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We demonstrate a single-molecule technique that is compatible with high-precision measurements and obtain the spectrum of three molecular ion species. While the current result yields a modest spectral resolution due to a broad light source, we expect the method to ultimately provide a resolution comparable to quantum logic methods with significantly less stringent requirements. Adaptations of this technique will prove useful in a wide range of precision spectroscopy arenas including the search for parity-violating effects in chiral molecules. 

Spectroscopy is a key analytical tool that provides valuable insight into molecular structure and is widely used to identify chemical samples. Tagging spectroscopy is a form of action spectroscopy in which the absorption of a single photon by a molecular ion is detected via the loss of a weakly attached, inert “tag” particle (e.g. He, Ne, N2).1–3 The absorption spectrum is derived from the tag loss rate as a function of incident radiation frequency. To date, all spectroscopy of gas phase polyatomic molecules has been restricted to large molecular ensembles, complicating spectral interpretation by the presence of multiple chemical and isomeric species. Here we present a novel tagging spectroscopic scheme to analyze the purest possible sample: a single gas phase molecule. We demonstrate this technique with the measurement of the infrared spectrum of a single tropylium (C7H7+ ) molecular ion; to our knowledge the first recorded spectrum of a single gas phase polyatomic molecule. Our method’s high sensitivity revealed spectral features previously unobserved using traditional tagging methods.4 Our approach in principle enables analysis of multi-component mixtures by identifying constituent molecules one at a time. Single molecule sensitivity extends action spectroscopy to rare samples, such as those of extraterrestrial origin,5,6 or to reactive reaction intermediates formed at number densities too low for traditional action methods.