Single molecule infrared spectroscopy in the gas phase.

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 (for example, He, Ne, N2)1-3. The absorption spectrum is derived from the tag loss rate as a function of incident radiation frequency. So far, all spectroscopy of gas phase polyatomic molecules has been restricted to large molecular ensembles, thus complicating spectral interpretation by the presence of multiple chemical and isomeric species. Here we present a novel tagging spectroscopic scheme to analyse the purest possible sample: a single gas phase molecule. We demonstrate this technique with the measurement of the infrared spectrum of a single gas phase tropylium (C7H7+) molecular ion. The high sensitivity of our method revealed spectral features not previously observed using traditional tagging methods4. Our approach, in principle, enables analysis of multicomponent mixtures by identifying constituent molecules one at a time. Single molecule sensitivity extends action spectroscopy to rare samples, such as those of extraterrestrial origin5,6, or to reactive reaction intermediates formed at number densities that are too low for traditional action methods.

Spectroscopy of Molecular Ions in Coulomb Crystals.

In this Perspective, we examine the use of laser-cooled atomic ions and sympathetically cooled molecular ions in Coulomb crystals for molecular spectroscopy. Coulomb crystals are well-isolated environments that provide localization and long storage times for sensitive measurements of weak signals and cold temperatures for precise spectroscopy. Coulomb crystals of molecular and atomic ions enable the detection of single-photon molecular ion transitions at a range of wavelengths by a change in atomic ion fluorescence at visible wavelengths. We give an overview of the state of the art from action spectroscopy to quantum logic spectroscopy for a wide range of molecular transitions from rotational sublevels separated by 10-7 cm-1 to rovibronic transitions at 25 000 cm-1. We emphasize how this system allows for unparalleled control of the molecular ion state for precision spectroscopy with applications in astrochemistry and fundamental physics. We conclude with an outlook of the use of this control in cold molecular ion reactions.

Rovibronic Spectroscopy of Sympathetically Cooled 40CaH.

We measure the rovibronic transitions X 1Σ+, v″ = 0, J″ → A 1Σ+, v' = 0-3, J' of CaH+ and obtain rotational constants for the A 1Σ+ state. The spectrum is obtained using two-photon photodissociation of CaH+ cotrapped with Doppler cooled Ca+. The excitation is driven by a mode-locked, frequency-doubled Ti:Sapph laser, which is then pulse shaped to narrow the spectral bandwidth. The measured values of the rotational constants are in agreement with ab initio theory.

Vibronic Spectroscopy of Sympathetically Cooled CaH.

We report the measurement of the 11 Σ→21 Σ transition of CaH+ by resonance-enhanced photodissociation of CaH+ that is co-trapped with laser-cooled Ca+ . We observe four resonances that we assign to transitions from the vibrational v=0 ground state to the v'=1-4 excited states based on theoretical predictions. A simple theoretical model that assumes instantaneous dissociation after resonant excitation yields results in good agreement with the observed spectral features except for the unobserved v'=0 peak. This discrepancy is attributed to an insufficient understanding of the dissociation process, and further experimental and theoretical studies are required to confirm the assignment. The resolution of our experiment is limited by the mode-locked excitation laser, but this survey spectroscopy enables future rotationally resolved studies with applications in astrochemistry and precision measurement.

The copper (II) ion as a carrier for the antibiotic capreomycin against Mycobacterium tuberculosis.

In recent years, the bacterium responsible for tuberculosis has been increasing its resistance to antibiotics resulting in new multidrug resistant Mycobacterium tuberculosis (MR-TB) and extensively drug-resistant tuberculosis (XDR-TB). In this study we use several analytical techniques including NMR, FT-ICR, TOF-MS, LC-MS and UV/Vis to study the copper-capreomycin complex. The copper (II) cation is used as a carrier for the antibiotic capreomycin. Once this structure was studied using NMR, FT-ICR, and MALDI-TOF-MS, the NIH-NIAID tuberculosis cell line for several Tb strains (including antibiotic resistant strains) were tested against up to seven variations of the copper-capreomycin complex. Different variations of copper improved the efficacy of capreomycin against Tb up to 250 fold against drug resistant strains of Tb.