroglie wavelength in the Angstrom regime despite a vast dose of 10{8} electrons/nm{2} accumulated over more than one hour.

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